Talk:List of metric units

Initial thread
There was no real duplication in scope because we have no article that describes (or purpots to describe) the concept metric units (as opposed to any one one metric systm of units). I recognize that my stub might have looked that way, but the scope was quite different. I will shortly upoload a more complete article, whose scope is all metric units, and not some limited subset. Dondervogel 2 (talk) 09:46, 13 February 2020 (UTC)
 * , fair enough. A key thing here is to link the two articles and explain their relationship.  Is your article a subset of the existing Metric Units article (or it is visa-verse)? Should they be merged?  Answering these types of questions will prevent others from redirecting in the future.  Best of luck with it. Britishfinance (talk) 09:59, 13 February 2020 (UTC)
 * I wouldn't describe it as a subset. The existing articles describing various metric systems (and there are many of these) tend to focus either on one metric system only (usually the SI system and its variants) or a limited selection of metric systems. What I have started on my sub-page is an article with a broader scope, in principle describing all metric units, regardless of which metric system they come from. You will see that I have focused on units outside the SI because many of these units are outside the scope of existing articles. Dondervogel 2 (talk) 11:45, 13 February 2020 (UTC)
 * I uploaded the article again, slightly refined. Please discuss on this talk page (or at Metric system) before reinstating the redirect. Dondervogel 2 (talk) 14:59, 16 February 2020 (UTC)

Move to List of metric units
, as this is a list, I wonder it it would be best move to List of metric units? Cheers, Polyamorph (talk) 15:30, 16 February 2020 (UTC)


 * Thanks for this suggestion. I do not object in principle, but let's hear what others think first. It could be that someone is willing to put effort into making a more encyclopaedic article of it.
 * Also, I think the logarithmic units stick out like a sore thumb. Perhaps they belong in a separate article (or list).
 * Dondervogel 2 (talk) 15:35, 16 February 2020 (UTC)


 * Thanks for the suggestion. I have performed the move from List of Metric units to List of metric units, as the 'metric system' is not commonly capitalised. Sauer202 (talk) 10:37, 12 April 2022 (UTC)

Moving logarithmic units to talk page: make sub-page
/Base-10 logarithmic units


 * I don't think I did that right, but I'm not sure how to fix it. My purpose was just to move the logarithmic units our of the article page so we can discuss where they belong. Help appreciated! Dondervogel 2 (talk) 17:40, 16 February 2020 (UTC)
 * I've now copied the content to my user page. I suggest deleting the stub at /Base-10 logarithmic units. I'd do this myself if I knew how. Dondervogel 2 (talk) 14:26, 18 February 2020 (UTC)
 * You can request speedy deletion by blanking the page and adding a template.  You need to do this yourself as the creator of the page.  Lithopsian (talk) 17:18, 18 February 2020 (UTC)
 * done - thanks for the tip! Dondervogel 2 (talk) 18:43, 18 February 2020 (UTC)

Proposed structure
I suggest the following structure for the article. Thoughts welcome. Dondervogel 2 (talk) 20:52, 16 February 2020 (UTC)


 * I've tweaked a bit. I would like to see the units listed under their categories, meaning that the final "List of metric units" section would disappear.  —Quondum 23:16, 24 March 2020 (UTC)


 * On second thoughts, I'm not sure that I like of mine.  This would be structuring it as a list class article, and perhaps Dondervogel2's proposal below interpreted as having discussion in most of the article and listing of details of individual units left to the end makes more sense: then it can become an actual article, giving the various meanings of "metric unit" and in what contexts they apply.  —Quondum 01:44, 29 May 2020 (UTC)


 * Can we agree that the gravitational metric system does not need a question mark or an asterisk or anything like that? I can provide (many) references if anyone insists, but if not, they would clutter this space unnecessarily. (They will of course be needed later, for the actual article.) --Reuqr (talk) 19:37, 1 June 2020 (UTC)


 * I suspect that the question mark was more asking whether tweaks to the structure would be needed. Force-based metric systems differ enough (and hopefully in a consistent way) from the mass-based systems that it would probably be better to give them their own top-level category rather than lumping them in with "other metric systems".  Looking at the mishmash of metric systems that arose, I'm kinda surprised (but glad) that SI eventually emerged from it all, despite its few remaining warts.  —Quondum 21:33, 1 June 2020 (UTC)

Units of the CGS system

 * CGS mechanical units
 * s, cm, g and their coherent derived units belong here
 * CGS subsystems (extensions to electromagnetism)
 * Electrostatic CGS (CGS-ESU)
 * Electromagnetic CGS (CGS-EMU)
 * Gaussian units
 * Heaviside–Lorentz units (possibly no unit names here —Quondum 17:14, 29 November 2022 (UTC))

Units of other metric systems
Examples:
 * Quadrant–eleventh-gram–second or hebdometre–undecimogramme–second system (QES or HUS)
 * Gravitational metric system (MKpS or MKfS)
 * Metre–tonne–second system of units (MTS)
 * MKSA – the direct precursor of the SI, extending MKS

Units that are power of 10 multiples or sub-multiples of SI units
Examples: dioptre, scandinavian mile, angstrom, fermi

Other multiples
Units that were defined as nondecimal multiples of SI units, e.g. metric carat, metric pound

Units that are commonly used with SI prefixes
Examples: bit, byte, dalton, electronvolt, watt-hour

List of metric units
Table of all metric units mentioned in the article

UCUM
has just pointed out to me the existence of UCUM, which provides a neat way to distinguish between metric and non-metric units. Many of the units listed in this page do not qualify as metric by UCUM's criteria. I propose we delete those from the main article, and move them to the Talk page. Counter-proposals are welcome. Dondervogel 2 (talk) 07:23, 15 March 2020 (UTC)
 * I see no evidence that UCUM is anything more than the pet project of Gunther Schadow and Clement J. McDonald. I propose we ignore it.
 * Further, since I seen no references in Unified Code for Units of Measure to independent reliable sources, perhaps that article should be sent to Articles for deletion. Jc3s5h (talk) 12:26, 15 March 2020 (UTC)
 * Looking closer, it seems some independent open-software organizations have implemented it. But still, I see no evidence that people who actually make, process, and read about measurements are using it.
 * Even if there are bona fide users, it's all about unambiguous machine-readable symbols for units. I see no process for the views of human, non-technical, readers being treated as stakeholders in the development process for this "code". So I see no reason to expect that what a Wikipedia reader thinks of as a metric unit will correspond to what Messrs. Schadow and McDonald think of as a metric unit. Jc3s5h (talk) 12:37, 15 March 2020 (UTC)
 * All valid points. Can you suggest other ways (ideally with RS) of deciding whether a unit is "metric"?
 * Here's my description of a metric unit. A metric unit is based on one or more units created by the Académie des sciences in the 1790s, or any of the successor organizations, including the present custodian of the system, the General Conference on Weights and Measures, provided the unit does not include in its definition a conversion factor to make it align with pre-metric units such as the US customary system or imperial units. Jc3s5h (talk) 14:59, 15 March 2020 (UTC)

Why are we here?

 * One should question whether the idea of a "metric unit" is a topic of encyclopaedic merit. I regard it as a synonym for "a unit in the metric system at hand".  Whenever someone refers to a "metric unit", I tend to assume that they mean a unit of the present-day SI.  As such, this could suggest that this article should be a redirect to SI.  —Quondum 17:13, 15 March 2020 (UTC)
 * But there are many metric units outside the SI. Many (by no means all) are listed in this article. Previously Metric units was a re-direct to Metric system. That didn't work because at that time Metric system was basically duplicating SI. That's no longer the situation, which weakens the case for this article, but what about metric units that fall outside the scope of Metric system? Dondervogel 2 (talk) 17:38, 15 March 2020 (UTC)
 * You seem to be starting from a preconception of what a metric unit is. Until we find a reliable source defining it, we cannot establish that it is notable.  The only definitions that I have been able to find online (admittedly after a very cursory search) do not meet our notability criteria.  If we find that it does have a reasonably established/notable usage, and that as you suggest it is more general than my definition, then we might find it to be something along the lines that Jc3s5h suggests, which I would refine as follows: a metric unit is a unit that is regarded as part of some metric system.  If so, I would structure the article by metric system.  —Quondum 18:11, 15 March 2020 (UTC)
 * I disagree that a metric unit has to be part of a metric system. Consider millimeters of mercury, a widely used unit for atmospheric pressure and blood pressure. It's a one-off unit; it isn't coherent with anything else. But it's metric. Jc3s5h (talk) 19:11, 15 March 2020 (UTC)
 * How about this definition?
 * A metric unit is one of the forty-five (not including the franc) weights and measures of the original metric system (1792) and logical extensions of the same
 * Dondervogel 2 (talk) 19:23, 15 March 2020 (UTC)

Apparently this is a definition from behind a paywall, written by a zealot for US customary units. So it isn't from a reliable source, nor from a source we can make much use of, since it's paywallled. The definition isn't very helpful in the Wikipedia environment because of the word "logical". It'll be tough to find two Wikipedia editors who can agree on what is or isn't logical. Jc3s5h (talk) 19:42, 15 March 2020 (UTC)
 * So it seems to come to this: we don't have a notable definition, and hence do not have grounds for the article. —Quondum 23:22, 15 March 2020 (UTC)
 * I disagree. The notability policy does not have a requirement that a notable definition be found that exactly covers the topic of the article, no less, no more. Jc3s5h (talk) 01:10, 16 March 2020 (UTC)
 * "Information on Wikipedia must be verifiable; if no reliable third-party sources can be found on a topic, then it should not have a separate article." Do we have any reliable sources about the topic of "metric units"?  At least sources that allow us to determine what is meant when WP refers to a "metric unit"?  The article's oft-cited 8th SI brochure uses the phrase once only, in a reference to the 1st CGPM of 1889: "The Prototype of the metre chosen by the CIPM. This prototype, at the temperature of melting ice, shall henceforth represent the metric unit of length."  I do not find this enough to figure out what the phrase means in general.  As I indicated, I actually did make a brief internet search, but ended up empty-handed (unless it is considered to be synonymous with "SI unit").  Perhaps you can do better?  —Quondum 01:52, 16 March 2020 (UTC)
 * I suggest this article is essentially a subset of "Appendix B" of NIST Special Publication 811 The NIST Guide for the use of the International System of Units. The main difference is this article does not include Imperial or US customary units. Jc3s5h (talk) 02:46, 16 March 2020 (UTC)

and how did we get here?

 * The existence of this article was triggered by an edit of mine pointing out that the Litre is a metric unit. The edit was reverted on the grounds that the litre is not an SI unit. This led to me and a couple of like-minded editors at Litre prepare a post for WP Physics. You can read for yourselves where that led, but my reasoning stands: Units like the litre, bar and tonne are clearly metric units (it is not hard to find sources that will confirm that), but they are not part of the SI, so where do they go? This reasoning is what led me to create this article, and now we need to draw a line somewhere. I’m just not sure where.
 * Also, I agree these units do not necessarily need an article of their own, but where would they go instead?
 * Dondervogel 2 (talk) 09:03, 16 March 2020 (UTC)

Proposed scope
We have multiple reliable sources that use the term "metric units" but they seem to use the term in different ways. This inconsistency is why we are struggling with the scope, but does not alter the fact that the concept of a "metric unit" is out there, so let's just explain this ambiguity in the article. If we agree it's an ambiguous concept I see no problem with saying that. How about we start by listing the sources that use the term, working out how the sources differ in interpretation (on Talk) and then explaining those differences (in the article). Dondervogel 2 (talk) 09:11, 16 March 2020 (UTC)
 * Thanks for the context – it helps a lot in framing what is to be achieved. I suggest that we try to define the scope while deliberately ignoring the semantics associated with the term "metric unit" in various contexts.  WP is, after all, not a dictionary.  It would be good to start the lead and the introduction of a statement of the purpose/coverage of the article, which seems to be to list units that roughly speaking fit into one of the metric systems well enough that they are sometimes thought of or even loosely referred to as metric units, and are often used together with units of such a system or as direct replacements.  The units already listed seem to fit this pattern: they are predominantly decimal-power-multiples of SI units (L, barn) or are units in one of the various metric systems (statC), or are frequently used in some area together with SI units (kW·h, mmHg, eV).  (I have some difficulty even calling the last group "metric units" in any sense.)  —Quondum 15:28, 16 March 2020 (UTC)
 * I also have reservations about those 3 units, which is (partly) what prompted my question where to draw the line. I have just added carat which I also think is borderline, and a similar unit (in the sense that neither is a power of ten multiple of the gram) is the metric pound. Are these two units in or out? And what criterion do we apply to make that determination? Dondervogel 2 (talk) 20:29, 22 March 2020 (UTC)
 * Along the lines of the spirit of the purpose of this article, it can be a sort of catch-all for what people refer to as "metric units" in various contexts. I'd suggest making this clear in the lead: "The term metric unit is used with different meanings ranging from SI unit to units related variously to a metric system in how they were defined.  This article lists units that part the modern metric system (SI), units of metric systems that preceded the SI, units that take metric prefixes, and units that were defined as small multiples of SI units."  Then we can follow this with the categories listed, and this leaves us free to add or remove categories since it is clear that this is just a collection of information and not the definition.  One new category here is units that commonly take SI prefixes, as suggested by UCUM (example: dalton).  The metric pound can be put in a category of units defined (or redefined) to be a convenient multiple of SI units.  If the article is organized at the top level by these categories, the question of what a metric unit is becomes relatively unimportant.  We will not need to justify the existence of the article or the concept "metric unit", because it is essentially a list-class article with lists of items in well-defined categories.  —Quondum 23:34, 23 March 2020 (UTC)


 * Sounds sensible. I support changes along these lines but have no time to contribute myself just now. Your criteria would give grounds for inclusion of eV (keV), mmHg (cmHg) and Wh (kWh). What about the yard (kyd) and dollar (kUSD) though? I often see those with an SI prefix but do not think of them myself as metric units. Dondervogel 2 (talk) 00:46, 24 March 2020 (UTC)
 * I agree with Dondervogel 2 about metric prefixes. It is becoming more and more common to see SI prefixes attached to units that have nothing whatever to do with the metric system. The dollar is a good example. Up through the 1970s, the abbreviation for 1000 dollars was M, inspired by the Roman numeral M. Nowadays k$ is a likely abbreviation, "M" would evoke a blank stare from anyone under 30 years of age, and Roman numerals are no longer a required subject in US schools. Jc3s5h (talk) 13:33, 24 March 2020 (UTC)
 * I agree that extending the pattern of use of metric prefixes to other units does not qualify a unit as a "metric unit" in this sense. This is a sort of colloquial usage.  The sense in which  this applies would be how UCUM defines it: a sort of "officially allowed" status.  Which is difficult to determine, of course.  The dalton is one that is sanctioned by the SI.  One thing we should avoid: the lure of summarizing common usage, even in scholarly papers.  (This makes me think of "atomic unit of temperature", which is mentioned by some papers but seems to have no authoritative source.)  I expect that we'll find very few units that have authoritative sources to fit a category "metric prefixes are sanctioned", and we should perhaps avoid any category that does not have a clear definition.  —Quondum 16:49, 24 March 2020 (UTC)
 * So what we need to get started is a list of categories. How about basing these on my previous proposal for the article structure (just revised based on this discussion)? Dondervogel 2 (talk) 18:19, 24 March 2020 (UTC)

List of sources that use the term "metric unit"

 * Dealing with Metric Unit Conversion: An Examination on Prospective Science Teachersâ€™ Knowledge of and Difficulties with Conversion
 * English unit to metric unit conversion device
 * Use Cases and Suitability Metrics for Unit Ontologies
 * Quantities, units and computing
 * Modeling sensor web autonomy
 * An Ontology-Based Approach for Product Entity Resolution on the Web
 * Units of Measure in Clinical Information Systems
 * The Unified Code for Units of Measure
 * Automatic noninvasive measurement of arterial blood pressure
 * Anatomy & Physiology Laboratory Manual and E-Labs E-Book

Angle as a base quantity
In thinking of equivalences/correspondences between quantities in different (metric) systems, it strikes me that the modern SI premise that angle is a dimensionless quantity and the associated counter-intuitive identity $rad = 1 = Hz/s$ did not necessarily hold in previous systems. It might not have been explicitly and formally recognized, but nevertheless, since the SI does not dictate this for all systems and all times (and it is by no means either necessary or even useful), angle may have been treated as an independent quantity. This would have implications on equalities/correspondences for units in this article. For example, to say $Hz = s^{−1}$ might not hold in a system where $Hz = cyc/s$ and $cyc$ is not treated as being unitless. This applies to wavenumber, frequency and even torque. As an example, I remember a publication from the earlier part of the 1900s referring to metric units of torque as $J⋅rad^{−1}$. Metrologists today continue to argue about this. It would be interesting to include this subtlety as information on it is found. —Quondum 14:22, 10 July 2020 (UTC)

Buckingham
I see that there is an article Buckingham (unit), which, if verified, would be another unit to add to this list. —Quondum 20:00, 20 November 2022 (UTC)


 * makes sense (added) Dondervogel 2 (talk) 00:50, 21 November 2022 (UTC)

Criteria/organisation
I'm getting sloppy, adding curie: even though its definition was essentially codified by the CGPM and it probably gets used with metric prefixes, it is probably one of those that does not belong. It probably does not help to fuss too much, but some units really do lie outside the boundaries of what we think of as metric units.

This makes me think that, rather than being too careful about what we include, may be we can try to organize these roughly by unit system, with a sort-of-metric category to catch the dubious ones. I can think of the following rough categories:
 * Those that are part of the SI
 * Those that are clearly named versions of SI units (i.e. those that can be expressed as a product of SI units, including prefixed units, but see next point)
 * Those that were defined to be a multiple of SI units, as a rough approximation to an existing unit. These can be argued to have been defined by the metric system (e.g. metric pound).  Some units under the previous point might really belong here (e.g. angstrom, tonne).  See last bullet.
 * Units that are gravitational metric system variants (using standard gravity, using kgf, etc.)
 * Units that are CGS variants (multiplying constants like c and &epsilon;0)
 * Units that are clearly defined in terms of something else originally and really do not fit, but one is tempted to mention because they get used rather like metric units (curie, eV, watt-hour, dalton) – may actually include everything under the third bullet. Adding this "trash can" catchall category is probably the main motivation for this suggestion.

Since the the total number of units is quite large, it may make sense to organize these into tables with columns, dispensing with many of the words. Thoughts? —Quondum 02:51, 22 November 2022 (UTC)


 * I agree the criteria for inclusion are unclear. We have discussed criteria previously but never reached consensus.
 * I also like the reasoning you present above, including the proposal for a table. I would change the order though, with CGS units higher up the ranking (3rd?).
 * I would place units obtained through multiplication by a constant of nature (eg electronvolt) above units obtained by some anthropocentric or terracentric "constant" such as gravitational acceleration.
 * And finally, the "metric pound" category is a slippery slope. If one accepts 500 g as a metric unit, why not also accept the "metric inch", defined as 25.4 mm? In other words, where to draw the line?
 * Dondervogel 2 (talk) 09:30, 22 November 2022 (UTC)


 * I didn't intend to imply an order of prioritization. My third and last bullets would be listed as, effectively "units that are not metric units but were defined with easy conversion to metric units in mind".  There are often prefixed by "metric" (e.g., metric pound, metric ton, metric carat, ...), and sometimes effectively displaced/redefined the unit that it was modelled on (e.g. carat, angstrom).  The idea of a section in the article of "units that one might think are but are not" is to explain the dissonance such as of "metric xxx" and the like not being a metric unit.
 * There is a historical (and conceptual) difference between units from systems that vied to become a coherent metric system (gravitational metric systems, CGS variants) and those more generally derived by multiplication with constants of nature, even though the line between these might be tricky. The SI is terracentric (the second and metre are terracentric, but so is standard gravity), so this is not an easy criterion.  Constants of nature are a terribly slippery slope: next we'll have claims of that Stoney units are some of the only the "real" metric units ... I would add these to the "not-metric units" category, unless they fall into one of the others.
 * This leaves me with the following categorization:
 * SI units
 * Units of notable wannabe-SI systems, refined by system with overlap: CGS-EMU, CGS-ESU, CGS-Gaussian, and any "metric" gravitational systems. These are mostly distinguished by the system of quantities on which they are based.  Just as Gaussian and SI systems differ on whether there is a constant in Coulomb's law, gravitational systems and SI differ on whether there is a constant in Newton's second law (I might even be able to source the latter in an older SI brochure, if my vague memory has any validity).
 * Units that originated as a name attached to some derived SI unit or if one of the wannabe metric systems (fermi, litre, barn, dioptre, stokes, sverdrup, micron, benz)
 * Units that are not metric units:
 * Those that are related to some constant of nature (electronvolt, dalton).
 * Those that are resized units from unrelated systems (carat, metric pound, Scandanavian mile).
 * This leaves a few difficult ones, like tonne, which could be a (a) renamed SI unit or (b) a resized unit that coincided with an SI unit. I would classify it as the latter, and hence not even as a metric unit, and hence it is only "metric" in the sense that a "metric pound" is.  And this all has an OR feeling about it, but it seems better to me in this sense than what we currently have.  —Quondum 14:20, 22 November 2022 (UTC)
 * OR is indeed a concern, but I see none in the first 3 bullets, which I fully support. Perhaps we should start there? Dondervogel 2 (talk) 14:40, 22 November 2022 (UTC)
 * Sounds good. I will likely spend time in the next week on this.  Feel free to go ahead if you feel so inclined.  Note that I have not worked through the refinement of the second bullet in my head yet.  —Quondum 14:48, 22 November 2022 (UTC)
 * I added enzyme unit, which I guess falls in the same category as watt hour.
 * I presently have neither the time to progress with restructuring nor the knowledge to turn into a table. I gladly leave both to you!
 * Dondervogel 2 (talk) 00:11, 23 November 2022 (UTC)
 * , if you're interested, in my sandbox I have created tables. The first table is my first bullet (SI units), and the second table is an amalgam of the next two bullets (units of other metric systems and named units based on those systems).  It is not that clear to me which units are "part" of a system and which are only defined in terms of it; for several units I haven't determined which metric system to attach them to.  The columns are not final.  There is clearly still quite a bit of work to do.  Any comments are welcome.  —Quondum 04:24, 27 November 2022 (UTC)
 * Thanks for the tip. I don't have time now to examine in any detail, but here's my reaction at first glance: I find it confusing to have length, mass, time, ... all in one table. I suggest making separate tables for each. Dondervogel 2 (talk) 08:54, 27 November 2022 (UTC)
 * There is still a lot of work to do, and I am toying with different organizational principles, and so have not ordered or split that table yet. At this point I am more interested in ensuring that I am not leaving necessary information out at the individual entry level.  It will take a while for me to beat it into a shape that I feel is sufficiently coherent, so you might want to look again in week or three.  —Quondum 13:16, 27 November 2022 (UTC)
 * Understood. Thank you for your efforts. Dondervogel 2 (talk) 14:11, 27 November 2022 (UTC)

Reshuffling of December 2022
Mostly I agree with your categories. I have 3 comments: Dondervogel 2 (talk) 01:07, 1 December 2022 (UTC)
 * 1) I did not see why kgf and related units are treated differently from (say) kWh. I moved them down to remind us to discuss pros and cons.
 * 2) I consider stremma to be a metric unit area. What is the counter-argument?
 * 3) I consider percent, permille and permyriad to be metric units of number, as is the number one (symbol 1). Again, what is the counter-argument?
 * My rationale is essentially as follows, possibly flawed.
 * The gravitational metric system (MKfS) seems to be a full mechanical metric system of units, similar to the mechanical CGS, with the same base unit principle, coherence of units, metric prefixes and the same system of quantities. The base unit the nominal weight of one litre of water instead of its mass, so it is as terracentric as the other metric systems.  Its units are not hybrid.
 * The stremma is a traditional unit with a long history; it was redefined to something specific and conveniently in line with the metric system as with many crude historical units. Its close coincidence with a decadic multiple was a historical accident that made the particular choice of redefinition natural but does not make it a metric unit.
 * Percent, etc., are decadic, but are no more metric than are 'one', 'ten', 'hundred'. They do not use metric prefixes.
 * The rutherford and lambert are still dubious. As you will see, my definition of "metric unit" basically draws the line at units of metric systems, or units that were names given to (no necessarily coherent) units of such systems.  This fits with a next step of arranging the metric units by metric system (CGS and its subsystems, MKS(A), etc.).
 * That said, this to experiment with classification structure. Once we have this reasonably agreed, the awkward unit classifications can be done.  I decided to start without the tables so that the contrast would be less unsettling; my sandbox experiment is a big jump.  —Quondum 02:49, 1 December 2022 (UTC)
 * OK, you've convinced me on the MKfS. Thank you for explaining.
 * I still don't see the difference compared with the Scandinavian mile or metric dunam. I think the stremma falls in the same category as these.
 * The units 1 and % are formally defined as part of the ISQ. Together they form part of the geometric series 1, 0.1, 0.01, 0.001, ... some of which have special symbols and some not. Given that 1 is the quintessential unit of number, it forms a central part of all other unit systems, including the SI, so why are they not metric units? I don't see the use of a prefix being a pre-requisite. Bottom line: let's keep them separate from B, kB, MB, ..., which do not feel like metric units to me (though we still don't know what that means! LOL).
 * Dondervogel 2 (talk) 09:23, 1 December 2022 (UTC)
 * I moved the MKpS units back into the metric section. They could slot back into their respective sub-sections, but I'm not sure whether this is worth the effort. What is your next planned move? Dondervogel 2 (talk) 09:57, 1 December 2022 (UTC)
 * I intend to subdivide the metric units by system next. There are many that fit into more than one system, which we could eventually deal with through a table.  There will be a few stragglers that I don't know where to fit or do not really belong to any specific system (e.g. jansky), and they would be put into a separate section.
 * stremma: This is not qualitatively different from Scandinavian mile or metric dunam. I have classified all these under "Nonmetric units".  Distinction between those that redefined the original unit and those that continued alongside it is less clear, and I have not worried about this too much: the purpose of mentioning any examples of these is in my mind to point out that "metric pound" etc. are like the imperial inch: nonmetric units.  The metric dunam may have continued alongside other dunams, whereas the stremma may not have been regarded as separate from its original, just more shaply defined.  I don't know, and have not looked closely.  Anyhow, I expect most of these to be removed, leaving mainly only the ones starting "metric ...".
 * You appear to be referring to ISO 80000 as the ISQ. They are not synonymous.  1 can be regarded as a metric unit, but no metric system defines its multiples as units.  A metric unit prefixed with a metric prefix is a metric unit, bit 1 is never prefixed.  SI and ISO 80000 define many units, but this does not in itself cause them to be metric units.  '%' is a symbol permitted for use with the SI, and related symbols may be defined in ISO 80000 (I have not looked), and this may be interpreted to mean that they are units.  There are distinctions between numbers and units: dm is a unit, but 10 m is not a unit, even if semantically they are identical.  This means that being a geometric series has no bearing.  We do not list bel (B) or decibel (dB), which suggests that we are not regarding ISO 80000-defined units as metric units unless part of a metric system.  I remain unpersuaded.
 * —Quondum 02:29, 2 December 2022 (UTC)
 * Forget the ISQ. ISO 80000-1:2009 has this to say about the unit one (1):
 * “The coherent SI unit for any quantity of dimension one is the unit one, symbol 1. It is generally not written out explicitly when such a quantity is expressed numerically.”
 * “In the case of certain such quantities, however, the unit one has special names and symbols that could be used or not, depending on the context.”
 * “Such special names and symbols may be used in expressions for derived units to facilitate distinction between quantities of different kinds, but having the same dimension.”
 * “Special names and symbols for the unit one may be combined with SI prefixes, but the unit one itself, or its symbol 1, may not. Instead, the numerical value may be expressed by using powers of 10.”
 * “NOTE It has been proposed to adopt a special name and symbol for the unit one and its symbol 1 for general use, which could be combined with prefixes.”
 * “In some cases, per cent, symbol %, where 1 % := 0,01, is used as a submultiple of the coherent unit one.”
 * “Also, per mil (or per mille), symbol ‰, where 1 ‰ := 0,001, is used as a submultiple of the coherent unit one.”
 * In other words (according to ISO 80000-1), one is not only a metric unit but a unit of the SI. I do not make that claim myself (I don't recall seeing BIPM refer to 1 as an SI unit - though it's worth checking), but ISO 80000-1 supports my position that 1, % and o/oo are units of number. The only question is whether they are also metric units, and there, as always, we founder on what we mean by "metric". Dondervogel 2 (talk) 09:23, 2 December 2022 (UTC)
 * Agreed that 1 is a unit of the SI, which I have not argued against. The 9th SI Brochure says:
 * 'Such quantities are simply numbers. The associated unit is the unit one, symbol 1 ...'
 * 'The unit one is the neutral element of any system of units – necessary and present automatically. There is no requirement to introduce it formally by decision.'
 * 'Prefix symbols can neither stand alone nor be attached to the number 1, the symbol for the unit one. Similarly, prefix names cannot be attached to the name of the unit one, that is, to the word “one”.'
 * 'The internationally recognized symbol % (percent) may be used with the SI.'
 * I would say that in "the unit one has special names and symbols that could be used or not, depending on the context", ISO 80000 is referring to the class of coherent dimensionless units, of which there are two clear examples: radian and steradian. This, with "Special names and symbols for the unit one may be combined with SI prefixes, but the unit one itself, or its symbol 1, may not.", ISO 80000 is saying that milliradian (mrad), microsterradian (μsr), etc. are units, but that millione (m1) is not.  This concurs with the SI brochure.  However, I disagree with your statement "... but ISO 80000-1 supports my position that 1, % and o/oo are units of number" on % and ‰: it says that these are submultiples of 1, but this does not imply that they are units.  0.5 m is a submultiple of the unit metre, but is not a unit.  —Quondum 13:38, 2 December 2022 (UTC)
 * More from ISO 80000-1
 * "Additional information, such as % (m/m) or % (V/V) shall therefore not be attached to the unit symbol %."
 * "The symbol of the unit shall be placed after the numerical value in the expression for a quantity, leaving a space between the numerical value and the unit symbol. It should be noted that this rule also applies to the units per cent, % and per mil, ‰."
 * "Any attachment to a unit symbol as a means of giving information about the special nature of the quantity or context of measurement under consideration is not permitted. EXAMPLE 2 ... w_B = 0,76 = 76 %, neither 0,76 (m/m) nor 76 % (m/m)"
 * Dondervogel 2 (talk) 16:30, 2 December 2022 (UTC)
 * Okay, this suggests that % and ‰ are units (we already have accepted that they have symbols). However, we also know that they are outside the SI.  SI and ISO 80000 define many units that are not part of the SI, and (I would argue) thus not "metric symbols", such as eV, Da, %, bit, byte, Np, bel, etc.  Many even take metric prefixes.  What makes % different from this group?  —Quondum 17:04, 2 December 2022 (UTC)
 * % and ‰ are related to 1 in the same way as cm and mm are related to m. That makes them metric units IMO. From your list I would argue that Np is also a metric unit. The others (apart from %) are not. Dondervogel 2 (talk) 17:13, 2 December 2022 (UTC)
 * Your arguments are getting into fuzzy regions of interpretation that promise to include disagreements. I would like to classify those in metric systems by system first.  —Quondum 18:29, 2 December 2022 (UTC)
 * Being related on a decadic scale to an SI unit seems a strong argument to me. But yes, in the end it’s just that. An opinion. And it will remain so for all non-SI units until we have a common understanding of the qualifier “metric”. The case for the neper is stronger.
 * Yes, things will become clearer once we have sensible categories in place. There we can agree :) Dondervogel 2 (talk) 19:29, 2 December 2022 (UTC)

Leaving aside  for the moment, I have (hopefully) arranged all units that fall under metric systems, leaving a large section § Metric units that are not part of the SI § Other metric units. Does the organization make sense so far, or are there any suggestions for better organizing principles that might be applied? Are there any metric units that should be reclassified? (Note: I have not yet started introducing tables, as I think this is a separate debate and exercise that would complicate things if done at the same time.) —Quondum 01:34, 11 December 2022 (UTC)


 * You're doing a great job. Thank you! I have made one suggestion (albeit with multiple edits) to regroup some units into the category 'Light', just before 'Radioactivity'. My purpose was to reduce the number of units under the inelegant heading 'Other'. That heading now only includes units about photons. Perhaps it could be renamed 'Photons' or similar? Dondervogel 2 (talk) 09:29, 11 December 2022 (UTC)
 * I made some further minor edits, including merging 'Other' with 'Light'. See what you think. Dondervogel 2 (talk) 11:13, 11 December 2022 (UTC)
 * I like all the changes that you made. I don't think that a heading referring to photons would work, for the reason that most of these units have a definition and interpretation that makes no reference to photons or quantum theory.  I would suggest calling a heading "Electromagnetic radiation" instead, and include what is currently under "Spectral irradiance".
 * I would like to reduce the number of headings under "Other metric units" by categorizing them along similar lines: by type of use or discipline rather than exact quantity or by corresponding SI dimension. Whether this will lead to a reduction remains to be seen, though.  —Quondum 14:15, 11 December 2022 (UTC)

A few more units
—Quondum 17:05, 29 November 2022 (UTC)
 * gamma (&gamma;) a unit of magnetic flux density corresponding to 1 nT (NIST Guide to the SI, Chapter 5: Units Outside the SI)
 * stattesla, statweber, stathenry (System of Electric Units – Francis B. Sildbee (1962), p. 172)
 * Does thomson (unit) (Th), a unit of mass-to-charge ratio, count? It would fit into the same "nonmetric" category as eV and Da.


 * I'd say the dalton and thomson are out of scope. On the other hand, we don't really have a scope because we haven't decided what we mean by "metric unit". Things will become clearer after the restructuring you proposed. Dondervogel 2 (talk) 18:33, 29 November 2022 (UTC)

Does the rayl belong under CGS or MKS?
We previously had the rayl as a CGS unit (defined as 1 (dyn/cm^2)/(cm/s) = 10 Pa s/m) but it now appears as MKS. I think it belongs under CGS (and not MKS, unless qualified as an 'MKS rayl'). What was the justification for moving it? Dondervogel 2 (talk) 12:54, 16 December 2022 (UTC)
 * I can't remember the progression, but the short answer is that there was no real reason, other than that the article is confusing and does not indicate which version was "real" and which was the (IMO stupid) adaptation from the "original". My inclination would be to include only the most notable one if there is a clear difference in notability/prevalence, with at most a footnote about the other.  I have not looked into what makes more sense.  Feel free to change.  —Quondum 13:13, 16 December 2022 (UTC)
 * I made a proposal - see what you think. Dondervogel 2 (talk) 13:23, 16 December 2022 (UTC)
 * I found a partial answer to my question on the ASA standards page, which defines MKS rayl and CGS rayl separately. This made me dig a little deeper, and in Morfey’s Dictionary of Acoustics I find
 * Rayl    – see SI RAYL. The original rayl was defined as a cgs unit of specific     acoustic impedance, equal to 1 dyn s cm^-3.
 * SI    rayl – 1 N s m^-3 := 1 Pa s m^-1; the unit of SPECIFIC ACOUSTIC IMPEDANCE     based on SI fundamental quantities. Not a recognixed SI derived unit,     despite its name
 * Conclusions:
 * ‘rayl’ and ‘CGS rayl’ are equivalent terms for 1 (dyn/cm^2)/(cm/s)
 * ‘MKS rayl’ and ‘SI rayl’ are equivalent terms for 1 Pa s/m
 * Dondervogel 2 (talk) 13:15, 16 December 2022 (UTC)
 * I suppose it won't help to rail against the stupidity of whoever repurposed the name ...
 * It seems possible that neither of them is necessarily an accepted unit of a system, any more than the benz and talbot are SI units. They may both belong under .  Then again, who is to know what these systems really were?  —Quondum 13:39, 16 December 2022 (UTC)
 * I just notice your clever pun. LoL. What brought me here was Rayl, which I think misleads the reader by pretending equivalence between "CGS rayl" and "MKS rayl", even seeming to put MKS first in places. Worse still, it capitalises the name "Rayl" Dondervogel 2 (talk) 09:57, 28 January 2024 (UTC)
 * Nice that you appreciate it. I seem to remember that sources were rather scarce and inconclusive.  As to the detail, I have little opinion (and even less knowledge), other than if there is a primary modern meaning and notation, that should be given greater weight.   —Quondum 13:54, 28 January 2024 (UTC)
 * It seems likely that the rayl is more often used alongside SI units than alongside CGS units, but only because SI units are far more common in acoustics than CGS. My arguments in 2022 were based on the origin of the unit, not modern use, which (fortunately!) is rare. Dondervogel 2 (talk) 14:23, 28 January 2024 (UTC)
 * A cursory search suggests that what you say is accurate, especially over the last few decades. The article Rayl might benefit from this perspective, since it gives the two usages nearly equal weight: the CGS unit might be better moved to a historical section.  —Quondum 15:57, 28 January 2024 (UTC)

Are the conversions exact?
One of the thoughts that prompted my self-revert was that, since the 2019 redefinition of the SI base units, the conversions may not be exact because now μ0 is no longer exactly $4 H/m$, and I was not going to analyze the detail at the time. The table at  may help, but may not be accurate. Throughout, we have implicitly redefined historical units in a somewhat arbitrary way as the SI develops, in the sense that we are treating the second, metre and gram as having the modern values. Since the defining equations ($F = q_{1}q_{2}⁄r^{2}$ in the CGS-ESU and $F = 2I_{1}I_{2}l⁄d$ in CGS-EMU) should not change, this establishes the exact relationship between the SI charge and the ESU charge. For all the conversions in which the electrical dimension does not cancel, I suspect that this makes the relationships inexact (not only those that I reverted). —Quondum 14:11, 16 December 2022 (UTC)


 * I hadn't realised this was an issue. If the 10/(4 pi) conversion is not exact, we should reflect that somehow. Is there a relevant RS? Dondervogel 2 (talk) 18:38, 16 December 2022 (UTC)


 * My approach would be to understand this properly first, so that we can frame the issue, and then seek sources. Unfortunately, most documents relating to CGS units will date to before the "new SI".  A typical conversion constant might be μ0/4π, which is where the almost-exact-power-of-10 might arise.  The equations above can be sourced.  The impact on accuracy maybe not so easily, but the implications can be spelled out for the reader in an article.  In this article, it might be sufficient to make a comment to sensitize the reader to it and "detune" the actual numbers (use a tilde or whatever).  We could maybe give the explicit formula for e.g. H/statH: easy enough to derive, but I'm not sure that it could be sourced in its final form.  —Quondum 19:16, 16 December 2022 (UTC)
 * Can we perhaps qualify the conversions with "before 2019 they would have been exact"? Dondervogel 2 (talk) 21:10, 16 December 2022 (UTC)
 * That's a neat solution. After all, most people are not interested in accuracy beyond the 10th decimal point ... —Quondum 21:25, 16 December 2022 (UTC)

Re-ordering of "non-metric" metric units
I re-ordered the "non-metric" units in a way I find helpful. Revert if you disagree. Dondervogel 2 (talk) 21:20, 25 December 2022 (UTC)

What is a metric unit?
We have yet to come up with a meaning of "metric unit" that satisfies WP:OR. I found this essay by Schadow et al 2002 that states "To be metric or not to be metric is a predicate assigned to each unit atom where it is defined", followed by:

I find it comforting to learn that the authors of the essay consider it "not trivial" to determine whether a unit is metric, as that explains why we are struggling. Thoughts welcome. Dondervogel 2 (talk) 22:41, 25 December 2022 (UTC)
 * The metric predicate accounts for the fact that there are units that are prefixed and others that are not. This helps to disambiguize the parsing of simple units into prefix and atom.
 * To determine whether a given unit atom is metric or not is not trivial. It is a cultural phenomenon, subject to change, just like language, the meaning of words and how words can be used. At one time we can clearly tell right or wrong useage of words, but these decisions may need to be revised with the passage of time.
 * Generally, metric units are those defined “in the spirit” of the metric system, that emerged in France of the 18th century and was rapidly adopted by scientists. Metric units are in general based on reproducible natural phenomena and are usually not part of a system of compareable units with different magintudes, especially not if the ratios of these units are not powers of 10. Instead, metric units use multiplier prefixes that magnifies or diminishes the value of the unit by powers of ten.
 * Conversely, customary units are in the spirit of the middle age as most of them can be traced back into the a time around the 10th century, some are even older from the Roman and Babylonian empires. Most customary units are based on the average size of human anatomical or botanic structures (e.g., foot, grain) and come in series of comparable units with ratios 1/2, 1/4, 1/12, 1/16, and others. Thus all customary units are non-metric
 * Not all units from ISO 1000 are metric as degree, minute and second of plane angle are non-metric as well as minute, hour, day, month, and year. The second is a metric unit because it is a part of the SI basis, although it used to be part of a series of customary units (originating in the Babylonian era).
 * Furthermore, for a unit to be metric it must be a quantity on a ratio scale where multiplication and division with scalars are defined. The Comit´e Consultatif d’Unit´es (CCU) decided in February 1995 that SI prefixes may be used with the degree Celsius. This statement has not been made explicitely before. This is an unfortunate decision because difference-scale units like the degree Celsius have no multiplication operation, so that the prefix value could be multiplied with the unit. Instead the prefix at non-ratio units scales the measurement value. One dekameter is 10 times of a meter, but there is no meaning to 10 times of 1 ◦C in the same way as 30 ◦C are not 3 times as much as 10 ◦C. See §22 on how The Unified Code for Units of Measures finds a way to accomodate this different use of prefixes at units such as the degree Celsius, bel or neper.

Is toe a metric unit?
See tonne of oil equivalent. I added it and then removed it again. Is it a metric unit? Dondervogel 2 (talk) 07:51, 19 March 2023 (UTC)

List of non-metric units
I boldly deleted a list of non-metric units, as it does not fit the title of the article. The deleted list is reproduced below in case we want to reinstate parts of the list.

Dondervogel 2 (talk) 09:50, 9 December 2023 (UTC)


 * Revisiting this list in April 2024, I'm finding it hard to convince myself that units like metric horsepower and metric carat and should be excluded from a list of metric units. What do others think? Dondervogel 2 (talk) 09:10, 3 April 2024 (UTC)

Hybrid units
Some nonmetric units arose as a combination of a nonmetric quantity combined with a metric unit. Examples include:

Metric unit combined with a unit permitted alongside the SI

 * The ampere-hour is a unit of electric charge equal to $3.6 kC$.
 * The watt-hour (W⋅h) is equal to $3.6 kJ$.
 * The watt-hour per day (W⋅h/d) is a unit of power equal to 3.6 kJ/(24 h) = 1/24 W.
 * The enzyme unit (U) is equal to one micromole per minute (50/3 nkat).

Other combinations

 * The normal litre per minute (NLPM) is approximately equal to (0.001/60) m3/s.
 * The standard litre per minute (SLPM) is approximately equal to (0.001/60) m3/s.
 * The centimetre of water (cmH2O) is approximately $98.067 Pa$.
 * The millimetre of mercury (mmHg) is approximately $133.322 Pa$.
 * The torr (Torr) is approximately 133.3224 pascals.
 * The centimetre of mercury (cmHg) is approximately $1.333 kPa$.
 * The electronvolt (eV) is equal to $1.602 J$ = $160.218 zJ$.
 * The calorie (cal) is $4.184 J$.
 * The langley (Ly) is a unit of energy density equal to 1 calorie per square centimetre ($41.84 kJ/m^{2}$).
 * The darcy (d) is a unit of permeability approximately equal to $9.869 m2$ ($0.987 um2$).

Traditional units standardized in terms of metric units
Further traditional units were standardized by defining them in terms of metric units, such as the imperial inch, almost always retaining their original name, and replaced the units as traditionally defined.

Nonmetric units that arose as approximations to traditional units that were adjusted for convenient conversion to metric units were typically named as for the traditional unit but qualified with the word "metric". The corresponding original traditional usually remained in use alongside the corresponding "metric" version.

Dimensionless

 * The neper (Np) is a unit of logarithmic ratio equal to 1.
 * The percent (%) is equal to one part in a hundred (0.01).
 * The permille (‰) is equal to one part in a thousand (0.001).
 * The permyriad (‱) is equal to one part in ten thousand ($0$).
 * The part per million (ppm) is equal to one part in a million ($0$).
 * The part per billion (ppb) is equal to one part in a billion ($0$).
 * The part per trillion (ppt) is equal to one part in a trillion ($0$).
 * The part per quadrillion (ppq) is equal to one part in a quadrillion ($0$).

Length

 * The metric inch is equal to twenty five millimetres ($25 mm$).
 * The cun is equal to one tenth of a chi (approximately 33.3333 mm).
 * The metric foot is equal to three hundred millimetres ($300 mm$).
 * The chi is equal to one third of a metre (approximately 333.333 mm).
 * The metric chain is equal to 20 m.
 * The metric lieue is equal to four kilometres ($4 km$).
 * The Scandinavian mile (mil) is equal to ten kilometres ($10 km$).

Area

 * The metric dunam is equal to one thousand metres squared ($1,000 m^{2}$).
 * The stremma is equal to one thousand metres squared (1000 m2).

Volume

 * The food labeling ounce is equal to 30 cm3.
 * The metric cup is equal to $250 cm^{3}$.

Mass

 * The metric carat (ct) is equal to $200 mg$.
 * The li is equal to one ten-thousandth of a jin (50 mg).
 * The fen is equal to one thousandth of a jin (500 mg).
 * The qian is equal to one hundredth of a jin (5 g).
 * The liang (or tael) is equal to one tenth of a jin (50 g).
 * The metric pound is equal to $500 g$.
 * The jin (or catty) is equal to 500 g.
 * The dan is equal to one hundred jin (50 kg).
 * The metric quintal (q) is equal to one hundred kilograms ($100 kg$).

Power

 * The donkey power is equal to 250 W.

Length

 * The mo is equal to one ten-thousandth of a shaku (approximately 0.0303030 mm).
 * The rin is equal to one thousandth of a shaku (approximately 0.303030 mm).
 * The bu is equal to one hundredth of a shaku (approximately 3.03030 mm).
 * The sun is equal to one tenth of a shaku (approximately 30.3030 mm).
 * The shaku is equal to 10/33 m (approximately 303.030 mm).
 * The jo is equal to ten shaku (approximately 3030.30 mm).
 * The metric mile is equal to $1.5 km$.

Area

 * The tsubo is equal to 400/121 metres squared (approximately 3.306 m2).

Volume

 * The sho is equal to 2401/1331 litres (approximately 1.804 dm3).

Mass

 * The hyakume is equal to one tenth of a kan (375 g).
 * The kan is equal to 15/4 kilograms (3.75 kg).

Power

 * The metric horsepower is equal to $75 kgf⋅m/s$ (approximately 735.499 W).

Date format
I notice that "metre" uses British spelling. But one date in the citations uses the month day, year format, and that format is usually used in the US. One date in the citations uses the YYYY-MM-DD format. What format would editors prefer? Jc3s5h (talk) 19:13, 12 January 2024 (UTC)


 * Unless in prose, my preferred date format is YYYY-MM-DD. Dondervogel 2 (talk) 21:45, 12 January 2024 (UTC)