Talk:International System of Units

Term in French
As a matter of background for the Introduction section, I think providing the French translation (Systéme international d'unités) can help explain how the abbreviation "SI" arose, so I added it to the article. My edit was reverted by someone saying "Not required in English Wiki (MOS:FOREIGN)". Because of the abbreviation and the prevalence of this term in international use, I suggest there is more justification for including this term than would ordinarily be the case for a foreign-language translation. I see that Britannica puts the French in boldface in its opening sentence. There is also a very close relationship between France and the SI system, e.g. as explained here. —⁠ ⁠BarrelProof (talk) 18:15, 16 December 2022 (UTC)


 * I agree that the French term is relevant here. If it is the origin of the abbreviation "SI" (is it?), especially so. Dondervogel 2 (talk) 18:42, 16 December 2022 (UTC)
 * It's useful to know the origin of the abbreviation, yes. It's already covered in the article though, in its own section - "Birth of the SI". I'm not sure it's important enough to repeat in the lead though. -- DeFacto (talk). 18:52, 16 December 2022 (UTC)
 * I somehow had not noticed that existing mention. However, that is buried pretty deeply within the article. I still think it might be worth putting into the Introduction, along with some mention of the origination of the system being led by France. (I had not tried to put it in the lead, although Britannica does.) FWIW, here is a page on Britannica Kids that directly says "SI" came from the French. —⁠ ⁠BarrelProof (talk) 19:05, 16 December 2022 (UTC)
 * Yes, it's a huge and unwieldly article here, which is difficult to follow or navigate. As well as a dedicated article, History of the metric system, there are two "History" sections in this article covering the role the French played in the development of the system. -- DeFacto (talk). 19:20, 16 December 2022 (UTC)
 * @⁠BarrelProof, @DeFacto: Thank you for the clarifying reference to kids.britannica.com. I will be very specific in response to the opening thrust of this discussion. Any thinking reader will see the SI abbreviation as being disassociated from any logic apparent with regard to the opening text of the article. Every standard and convention has a history. To add a phrase making clear the origin of the SI abbreviation—in the article, rather than in a footnote—is a way to avoid reader confusion and wasted time. That there are other issues with the article's length and complexity: about this there is no quarrel. But a simple change, immediately (adding "SI, deriving historically from its original name in French, Systéme international d'unités...") will avoid continued reader pause. Remember, even following WP markup conventions for notes and footnotes is a learned behaviour, not always brought in to the first reading of an article. And this is the sort of article that might draw young readers. (A retired academic.) 2601:246:C700:F5:18B0:5EF:D03:EBA0 (talk) 16:36, 5 June 2023 (UTC)
 * People should be used to it by now. We have ones like NATO OTAN and such. Even more, CERN is the European Center for Nuclear Research, without any confusion. Gah4 (talk) 07:14, 6 June 2023 (UTC)
 * Yes, it's easy enough to include a very brief mention of the words Système International; it doesn't require that we expand the body text with digressions into explaining that it's French or what it means; the words are so similar in English and we have a very full footnote too. Indeed, we can save a little padding: "in all languages" is not supported by the source, the SI Brochure, and quite right too, as it would be immensely difficult to prove even if we had confidence that all human languages are now known, and we've already asserted internationally anyway. I'll edit the sentence accordingly. NebY (talk) 18:36, 16 June 2023 (UTC)
 * The abbreviation is supported by the SI brochure. On page 165: The 11th Conférence Générale des Poids et Mesures (CGPM) [...] decides [...] the international abbreviation of the name of the system is: SI. -- Jähmefyysikko (talk) 18:44, 16 June 2023 (UTC)
 * Yes indeed, and we can and do say that it's internationally known by that abbreviation. We can't make the stronger claim that "SI" is used "in all languages", and the SI brochure doesn't make that claim either. NebY (talk) 19:00, 16 June 2023 (UTC)
 * I see that this one has been fixed. But then notice, in the very next sentence, no note of CGPM and its naming. Gah4 (talk) 12:00, 17 January 2024 (UTC)
 * Please be specific. There are lots of "next sentences" here. Johnjbarton (talk) 17:06, 17 January 2024 (UTC)
 * There is a note in the #Controlling authority section that explains the CGPM derivation from French. To me that seems less important than explaining SI, so it doesn't need to be in the lead section. —⁠ ⁠BarrelProof (talk) 17:41, 17 January 2024 (UTC)
 * My choice would have been to put the official name in the lead, and explain the other later, but either way is fine. Gah4 (talk) 18:13, 17 January 2024 (UTC)
 * OK, but it doesn't explain it there. Maybe people are used to French acronyms for English terms by this point, though. As above, should we (and I am not saying one or the other) give the French name, too? Gah4 (talk) 18:17, 17 January 2024 (UTC)
 * Ok take a look, I did something different in the intro: use BIPM with explicit call out on the French origin of the acronym. I think this is better aligned with the article, which uses extensive refs to the brochure. Johnjbarton (talk) 18:32, 17 January 2024 (UTC)
 * My choice would have been to put the official name in the lead, and explain the other later, but either way is fine. Gah4 (talk) 18:13, 17 January 2024 (UTC)
 * OK, but it doesn't explain it there. Maybe people are used to French acronyms for English terms by this point, though. As above, should we (and I am not saying one or the other) give the French name, too? Gah4 (talk) 18:17, 17 January 2024 (UTC)
 * Ok take a look, I did something different in the intro: use BIPM with explicit call out on the French origin of the acronym. I think this is better aligned with the article, which uses extensive refs to the brochure. Johnjbarton (talk) 18:32, 17 January 2024 (UTC)

Internal duplications
There used to be an Introduction section which competed with the rest of the article. First, the lead is supposed to be the intro (MOS:INTRO). Second, two sections titled in the same way (e.g., #History) cannot receive different incoming links. Third, many Intro subsections were very long, some were even longer (in prose) than the corresponding section outside of the Intro section, which contained mostly lists (via templates). If the intent was to create an introductory article, then it should have been split into Introduction to the International System of Units instead. It felt as if someone didn't like the existing article and decided to write a new one, except it was left in a subsection. fgnievinski (talk) 04:46, 14 August 2023 (UTC)

External duplications
This article is too long and should not duplicate content in subtopics, such as SI base units, SI derived units, SI prefix, etc. Other notable subtopics could be mover into the respective main articles, e.g., SI. fgnievinski (talk) 05:13, 14 August 2023 (UTC)

Gram per cubic centimetre
There is some disagreement about the meaning of "gram per cubic centimetre" and "kilogram per cubic metre". Editors are invited to comment at the g/cm^3 talk page. Dondervogel 2 (talk) 13:56, 27 October 2023 (UTC)

More citations needed?
This article was recently tagged "More citations needed", but it has 97 references and not a single "citation needed". Can you help me understand the reasoning? @AirshipJungleman29 @CactiStaccingCrane Johnjbarton (talk) 02:56, 14 January 2024 (UTC)
 * I'm not sure I understand your question ; it is not a requirement to have "citation needed" tags in an article when adding a "more citations needed" banner, and the number of references is not really relevant if there is significant uncited material in the article. If it helps, I'll add citation needed tags, so you can see the unsourced material easier. AirshipJungleman29 (talk) 02:59, 14 January 2024 (UTC)
 * Yes, please. In a Start class article "More citations needed" is a good hint. In a B class article, it is only frustrating. Johnjbarton (talk) 03:04, 14 January 2024 (UTC)

Proposed improvements.
This article is one of the top 20 most viewed Physics articles that are assessed a C rating.


 * The intro is way too long. Some good material here could go into Units section.
 * Move Units up, Organizational down.
 * Much of the history duplicates a GA article: reduce to a summary plus SI-specific history.
 * Unpack most of the footnotes. If they aren't important enough to be in the article, delete them.
 * Apply WP:NOTGUIDE to many sections. I think a lot will not survive.

After removing material we don't think contributes to Wikipedia goal for this article, then we need to add citations to the rest. Johnjbarton (talk) 03:56, 14 January 2024 (UTC)


 * I finished my History butchery. The new lean mean version ready for review. Johnjbarton (talk) 03:19, 16 January 2024 (UTC)
 * Ok my big bunch of butchery is done. I have removed more than half of the material over all.
 * I don't think the end result is great. The text that is here is mostly the result of slicing and dicing. Every reference I added I checked but I did not check ever existing one. However three things have been accomplished: 1) the article describes SI, not some hybrid pre-post 2019 thing. 2) the outline is IMO much clearer 3) an amazing amount of duplicate material is gone. Johnjbarton (talk) 01:46, 18 January 2024 (UTC)

Why is the decibel special?
As this article is about the SI system, I don't think any other units are notable here without an important and specific connection to the SI.

In a recent edit @Dondervogel 2 added the decibel. I don't see how it is special and think it should be removed. Johnjbarton (talk) 17:11, 16 January 2024 (UTC)


 * The 9th edition of the SI brochure contains a Table 8 which lists non-SI units accepted for use with SI units. The decibel is listed in the table, along with other units that are mentioned in the article, such as the hour, the degree of angle, and the litre. Jc3s5h (talk) 18:01, 16 January 2024 (UTC)
 * Ok but we have an entire article Non-SI units mentioned in the SI devoted to that topic. Just mentioning decibel in that spot in this article is odd. Johnjbarton (talk) 18:25, 16 January 2024 (UTC)
 * What makes the decibel special IMO is that it is undefined. It seems notable that the BIPM permits use of (or even mentions) a unit that does not have a definition. Dondervogel 2 (talk) 18:42, 16 January 2024 (UTC)
 * The 9th edition of the SI brochure, page 146, has this paragraph:
 * "Table 8 also includes the units of logarithmic ratio quantities, the neper, bel and decibel. They are used to convey information on the nature of the logarithmic ratio quantity concerned. The neper, Np, is used to express the values of quantities whose numerical values are based on the use of the neperian (or natural) logarithm, ln = loge. The bel and the decibel, B and dB, where 1 dB = (1/10) B, are used to express the values of logarithmic ratio quantities whose numerical values are based on the decadic logarithm, lg = log10. The statement LX = m dB = (m/10) B (where m is a number) is interpreted to mean that m = 10 lg(X/X0). The units neper, bel and decibel have been accepted by the CIPM for use with the International System, but are not SI units."
 * In what way is the decibel undefined? Jc3s5h (talk) 19:02, 16 January 2024 (UTC)
 * The key bit is "are not SI Units". Defined or undefined is all about the decibel, where the topic gets a whole page. It does not belong in this article. Johnjbarton (talk) 19:06, 16 January 2024 (UTC)
 * I disagree. Just try to remove every mention of hour, minute (time or angle), arcsecond, degree of angle, and litre. I don't think such edits would be accepted by the editing community, and I would not agree with them. Jc3s5h (talk) 19:10, 16 January 2024 (UTC)
 * ok I put those back and made decibel not special. Johnjbarton (talk) 19:26, 16 January 2024 (UTC)
 * The decibel was defined in ISO 80000-3 but it's being moved to another section; it will be in ISO 80000-15 when that's released but it's already not in the latest ISO 80000-3. It's arguable whether this is noteworthy or a temporary glitch during which the definition is known and not about to change but not documented in a current ISQ standard. NebY (talk) 20:04, 16 January 2024 (UTC)
 * The ISO is not one of the organizations that defines or controls the International System of Units, although their views may be considered. Like most other ISO standards, ISO 80000-3 is fairly expensive so doesn't get used much outside of large organizations where readers can get their organization to pay for a copy. Why shouldn't we just ignore ISO 80000-3? Jc3s5h (talk) 20:19, 16 January 2024 (UTC)
 * There may be times we'd want to take note of it, but I agree, not in a way that leads to telling the general reader the decibel's a vague undefined unit. Glad that's gone - thanks @Johnjbarton. NebY (talk) 20:34, 16 January 2024 (UTC)
 * The price to purchase ISO 80000-3 is completely irrelevant because it is free to browse. Dondervogel 2 (talk) 22:34, 16 January 2024 (UTC)
 * When do you expect IEC 80000-15 to be published? Dondervogel 2 (talk) 22:32, 16 January 2024 (UTC)

Lexicographic conventions
We have a long section called "Lexicographic conventions" much of which reads like a guide contrary to WP:NOTGUIDE.

I propose to reduce the General rules subsection to a sentence and a for-example discussion of the figure. I think that gives the general idea without the detail. Any real use of the detail would need to consult the brochure anywayl. Johnjbarton (talk) 19:32, 16 January 2024 (UTC)


 * I cut down the general rules to a paragraph after moving some bits up in the section. Johnjbarton (talk) 19:00, 17 January 2024 (UTC)

Realisation of units
The section "Realisation of units" has a lot of duplicate material. Johnjbarton (talk) 20:25, 16 January 2024 (UTC)
 * we don't need another list of seven units
 * "Specifying fundamental constants vs. other methods of definition" this subsection is almost entirely historical material.


 * I cut the redundant material from this section. Johnjbarton (talk) 01:39, 18 January 2024 (UTC)

Defining constants
I think the weakest part of the article now is discussion of the defining constants and how they relate to the base units. This is the heart of the (new) SI. Johnjbarton (talk) 01:56, 18 January 2024 (UTC)

Error in equation
A bit down in the text on the page is the following equation:

1 kg = (299792458)2/(6.62607015×10−34)(9192631770)h ΔνCs/c2. Not sure if the paste worked as intended but you can find the original equation on the page. The problem is obvious, the units on the fraction is 1/kg rather than kg and it is inverse of the equation that follows The speed of light squared is in the nominator rather than the denominator etc. It appears that nominator and denominator has switched places in the numeric fraction and also there ought to be an equal sign (=) between that fraction and the formula that follows it. Right now they are all placed together making the resulting value 1 without any units.

Please fix. AlfSalte (talk) 04:45, 28 April 2024 (UTC)


 * The equation in the article
 * is given on page 131 of the authoritative source, the SI Brochure (9th Edn). You can also see it shown with the denominators in a different order on the BIPM's page on the kilogram. NebY (talk) 12:16, 28 April 2024 (UTC)
 * The equation appears to be copied correctly from the source. This is the official source. If you don't like it, take it up with the government in your country, and beg them to have your country's representative to the General Conference on Weights and Measures complain about it. Jc3s5h (talk) 12:36, 28 April 2024 (UTC)
 * The equation appears to be copied correctly from the source. This is the official source. If you don't like it, take it up with the government in your country, and beg them to have your country's representative to the General Conference on Weights and Measures complain about it. Jc3s5h (talk) 12:36, 28 April 2024 (UTC)