User:Lklundin/sandbox

= Draft: SI-writing of SI-units =

Unit choice and order
Quantities are typically expressed using an appropriate "primary unit", displayed first, followed by a conversion in parentheses, when appropriate, e.g. 200 km. For details on when to provide a conversion, see the section. The choice of primary units depends on the circumstances, and should respect the principle of "strong national ties", where applicable:
 * In most articles, including all scientific articles, the primary units chosen will be SI units, non-SI units officially accepted for use with the SI, or such other units as are conventional in reliable-source discussions of the article topic (such as revolutions per minute (rpm) for angular speed, hands for heights of horses, et cetera).
 * In non-scientific articles relating to the United States, the primary units are US customary, e.g. 97pounds (44kg).
 * In non-scientific articles relating to the United Kingdom, the primary units for most quantities are metric or other internationally used units, except that:
 * the primary units for distance/ length, speed and fuel consumption are miles, miles per hour, and miles per imperial gallon (except for short distances or lengths, where miles are too large for practical use);
 * the primary units for personal height and weight are feet /inches and stones/ pounds;
 * imperial pints are used for quantities of draught beer/ cider and bottled milk;
 * UK engineering-related articles, including all bridges and tunnels, generally use the system of units that the topic was drawn-up in (but road distances are given in imperial units, with a metric conversion).

Special considerations:
 * Quantities set via definition (as opposed to measured quantities) should be given first in the units used in the definition, even if this makes the structure of presentation inconsistent: During metrification, the speed limit was changed from 30miles per hour (48kilometers/hour) to 50km/h (31mph).
 * This may benefit from a slightly non-standard structure, such as ...from 30miles per hour (about 48kilometers/hour) to 50km/h (about 31mph). In this sort of case, using "about" can help make clear which is the statutory, exact value.
 * Nominal quantities (e.g. "2 &times; 4" lumber) require consideration of whether the article is concerned with the item's actual dimensions or merely with its function. In some cases only the nominal quantity may suffice; in others it may be necessary to give the nominal size (often in non-SI units), the actual size in non-SI units, and the actual size in SI units.
 * Where the primary units in the article are different from the primary units in the source, ensure that the precision of the converted quantity in the article is comparable to the precision of the quantity from the source (see below). The convert template has a   flag, which tells it to treat the converted unit as primary and the original unit as secondary, for use in such situations.

Unit names and symbols

 * Definitions:
 * Examples of unit names: foot, meter, kilometer.
 * Examples of unit symbols: ft, m, km.


 * Unit names and symbols should follow the practice of reliable sources.
 * For SI-units it is recommended to write a physical quantity (the combination of a number and a physical unit) using arabic numerals and the relevant unit symbol, e.g. 2 kg. However, when a value is used in a descriptive or literary manner it is preferable to use the unit name rather than its symbol. For example 'the kitchen lamp was designed to take three 60-watt light bulbs' or 'their old-fashioned camera uses rolls of 35-millimeter film'. This follows the recommendation of the SI-standard.
 * In prose, non-SI unit names should be given in full if used only a few times, but symbols may be used when a unit (especially one with a long name) is used repeatedly, after spelling out the first use (e.g. Up to 15ounces of filler is used for a batch of 250oz).
 * Certain unit names (e.g. &deg;C) need never be written in full unless required stylistically (automatic conversion of degrees Celsius to degrees Fahrenheit).


 * A SI-unit name rather its symbol should be used whenever the unit appears without a quantity. For example: "Some jurisdictions mandate that goods are sold in metric quantities such as kilogram or liter and expressly forbid the use of non-metric units".
 * Where space is limited, such as in tables, infoboxes, parenthetical notes, and mathematical formulas, unit symbols are preferred.
 * Units unfamiliar to general readers should appear with a wikilink to its definition the first time it is used, e.g. energies were initially 2.3 MeV, but eventually rose to 6 MeV.
 * Ranges use unspaced ndash if only one unit symbol is used at the end (e.g. 5.9–6.3kg), and spaced en dash (snd) if two symbols are used (e.g. 3μm – 1mm); ranges in prose may be specified using either unit symbol or unit names, and units may be stated either after both numerical values or after the last (e.g. from 5.9 to 6.3kilograms, from 5.9kilograms to 6.3kilograms, from 5.9 to 6.3kg and from 5.9kg to 6.3kg are all acceptable).
 * Length–width, length–width–height and similar dimensions may be separated by the multiplication sign (×) or the word by.
 * With the multiplication sign, each number should be followed by a unit name (if non-SI) or symbol (if appropriate):
 * &bull;    1m &times; 3m &times; 6m or (1 &times; 3 &times; 6)m, not 1 &times; 3 &times; 6 m or 1 &times; 3 &times; 6m3
 * &bull;    a metal plate 1ft &times; 3ft &times; 0.25in
 * &bull;    a railroad easement 10ft &times; 2.5mi
 * With by the unit need be given only once: 1 by 3 by 6 feet or 1 by 3 by 6 m
 * The unspaced letter x may be used in common terms such as 4x4.

Note to table:

Specific units

 * The following table lists only units that need special attention.
 * The SI standard should be consulted for guidance on use of other SI units. "Chapter 4" tables 6, 7, 8, and 9 give additional guidance on non-SI units.

Quantities of bytes and bits
In quantities of bits and bytes, the prefixes kilo (symbol k or K), mega (M), giga (G), tera (T), etc. are ambiguous. They may be based on a decimal system (like the standard SI prefixes), meaning 103, 106, 109, 1012, etc., or they may be based on a binary system, meaning 210, 220, 230, 240, etc. The binary meanings are more commonly used in relation to solid-state memory (such as RAM), while the decimal meanings are more common for data transmission rates, disk storage and in theoretical calculations in modern academic textbooks.

Follow these recommendations when using these prefixes in Wikipedia articles:


 * Following the SI standard, a lower-case k should be used for "kilo-" whenever it means 1000 in computing contexts, whereas a capital K should be used instead to indicate the binary prefix for 1024 according to JEDEC. (If, under the exceptions detailed further below, the article otherwise uses IEC prefixes for binary units, use Ki instead).
 * Do not assume that the binary or decimal meaning of prefixes will be obvious to everyone. Explicitly specify the meaning of k and K as well as the primary meaning of M, G, T, etc. in an article (BDprefix is a convenient helper). Consistency within each article is desirable, but the need for consistency may be balanced with other considerations.
 * The definition most relevant to the article should be chosen as primary for that article, e.g. specify a binary definition in an article on RAM, decimal definition in an article on hard drives, bit rates, and a binary definition for Windows file sizes, despite files usually being stored on hard drives.
 * Where consistency is not possible, specify wherever there is a deviation from the primary definition.
 * Disambiguation should be shown in bytes or bits, with clear indication of whether in binary or decimal base. There is no preference in the way to indicate the number of bytes and bits, but the notation style should be consistent within an article. Acceptable examples include:
 * &bull;    A 64MB (64 &times; 10242-byte) video card and a 100GB (100 &times; 10003-byte) hard drive
 * &bull;    A 64MB (64 &times; 220-byte) video card and a 100GB (100 &times; 109-byte) hard drive
 * &bull;    A 64MB (67,108,864-byte) video card and a 100GB (100,000,000,000-byte) hard drive


 * Avoid inconsistent combinations such as A 64MB (67,108,864-byte) video card and a 100GB (100 &times; 10003-byte) hard drive. Footnotes, such as those seen in [//en.wikipedia.org/w/index.php?title=Power_Macintosh_5500&oldid=218088888 Power Macintosh 5500], may be used for disambiguation.
 * Unless explicitly stated otherwise, one byte is eight bits (see History of byte).

The IEC prefixes kibi-, mebi-, gibi-, etc. (symbols Ki, Mi, Gi, etc.) are rarely used, even in technical articles, so are generally not to be used except:
 * when the majority of cited sources on the article topic use IEC prefixes,
 * in a direct quote using the IEC prefixes,
 * when explicitly discussing the IEC prefixes,
 * in articles in which both types of prefix are used with neither clearly primary, or in which converting all quantities to one or the other type would be misleading or lose necessary precision, or declaring the actual meaning of a unit on each use would be impractical.

Unit conversions
Where English-speaking countries use different units for the same quantity, follow the "primary" quantity with a conversion in parentheses: the Mississippi River is 2320 mi long; the Murray River is 2375 km long. In science-related articles, however, supplying such conversion is not required unless there is some special reason to do so.
 * Where an imperial unit is not part of the US customary system, or vice-versa—‌and in particular, where those systems give a single term different definitions—‌a double conversion may be appropriate: Rosie weighed 19 st, The car had a fuel economy of 5 L/100km
 * Generally, conversions to and from metric units and US or imperial units should be provided, except:
 * When inserting a conversion would make a common or linked expression awkward (The four-minute mile).
 * When units are part of the subject of a topic—‌nautical miles in articles about the history of nautical law (5 nautical miles), SI units in scientific articles (a 600-kilometer asteroid), yards in articles about American football—‌it can be excessive to provide conversions every time a unit occurs. It could be best to note that this topic will use the units (possibly giving the conversion factor to another familiar unit in a parenthetical note or a footnote), and link the first occurrence of each unit but not give a conversion every time it occurs.
 * Converted quantity values should use a level of precision similar to that of the source quantity value, so the Moon is 380,000km (240,000mi) from Earth, not (236,121mi). Small numbers may need to be converted to a range where rounding would cause a significant distortion, so one mile (1–2km), not one mile (2km). Be careful especially when your source has already converted from the units you're now converting back to.  This may be evidenced by multiples of common conversion factors in the data, such as 160 km (from 100 miles). See false precision.
 * Conversion templates can be used to convert and format many common units, including convert, which includes non-breaking spaces.
 * In a direct quotation, always retain the source units. Any conversions can be supplied either in the quote itself (in square brackets, following the original measurement) or in a footnote. See footnoting and citing sources.
 * Units attention may be added to articles needing general attention regarding choice of units and unit conversions.