User:Lklundin/sandbox

Draft: SI-writing of SI-units

Units of measurement

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 (120 mi). For details on when to provide a conversion, see the section § Unit conversions. 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. 97 pounds (44 kg).
• 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 30 miles per hour (48 kilometers/hour) to 50 km/h (31 mph).
  • This may benefit from a slightly non-standard structure, such as ...from 30 miles per hour (about 48 kilometers/hour) to 50 km/h (about 31 mph). In this sort of case, using "about" can help make clear which is the statutory, exact value.
• Nominal quantities (e.g. "2 × 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 § Unit conversions below). The {{convert}} template has a |disp=flip 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 15 ounces of filler is used for a batch of 250 oz).

• Certain unit names (e.g. °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. 5.9–6.3 kg), and spaced en dash ({{snd}}) if two symbols are used (e.g. 3 μm – 1 mm); 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 6.3 kilograms, from 5.9 kilograms to 6.3 kilograms, from 5.9 to 6.3 kg and from 5.9 kg to 6.3 kg 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):

  •  1 m × 3 m × 6 m or (1 × 3 × 6) m, not 1 × 3 × 6 m or 1 × 3 × 6 m³

  •  a metal plate 1 ft × 3 ft × 0.25 in

  •  a railroad easement 10 ft × 2.5 mi

• 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.

Unit names and symbols—General guidelines

Aspect	Guideline	Acceptable	Unacceptable
Spelling	The spelling of certain unit names (some of which are listed in § Specific units, below) varies with the variety of English followed by the article.
Format	Do not spell out numbers before unit symbols ...	5 min	five min
	... but words or numerals may be used with unit names.	five minutes 5 minutes
	Values not accompanied by units are usually given in figures.	Set the control to 10. These go to 11.	Set the control to ten. These go to eleven.
	Write unit names and symbols in upright roman type.	10 m 29 pounds	10 m 29 pounds
	Unit names are given in lower case except: where any word would be capital­ized; where otherwise specified in the SI brochure; where otherwise specified in this Manual of Style.	He walked several miles. Miles of trenches were dug.	A Gallon equals 4 Quarts.
	Except as listed in the "Specific units" table below, unit symbols are uncapi­tal­ized unless they are derived from a proper name, in which case the first letter (of the base unit name, not of any prefix) is capitalized.	8 kg 100 kPa	8 Kg 100 kpa
	Unit symbols are undotted.	38 cm	38 cm.
	Except as shown in the "Specific units" table below, a space appears between a numeric value and a unit name or symbol. In the case of unit symbols,   (or {{nowrap}} ) should be used to prevent linebreak.	29 kg Markup: 29 kg	29kg
	To form a value and a unit name into a compound adjective use a hyphen or hyphens ...	a five-day holiday a five-cubic-foot box a 4-inch blade This hyphenation is optional if confusion could result, e.g. because the hyphenated construction has another meaning in the context: Nine-ball is a nine ball game, while eight-ball is a fifteen ball game with the 8 ball as the game-winning ball.
	... but a non-breaking space (never hyphen) separates a value and unit symbol.	a blade 10 cm long	a 10-cm blade
Plurals	SI unit names are pluralized by adding s or es...	1 ohm, 10 ohms
	... except for these irregular forms.	1 henry, 10 henries 1 hertz, 10 hertz 1 lux, 10 lux 1 siemens, 10 siemens	10 henrys 10 hertzes 10 luxes
	Some non-SI units have irregular plurals.	1 foot, 10 feet 1 stratum, 10 strata (unusual)	10 foots 10 stratums
	Unit symbols (in any system) are identical in singular and plural.	grew from 1 in to 2 in grew from 1 inch to 2 inches grew from one to two inches	grew from 1 in to 2 ins
Powers	Format exponents using <sup>, not special characters.	km2 Markup: km<sup>2</sup>	km² Markup: km&#178;
	Or use squared or cubed (after the unit being modified).	ten feet per second squared	ten feet per squared second
	For areas or volumes only, square or cubic may be used (before the unit being modified).		ten feet per square second
		ounces per square inch
	The abbreviations sq and cu may be used for US customary and imperial units but not for SI units.	15 sq mi 3 cu ft	15 sq km 3 cu m
Products	Indicate a product of unit names with either a hyphen or a space.	foot-pound foot pound	footpound foot·pound
	Indicate a product of unit symbols with &middot; or &nbsp; (Note: {{middot}} is not equivalent to &middot;.)	ms = millisecond m·s or m s = metre-second.
	Exception: In some topic areas such as power engineer­ing, certain products take neither space nor &middot;. Follow the practice of reliable sources in the article's topic area.	Wh, VA, Ah kWh, MVA, GAh
	To pluralize a product of unit names, pluralize only the final unit. (Unit symbols are never pluralized.)	ten foot-pounds	ten feet-pounds
Ratios Rates Densities	Indicate a ratio of unit names with per.	meter per second	meter/second
	Indicate a ratio of unit symbols with a slash (followed by either a single symbol or a parenthesized product of symbols‍—‌do not use multiple slashes); or use −1, −2, etc.	metre per second m/s m·s−1	mps
		kg/(m·s)	kg/m/s kg/m·s
	To pluralize a ratio of unit names, pluralize only the "numerator" unit. (Unit symbols are never pluralized.)	ten newton-metres per second 10 N·m/s
	Some of the special forms used in the imperial and US customary systems are shown here ...	mph = miles per hour mpg = miles per gallon psi = pounds per square inch
	... but only the slash or negative exponent notations are used with SI (and other metric) units.	g/m2 g·m−2 km/h km·h-1	gsm kph
Prefixes	Prefixes should not be separated by a space or hyphen.	The kilopascal	The kilo pascal The kilo-pascal
	Prefixes are added without contraction, except as shown here:	kilohm megohm hectare	kiloohm megaohm hectoare
	centi-, deci-, deca-, and hecto- should be avoided; exceptions include centimetre, decibel, hectolitre, hectare, hectopascal.	100 metres 0.1 km	1 hectometre
	Do not use M for 103, MM for 106, or B for 109.	3 km 8 MW 125 GeV	3 Mm 8 MMW 125 BeV
Mixed units	Mixed units are traditionally used with the imperial and US customary systems …	1 ft 6 in 1 foot 6 inches 1.5 ft 18 in 1 US fl pt 8 oz
	... and in expressing time durations ...	1:30:07 1:30  1 hr 30 min 7 sec 1 h 30 m 7 s 1h 30m 07s	1 h 30 min 7 s 1 hr 30 m 7 sec 1:30′07″
	… but are not normally used in SI.	1.33 m 133 cm	1 m 33 cm
	No comma.	6 lb 3 oz	6 lb, 3 oz

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.

Guidelines on specific units

Group	Name	Symbol	Comment
Length, Speed	inch	in	Do not use ′ (′), ″ (″), apostrophe/​single quote (') or double quote (")
	foot	ft
	foot per second	ft/s (not fps)
	hand	h or hh	Equal to 4 inches; used in measurement of horses. A dot may be followed by additional inches e.g. 16.2 hh indicates 16 hands 2 inches.
	knot	kn (not kt or kN)
	metre meter (U.S.)	m
	micron	μm (not μ)	Markup: μm  Link to micrometre (for which micron is a synonym) on first use.
	mile	mi	In nautical and aeronautical contexts use statute mile rather than mile to avoid confusion with nautical mile.
	mile per hour	mph
	nautical mile	nmi or NM (not nm)
Volume, Flow	cubic centimetre cubic centimeter (U.S.)	cm3	Markup: cm<sup>3</sup>
		cc	Non-SI symbol used for certain engine displacements; link to cubic centimetre on first use.
	imperial fluid ounce	imp fl oz	US or imperial/imp must be specified; fluid/fl must be specified, except with gallon. (Without fluid, ounce is ambiguous – versus avoirdupois ounce or troy ounce – and pint or quart is ambiguous – versus US dry pint or US dry quart.)
	imperial fluid pint	imp fl pt
	imperial fluid quart	imp fl qt
	imperial gallon	imp gal
	US fluid ounce	US fl oz
	US fluid pint	US fl pt
	US fluid quart	US fl qt
	US gallon	US gal
	cubic foot	cu ft (not cf)	Write five million cu ft or 5,000,000 cu ft, not 5 MCF.
	cubic foot per second	cu ft/s (not cfs)
	litre liter (U.S.)	l or L	The symbol l in isolation (i.e. not in such forms as ml) is easily mistaken for the digit 1.
Mass, Force, Density, Pressure	long ton	long ton	Spell out in full.
	short ton	short ton
	pound per square inch	psi
	tonne metric ton (U.S.)	t (not mt or MT)
	troy ounce	oz t	t or troy must be specified. Articles about precious metals, black powder, and gemstones should always specify whether ounces and pounds are avoirdupois or troy.
	troy pound	lb t or troy
	carat	carat	Used to express masses of gemstones and pearls.
Purity	carat or karat	k or Kt	A measure of purity for gold alloys. (Do not confuse with the unit of mass with the same spelling.)
Time	second	s (not sec)	Do not use &prime; (′), &Prime; (″), apostrophe (') or quote (") for minutes or seconds. Use m for minute only where there is no danger of confusion with meter, as in the hours–minutes–seconds formats for time durations described in the Unit names and symbols table.
	minute	min
	hour	h
	year	a	Use a only with an SI prefix (a rock formation 540 Ma old, not Life expectancy rose to 60 a).
		y or yr
Information	bit	bit (not b or B)	See also § Quantities of bytes and bits, below.
	byte	B or byte (not b or o)
	bit per second	bit/s (not bps)
	byte per second	B/s or byte/s (not Bps or Bps)
Angle
	arcminute	′	Markup: &prime;  (not apostrophe/​single quote '). No space between numerals and symbol (47′, not 47 ′)
	arcsecond	″	Markup: &Prime;  (not double-quote "). No space between numerals and symbol (22″, not 22 ″)
	degree	°	Markup: &deg;  (not masculine ordinal º or ring ̊ ). No space between numerals and symbol (23°, not 23 °)
Temperature	degree		Markup: &deg;. Nonbreaking space ({{nbsp}}) between numerals and symbol (40 °, not 40°; 12 °C, not 12°C, nor 12° C)
	degree Celsius (not degree centigrade)	°C (not C)
Energy	gram calorie small calorie	cal	In certain subject areas calorie is convention­ally used alone. Articles following this practice should specify either gram calorie (or small calorie) or kilogram calorie (or large calorie) on first use; providing conversions to SI units (usually gram calories to joules and kilogram calories to kilojoules) may also be useful. A kilogram calorie is 1000 gram calories, and is therefore also a kilocalorie (kcal); other SI prefixes may be used with the gram calorie (e.g., Mcal) but not with the kilogram calorie (do not use kCal or MCal).
	kilogram calorie large calorie (not Calorie)	Cal

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:

  •  A 64 MB (64 × 1024²-byte) video card and a 100 GB (100 × 1000³-byte) hard drive

  •  A 64 MB (64 × 2²⁰-byte) video card and a 100 GB (100 × 10⁹-byte) hard drive

  •  A 64 MB (67,108,864-byte) video card and a 100 GB (100,000,000,000-byte) hard drive

• Avoid inconsistent combinations such as A 64 MB (67,108,864-byte) video card and a 100 GB (100 × 1000³-byte) hard drive. Footnotes, such as those seen in 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 2,320 miles (3,734 km) long; the Murray River is 2,375 km (1,476 mi) 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 stone (266 lb; 121 kg), The car had a fuel economy of 5 L/100 km (47 mpg_‑US; 56 mpg_‑imp)
• 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 380,000 km (240,000 mi) from Earth, not (236,121 mi). Small numbers may need to be converted to a range where rounding would cause a significant distortion, so one mile (1–2 km), not one mile (2 km). 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.