Metric prefix

{{Short description|Order of magnitude indicator}} {{For|use of measurement as a form of social power|metric power}} {{use British English|date=November 2022}} {{use dmy dates|date=November 2022|cs1-dates=y}}

A '''metric prefix''' is a unit prefix that precedes a basic unit of measure to indicate a multiple or submultiple of the unit. All metric prefixes used today are decimal. Each prefix has a unique symbol that is prepended to any unit symbol. The prefix ''kilo'', for example, may be added to ''gram'' to indicate {{em|multiplication}} by one thousand: one kilogram is equal to one thousand grams. The prefix ''milli'', likewise, may be added to ''metre'' to indicate {{em|division}} by one thousand, so one millimetre is equal to one thousandth of a metre.

Decimal multiplicative prefixes have been a feature of all forms of the metric system, with six of these dating back to the system's introduction in the 1790s. Metric prefixes have also been used with some non-metric units. The '''SI prefixes''' are metric prefixes that were standardised for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 2022.<ref name="prefixes">{{Cite web |title=SI prefixes |url=http://www.bipm.org/en/si/prefixes.html |url-status=deviated |archive-url=https://web.archive.org/web/20140912222624/http://www.bipm.org/en/si/prefixes.html |archive-date=12 September 2014 |access-date=13 June 2023 |website=Bipm.org |publisher=Bureau International des Poids et Mesures}}</ref><ref name="res2022">{{Cite web |date=18 November 2022 |title=Resolutions of the General Conference on Weights and Measures (27th meeting) |url=https://www.bipm.org/documents/20126/64811223/Resolutions-2022.pdf/281f3160-fc56-3e63-dbf7-77b76500990f |url-status=live |archive-url=https://web.archive.org/web/20230613084209/https://www.bipm.org/documents/20126/64811223/Resolutions-2022.pdf/281f3160-fc56-3e63-dbf7-77b76500990f |archive-date=13 June 2023 |access-date=18 May 2024 |publisher=Bureau International des Poids et Mesures |language=fr, en |format=PDF}}</ref> Since 2009, they have formed part of the ISO/IEC 80000 standard. They are also used in the Unified Code for Units of Measure (UCUM).

==List of SI prefixes== {{hatn|Several terms redirect here including "Femto", "Ronna-", "Yotta", and "Zepto". For the character Femto, see Griffith (''Berserk''). For the name, see Ronna. For the prize-linked savings account, see Yotta Technologies. For the Finnish musician, see Yotto. For similar terms, see Yota (disambiguation). For the Indian Q-commerce company, see Zepto (company). For other uses, see Atto, Exa (disambiguation), Peta, Pico, Quetta (disambiguation), Ronto, and Tera.}} The BIPM specifies twenty-four prefixes for the International System of Units (SI). {{Anchor|SI prefixes table}} {{SI prefixes (infobox)}}

The first uses of prefixes in SI date back to the definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use, and were recognised by the 1947 IUPAC 14th International Conference of Chemistry<ref>{{Cite work |url=https://archive.org/details/iupaccomptesrend14iupa |title=Comptes rendus de la Quatorzième Conférence |date=24 July 1947 |publisher=IUPAC |location=London |language=fr}}</ref> before being officially adopted for the first time in 1960.<ref name="res12">{{Cite web |year=1960 |title=Resolution 12 of the 11th meeting of the CGPM |url=http://www.bipm.org/en/CGPM/db/11/12/ |url-status=deviated |access-date=2023-09-12 |website=Bipm.org |publisher=Bureau International des Poids et Mesures}}</ref>{{New archival link needed|date=April 2026}}

The prefixes that were most recently adopted are ''ronna'', ''quetta'', ''ronto'', and ''quecto''. These prefixes were adopted in 2022, after a proposal from British metrologist Richard J. C. Brown. Before 2022, Q/q and R/r were the only Latin letters available for abbreviations, with all other Latin letters either being: already used for other prefixes (a, c, d, E, f, G, h, k, M, m, n, P, p, T, Y, y, Z, z); already used for SI units (including: SI base units, SI derived units, non-SI units mentioned in the SI; A, B, C, d, F, g, H, h, J, K, L, m, N, S, s, T, t, u, V, W); easily confused with mathematical symbols ({{Mono|I}} and {{Mono|l}} are easily confused with {{Mono|1}}, {{Mono|O}} and {{Mono|o}} are easily confused with {{Mono|0}}, {{Mono|X}} and {{Mono|x}} are easily confused with {{Mono|×}}); or a combination (some symbols are used for both, e.g. metre and milli- both use {{Mono|m}}). The large prefixes ''ronna'' and ''quetta'' were adopted in anticipation of needs for use in data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were also added, even without such a driver, in order to maintain symmetry.<ref name="earth-electron">{{citation |last=Sample |first=Ian |title=Earth weighs in at six ronnagrams as new prefixes picked for big and small |date=2022-11-18 |newspaper=The Guardian |url=https://www.theguardian.com/science/2022/nov/18/earth-six-ronnagrams-new-prefixes-big-and-small |access-date=2022-12-14}}</ref>

The prefixes from ''peta'' to ''quetta'' are based on the Ancient Greek or Ancient Latin numbers from 5 to 10, referring to the 5th through 10th powers of 103. The initial letter ''h'' has been removed from some of these stems and the initial letters ''z'', ''y'', ''r'', and ''q'' have been added, ascending in reverse alphabetical order, to avoid confusion with other metric prefixes.

===Rules=== * The symbols for the units of measure are combined with the symbols for each prefix name. The SI symbols for kilometre, kilogram, and kilowatt, for instance, are km, kg, and kW, respectively. (The symbol for ''kilo'' is k.) Except for the early prefixes of ''kilo'', ''hecto'', and ''deca'', the symbols for the prefixes for multiples are uppercase letters, and those for the prefixes for submultiples are lowercase letters.<ref>{{Cite web |title=Metric Prefixes and SI Units |url=https://learn.sparkfun.com/tutorials/metric-prefixes-and-si-units/all |access-date=26 January 2020 |website=learn.sparkfun.com |series=tutorials}}</ref> * All of the metric prefix symbols are made from upper- and lower-case Latin letters except for the symbol for ''micro'', which is uniquely a Greek letter mu (μ). * The prefix symbols are always prepended to the symbol for the unit without any intervening space or punctuation.<ref>{{Cite web |date=September 2004 |orig-date=February 1998 |title=SI Unit rules and style conventions checklist |url=https://physics.nist.gov/cuu/Units/checklist.html |website=nist.gov |publisher=NIST}}</ref> This distinguishes a prefixed unit symbol from the product of unit symbols, for which a space or mid-height dot as separator is required. So, for instance, while 'ms' means millisecond, 'm s' or 'm·s' means metre-second. * Prefixes corresponding to an integer power of one thousand are generally preferred; the prefixes corresponding to tens (deci-, deca-) and hundreds (centi-, hecto-) are less common and are disfavoured in certain fields. Hence, 100 m is preferred over 1 hm (hectometre) or 10 dam (decametres). The prefixes ''deci-'' and ''centi-'', and less frequently ''hecto'' and ''deca'', are generally used for informal purposes; the centimetre (cm) is especially common. Some modern building codes require that the millimetre be used in preference to the centimetre, because "use of centimetres leads to extensive usage of decimal points and confusion".<ref>{{Cite report |url=http://wbdg.org/ccb/GSAMAN/mdg.pdf |title=Metric Design Guide |date=September 1995 |publisher=U.S. General Services Administration |id=PBS-PQ260 |access-date=21 April 2018 |archive-url=https://web.archive.org/web/20111215115519/http://wbdg.org/ccb/GSAMAN/mdg.pdf |archive-date=15 December 2011 |via=National Institute of Building Sciences |series=Public Buildings Service}}</ref> These prefixes are also commonly used to create metric units corresponding to older conventional units, for example hectares and hectopascals. * Prefixes may not be used in combination on a single symbol. This includes the case of the base unit kilogram, which already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg). * During mathematical operations, prefixes are treated as multiplicative factors. For example, 5 km is treated as 5000 m, which allows all quantities based on the same unit to be factored together even if they have different prefixes. * A prefix symbol attached to a unit symbol is included when the unit is raised to a power. For example, 1 km2 denotes 1 km × 1 km = 106 m2, not 103 m2.

==Usage==

===Examples=== * The mass of an electron is about 1 rg (rontogram).<ref name="earth-electron"/>{{efn|{{physconst|me|symbol=yes}}. Converting to grams gives {{val|9.1093837015|e=-28|u=g}}. Rounding to the nearest power of ten gives {{val|1|e=-27|u=g}}, or 1 rg.}} * The mass of 1 litre of water is about 1 kg (kilogram).<ref>{{Cite web |date=15 March 2024 |title=Kilogram | mass, weight, SI unit | Britannica |url=https://www.britannica.com/science/kilogram}}</ref> * The mass of the Earth is about 6 Rg (ronnagrams).<ref name="earth-electron"/> * The mass of Jupiter is about 2 Qg (quettagrams).<ref name="earth-electron"/>

===Examples of powers of units with metric prefixes=== * 1 km2 means one square kilometre, or the area of a square of {{val|1000|u=m}} by {{val|1000|u=m}}. In other words, an area of {{val|1000000}} square metres and not {{val|1000}} square metres. * 2 Mm3 means two cubic megametres, or the volume of two cubes of {{val|1000000|u=m}} by {{val|1000000|u=m}} by {{val|1000000|u=m}}, i.e. {{val|2|e=18|u=m3}}, and not {{val|2000000}} cubic metres ({{val|2|e=6|u=m3}}).

===Examples with prefixes and powers=== * {{val|5|u=mV}} × {{val|5|u=mA}} = {{val|5|e=-3|u=V}} × {{val|5|e=-3|u=A}} = {{val|25|e=-6|u=V⋅A}} = {{val|25|u=uW}}. * {{val|5.00|u=mV}} + {{val|10|u=uV}} = {{val|5.00|u=mV}} + {{val|0.01|u=mV}} = {{val|5.01|u=mV}}. * {{val|5|u=cm}} = {{val|5|e=-2|u=m}} = {{nowrap|5 × 0.01 m}} = 0.05 m. * {{val|9|u=km2}} = {{nowrap|9 × (103 m)2}} = {{nowrap|9 × (103)2 × m2}} = {{val|9|e=6|u=m2}} = {{nowrap|9 × {{val|1000000|u=m2}}}} = {{val|9000000|u=m2}}. * 3 MW = {{val|3|e=6|u=W}} = 3 × {{val|1000000|u=W}} = {{val|3000000|u=W}}.

==Application to units of measurement== The use of prefixes can be traced back to the introduction of the metric system in the 1790s, long before the 1960 introduction of the SI.<ref>{{Cite journal |date=2010-01-13 |title=Metric (SI) Prefixes |url=https://www.nist.gov/pml/owm/metric-si-prefixes |journal=NIST |language=en}}</ref> The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidyne and milligauss). Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time.<ref>{{SIbrochure9th|page=145}}</ref>

===Metric units===

====Mass==== The units kilogram, gram, milligram, microgram, and smaller are commonly used for measurement of mass. However, megagram, gigagram, and larger are rarely used; tonnes (and kilotonnes, megatonnes, etc.) or scientific notation are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".

The kilogram is the only coherent unit of the International System of Units that includes a metric prefix.{{refn|{{SIbrochure9th}}}}{{rp|p=144}}

====Volume==== The litre (equal to a cubic decimetre), millilitre (equal to a cubic centimetre), microlitre, and smaller are common. In Europe, the centilitre is often used for liquids (e.g. bottles or servings of wine), and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres).<ref>{{Cite web |date=2023-12-18 |title=Understanding Hectoliter Weight: What It Reveals About Wheat - Agriculture Notes by Agriculture.Institute |url=https://agriculture.institute/wheat-maize-coarse-grains-milling/hectoliter-weight-wheat-importance/ |access-date=2026-02-23 |language=en-US}}</ref>

Larger volumes are usually denoted in kilolitres, megalitres or gigalitres, or else in cubic metres (1 cubic metre = 1 kilolitre) or cubic kilometres (1 cubic kilometre = 1 teralitre). For scientific (other than medical) purposes, the SI unit of cubic metre is usually used, with scientific notation rather than prefixes.{{cn|date=November 2022}}

====Length==== The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by the older non-SI name ''micron'', which is officially deprecated. In some fields, such as chemistry, the ångström (0.1 nm) has been used commonly instead of the nanometre. The femtometre, used mainly in particle physics, is sometimes called a fermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the solar radius, astronomical units, light years, and parsecs, and less commonly large multiples (e.g. millions) of kilometres; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.<ref>{{Cite web |title=The International System of Units (SI) |url=https://www.bipm.org/documents/20126/41483022/SI-Brochure-9-EN.pdf/2d2b50bf-f2b4-9661-f402-5f9d66e4b507?version=4.0&t=1736179667753&download=true |access-date=2025-06-28 |website=International Bureau of Weights and Measures}}</ref>

====Time==== {{see also|Metric time|Orders of magnitude (time)}}

Prefixes for the SI standard unit second are most commonly encountered for quantities less than one second. For larger quantities, the system of minutes (60 seconds), hours (60 minutes) and days (24 hours) is accepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardised to {{val|86,400}} seconds so as not to create issues with the irregular leap second.<ref>{{Cite web |title=Leap Seconds |url=https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/ |archive-url=http://web.archive.org/web/20250906201039/https://www.cnmoc.usff.navy.mil/Our-Commands/United-States-Naval-Observatory/Precise-Time-Department/Global-Positioning-System/USNO-GPS-Time-Transfer/Leap-Seconds/ |archive-date=2025-09-06 |access-date=2025-12-25 |website=www.cnmoc.usff.navy.mil |language=en-US}}</ref>

Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in astronomy, the Julian year or ''annum'' (a) is a standardised variant of the year, equal to exactly {{val|31557600}} seconds ({{sfrac|365| 1 |4}} days). The unit is so named because it was the average length of a year in the Julian calendar. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum (Ma) or gigaannum (Ga).{{cn|date=November 2022}}

====Angle==== The SI unit of angle is the radian, but degrees, as well as arc-minutes and arc-seconds, see some scientific use in fields such as astronomy.<ref>Barbieri, C., & Bertini, I. (2020). Fundamentals of Astronomy (2nd ed.), p. 1-2. CRC Press. https://doi.org/10.1201/9780429287305</ref>

====Temperature==== Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states:<ref name="Special Publication 811"/> "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit name ''degree Celsius''. For example, 12 m°C (12 millidegrees Celsius) is acceptable." In practice, it is more common for prefixes to be used with the kelvin when it is desirable to denote extremely large or small absolute temperatures or temperature differences. Thus, temperatures of star interiors may be given with the unit of MK (megakelvin), and molecular cooling may be given with the unit mK (millikelvin).{{cn|date=November 2022}}

====Energy==== In use the joule and kilojoule are common, with larger multiples seen in limited contexts. In addition, the kilowatt-hour, a composite unit formed from the kilowatt and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour).{{cn|date=November 2022}}

Several definitions exist for the non-SI unit calorie. Distinguished are gram calories and kilogram calories. One kilogram calorie, which equals one thousand gram calories, often appears capitalised and without a prefix (i.e. ''Cal'') when referring to "dietary calories" in food.<ref>{{Cite web |last1=Conn |first1=Carole |last2=Kravitz |first2=Len |title=Remarkable Calorie |url=http://www.unm.edu/~lkravitz/Article%20folder/remarkablecalorie.html |access-date=22 May 2017 |publisher=University of New Mexico}}</ref> It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal.

===Non-metric units=== Metric prefixes are widely used outside the metric SI system. Common examples include the megabyte and the decibel. Metric prefixes rarely appear with imperial or US units except in some special cases (e.g., microinch, kilofoot, kilopound). They are also used with other specialised units used in particular fields (e.g., megaelectronvolt, gigaparsec, millibarn, kilodalton). In astronomy, geology, and palaeontology, the year, with symbol 'a' (from the Latin ''annus''), is commonly used with metric prefixes: ka, Ma, and Ga.<ref>{{Cite encyclopedia |title=Ga |encyclopedia=Encyclopedia of Astrobiology |publisher=Springer Science & Business Media |date=2011-05-26 |page=621 |lang=en |isbn=978-3-642-11271-3 |last2=Amils |first2=Ricardo |last3=Cleaves |first3=Henderson James |last1=Gargaud |first1=Muriel |df=dmy-all}}</ref>

Official policies about the use of SI prefixes with non-SI units vary slightly between the International Bureau of Weights and Measures (BIPM) and the American National Institute of Standards and Technology (NIST). For instance, the NIST advises that "to avoid confusion, prefix symbols (and prefix names) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) ° (degree), {{'}} (minute), and {{''}} (second),<ref name="Special Publication 811">{{Cite report |url=http://physics.nist.gov/Pubs/SP811/sec06.html |title=Special Publication 811 |last1=Thompson |first1=Ambler |last2=Taylor |first2=Barry N. |date=March 2008 |publisher=National Institute of Standards and Technology |edition=2008 |access-date=21 June 2018 |via=nist.gov |lang=en}}</ref> whereas the BIPM adds information about the use of prefixes with the symbol ''as'' for arcsecond when they state: "However astronomers use milliarcsecond, which they denote mas, and microarcsecond, μas, which they use as units for measuring very small angles."<ref>{{Cite report |url=http://www.bipm.org/en/publications/si-brochure/chapter3.html |title=The International System of Units (SI) |publisher=International Bureau of Weights and Measures |access-date=5 March 2017 |series=SI Brochure}}</ref>

==Non-standard prefixes== {{see also|Unit prefix#Unofficial prefixes}} [[File:Myriameterstein36RüdesheimRhein.JPG|thumb|Distance marker on the Rhine at Rüdesheim: 36 (XXXVI) myriametres from Basel. The stated distance is 360 km; the decimal mark in German is a comma.]]

===Obsolete metric prefixes=== Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.<ref>{{Cite web |date=13 May 1866 |title=H.R. 596, An Act to authorize the use of the metric system of weights and measures |url=http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html |url-status=dead |archive-url=https://web.archive.org/web/20150705015307/http://lamar.colostate.edu/~hillger/laws/metric-act-bill.html |archive-date=5 July 2015 |publisher=29th Congress of the United States, Session 1}}</ref><ref name="Brewster_1830">{{Cite book |last=Brewster |first=David |url=https://books.google.com/books?id=0bIkTUZAbxcC |title=The Edinburgh Encyclopædia |date=1830 |publisher=William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J. M. Richardson |volume=12 |location=Edinburgh, UK |page=494 |access-date=9 October 2015}}</ref><ref name="Brewster_1832">{{Cite book |last=Brewster |first=David |url=https://books.google.com/books?id=17RGAQAAIAAJ&pg=PA572 |title=The Edinburgh Encyclopaedia |date=1832 |publisher=Joseph and Edward Parker |edition=1st American |volume=12 |access-date=9 October 2015}}</ref>{{page needed|reason=Also specify entry using Template:Cite encyclopedia instead|date=August 2025}} The decimal prefix for ten thousand, ''myria-'' (sometimes spelt ''myrio-''), and the early binary prefixes ''double-'' (2×) and ''demi-'' ({{sfrac|1|2}}×) were parts of the original metric system adopted by France in 1795,<ref name="fr">{{Cite book |title=L'histoire du mètre |language=fr |trans-title=The History of the Metre |section=La loi du 18 Germinal an 3 |quote=Décision de tracer le mètre, unité fondamentale, sur une règle de platine. Nomenclature des « mesures républicaines ». Reprise de la triangulation |access-date=2015-10-12 |section-url=http://histoire.du.metre.free.fr/fr/Pages/Sommaire/06.htm |archive-url=https://web.archive.org/web/20221126164814/http://histoire.du.metre.free.fr/fr/Pages/Sommaire/06.htm |archive-date=2022-11-26 |url-status=live |via=histoire.du.metre.free.fr |trans-quote=''The Law of 18 Germinal [month], Year 3: Decision to draw the fundamental unit metre on a platinum ruler. Nomenclature of "republican measures". Resumption of the triangulation''}}</ref>{{efn|{{lang|fr|"Art. 8. Dans les poids et mesures de capacité, chacune des mesures décimales de ces deux genres aura son double et sa moitié, afin de donner à la vente des divers objets toute la commodité que l'on peut désirer. Il y aura donc le double-litre et le demi-litre, le double-hectogramme et le demi-hectogramme, et ainsi des autres."}} ("Art. 8. In the weights and measures of capacity, each of the decimal measures of these two kinds will have its double and its half, in order to give to the sale of the various articles all the convenience that one can desire. There will therefore be the double-litre and the half-litre, the double-hectogram and the half-hectogram, and so on.")<ref name="fr"/> }} but were not retained when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.

Other metric prefixes used historically include hebdo- ({{10^|7}}) and micri- ({{10^|−14}}).

=== Double prefixes=== Double prefixes have been used in the past, such as ''micromillimetres'' or ''millimicrons'' (now nanometres), ''micromicrofarads'' (μμF; now picofarads, pF), ''kilomegatonnes'' (now gigatonnes), ''hectokilometres'' (now 100 kilometres) and the derived adjective ''hectokilometric'' (typically used for qualifying the fuel consumption measures).<ref name="Rowlett_2008_Millimicro">{{Cite dictionary |year=2008 |title=millimicro- |dictionary=How Many? A dictionary of units of measurement |publisher=University of North Carolina at Chapel Hill |url=http://www.unc.edu/~rowlett/units/dictM.html |access-date=29 August 2016 |last=Rowlett |first=Russ |orig-year=2000 |url-status=live |archive-url=https://web.archive.org/web/20160829225351/https://www.unc.edu/~rowlett/units/dictM.html |archive-date=29 August 2016}}</ref> These are not compatible with the SI.

Other obsolete double prefixes included "decimilli-" ({{10^|−4}}), which was contracted to "dimi-"<ref>{{Cite book |last=Danloux-Dumesnils |first=Maurice |url=https://books.google.com/books?id=ElHAAAAAIAAJ |title=The Metric System: A critical study of its principles and practice |date=1969 |publisher=The Athlone Press |isbn=9780485120134 |page=34 |access-date=9 October 2015}} (a translation of the French original {{lang|fr|Esprit et bon usage du système métrique}}, 1965 )</ref> and standardised in France up to 1961.

There are no more letters of the Latin alphabet available for new prefixes (all the unused letters are already used for units). As such, Richard J. C. Brown (who proposed the prefixes adopted for {{10^|±27}} and {{10^|±30}}) has proposed a reintroduction of compound prefixes (e.g. ''kiloquetta-'' for {{10^|33}}) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always be ''quetta-'' or ''quecto-''. This usage has not been approved by the BIPM.<ref>{{Cite journal |last=Brown |first=Richard J. C. |date=27 April 2022 |title=Reply to 'Facing a shortage of the Latin letters for the prospective new SI symbols: Alternative proposal for the new SI prefixes' |journal=Accreditation and Quality Assurance |volume=27 |issue=3 |pages=143–144 |doi=10.1007/s00769-022-01499-7 |s2cid=248397680}}</ref><ref>{{Cite journal |last1=Brown |first1=Richard J.;C. |year=2019 |title=Considerations on compound SI prefixes |journal=Measurement |volume=140 |pages=237–239 |bibcode=2019Meas..140..237B |doi=10.1016/j.measurement.2019.04.024 |s2cid=146092009}}</ref>

==Similar symbols and abbreviations== In written English, the symbol ''K'' is often used informally to indicate a multiple of thousand in many contexts. For example, one may talk of a ''40K salary'' ({{val|40,000}}), or call the Year 2000 problem the ''Y2K problem''. In these cases, an uppercase K is often used with an implied unit (although it could then be confused with the symbol for the kelvin temperature unit if the context is unclear). This informal postfix is read or spoken as "thousand", "grand", or just "k".

The financial and general news media mostly use m or M, b or B, and t or T as abbreviations for million, billion (109) and trillion (1012), respectively, for large quantities, typically currency<ref>{{Cite news |date=13 February 2012 |title=Obama unveils $3.8T budget proposal |url=https://www.cbc.ca/news/business/obama-unveils-3-8t-budget-proposal-1.1283997 |access-date=1 March 2012 |publisher=Canadian Broadcasting Corporation |agency=Associated Press}}</ref> and population.<ref>{{Cite web |title=More than 65M Flock to Discovery's Planet Earth |url=http://www.multichannel.com/article/128853-More_than_65M_Flock_to_Discovery_s_Planet_Earth.php |access-date=1 March 2012 |publisher=Multichannel.com}}</ref>

The medical and automotive fields in the United States use the abbreviations ''cc'' or ''ccm'' for cubic centimetres. One cubic centimetre is equal to one millilitre.

For nearly a century,{{vague|date=December 2024}} engineers used the abbreviation ''MCM'' to designate a "thousand circular mils" in specifying the cross-sectional area of large electrical cables. Since the mid-1990s, ''kcmil'' has been adopted as the official designation of a thousand circular mils, but the designation ''MCM'' still remains in wide use. A similar system is used in natural gas sales in the United States: ''m'' (or ''M'') for thousands and ''mm'' (or ''MM'') for millions (thousand thousands) of British thermal units or therms, and in the oil industry,<ref name="pesa">{{Cite web |last=Purcell |first=P. |year=2007 |title=Disambiguating M |url=http://www.pesa.com.au/publications/pesa_news/june_july_07/pesanews_8830.html |url-status=dead |archive-url=https://web.archive.org/web/20120325192835/http://www.pesa.com.au/publications/pesa_news/june_july_07/pesanews_8830.html |archive-date=2012-03-25 |access-date=1 March 2012 |work=PESA News |issue=88}}</ref> where ''MMbbl'' is the symbol for "millions of barrels". These usages of the capital letter ''M'' for "thousand" in MCM is from Roman numerals, in which ''M'' means 1000.<ref>{{Cite web |date=4 August 2015 |title=What Is The Difference Between MCM and Kcmil? |url=https://www.reference.com/home-garden/difference-between-mcm-kcmil-32b016f3e6b497b6 |access-date=5 September 2016 |website=Reference}}</ref><ref name=pesa />

==Typography==

===Micro symbol=== {{main|Micro-}}

When the prefixes ''mega-'' and ''micro-'' were adopted in 1873, it was necessary to use a symbol other than upper and lowercase M to distinguish the prefixes. Eventually the Greek letter mu (μ) was adopted.

With the lack of a {{angbr|μ}} key on most typewriters, as well as computer keyboards, various other abbreviations remained common, including "mc", "mic", "M", and "u".

From about 1960 onwards, "u" prevailed in type-written documents.{{efn|Sometimes the symbol 'u' is marked by adding a downstroke using a pen or pencil, or a slash '/u'.}} Because ASCII, EBCDIC, and other common encodings lacked code-points for {{angbr|μ}}, this tradition remained even as computers replaced typewriters.

The standard ISO 8859-1 included the {{angbr|μ}} symbol for ''micro-'' at the codepoint {{code|0xB5}}. The whole of ISO 8859-1 was incorporated into Unicode, which lists the Greek letter at {{U+|03BC}}. Many fonts that support both characters (micro sign and lowercase mu) render them identically, but because the micro sign and the Greek lower-case letter have different applications ({{code|0xB5}} is not used in Greek text), some fonts render them differently, e.g. Linux Libertine and Segoe UI.{{cn|date=October 2024}}

===Keyboard entry=== Most English-language keyboards do not have a {{angbr|μ}} key, so it is necessary to use a key-code; this varies depending on the operating system, physical keyboard layout, and user's language.

On Microsoft Windows systems:

* Arbitrary Unicode codepoints can be entered in decimal with: {{key press|Alt}} sustained, {{key press|0|1|8|1|chain= }}, and releasing {{key press|Alt}}. A leading "0" is required (this registers as the corresponding Unicode hexadecimal code-point, {{code|0xB5}} is equal to {{code|181}}), or * Arbitrary Unicode codepoints can be entered in hexadecimal as: {{key press|Alt|+|b|5}} (up to 5 hexadecimal characters, not counting the leading '+', upper or lower case), or * In the tradition of MS-DOS, IBM code page 437 one can also enter old code-points in decimal: {{key press|Alt|2|3|0|chain= }} (the leading zero must be omitted);

On MacOS systems, code-point U+00B5 can be entered as either {{key press|Opt|m}} or {{key press|Opt|Y}}.

On Linux systems:

* Under X11, when a Compose key has been enabled: {{key press|Compose|m|u}} * Under X11, with ''ibus'' version 1.5.19 (or higher) active, and a non-composing input method selected: The default keybinding for starting codepoint input is {{key press|Ctrl|Shift|u}}. The key sequence {{key press|Ctrl|Shift|u}} {{key press|b|5|space|chain= }} then produces the micro sign (U+00B5). * On the VGA console's virtual terminals, arbitrary Unicode codepoints can be entered in decimal as: {{key press|Alt}} sustained, {{key press|1|8|1|chain= }}, and releasing {{key press|Alt}}. A leading "0" is not required. * The code-point U+00B5 can be entered as {{key press|right Alt|m}} (provided the right alt key is configured to act as AltGr).

===Typesetting in LaTeX=== The LaTeX typesetting system features an ''SIunitx'' package in which the units of measurement are spelled out, for example, <code>\qty{3}{\tera\hertz}</code> formats as "3 THz".<ref>{{citation |last=Kottwitz |first=Stefan |title=LaTeX Cookbook |date=2015-10-28 |pages=158–9 |publisher=Packt Publishing Ltd |isbn=978-1-78439-630-5}}</ref>

==See also== * {{anli|Binary prefix}} * CJK Compatibility – Unicode block including precomposed unit symbols with prefixes, such as {{Unichar|3371}} * {{anli|E1 series (preferred numbers)}} * {{anli|Engineering notation}} * {{anli|Indian numbering system}} * {{anli|International vocabulary of metrology}} * {{anli|ISO/IEC 80000}} * {{anli|Numeral prefix}} * {{anli|Order of magnitude}} * {{anli|Orders of magnitude (data)}} * {{anli|RKM code}} * {{anli|Unified Code for Units of Measure}}

==Footnotes== {{notelist}}

==References== {{reflist|30em}}

==External links== * [https://www.bipm.org/en/about-us/ International Bureau of Weights and Measures] (BIPM) * [https://www.bipm.org/en/measurement-units/si-prefixes SI prefixes at BIPM] * [http://physics.nist.gov/cuu/Units/prefixes.html US NIST ''Definitions of the SI units: The twenty SI prefixes''] * [http://physics.nist.gov/cuu/Units/binary.html US NIST ''Definitions of the SI units: The binary prefixes'']

Category:Metric prefixes Category:Numeral systems