For use of measurement as a form of social power, seemetric power.
Ametric prefix is aunit prefix that precedes a basic unit of measure to indicate amultiple or submultiple of the unit. All metric prefixes used today aredecadic. Each prefix has a unique symbol that is prepended to any unit symbol. The prefixkilo, for example, may be added togram to indicatemultiplication by one thousand: onekilogram is equal to one thousand grams. The prefixmilli, likewise, may be added tometre to indicatedivision by one thousand; one millimetre is equal to one thousandth of a metre.
Decimal multiplicative prefixes have been a feature of all forms of themetric 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. TheSI prefixes are metric prefixes that were standardised for use in theInternational System of Units (SI) by theInternational Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 2022.[1][2] Since 2009, they have formed part of theISO/IEC 80000 standard. They are also used in theUnified Code for Units of Measure (UCUM).
"Femto", "Ronna-", "Yocto", "Yotta", and "Zepto" redirect here. For the character, seeGriffith (Berserk). For the name, seeRonna. For the Linux Foundation open-source project, seeYocto Project. For the prize-linked savings account, seeYotta Technologies. For the Finnish musician, seeYotto. For similar terms, seeYota (disambiguation). For the Indian Q-commerce company, seeZepto (company).
^Prefixes adopted before 1960 already existed before SI. The introduction of theCGS system was in 1873.
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 1947IUPAC 14th International Conference of Chemistry[5] before being officially adopted for the first time in 1960.[6]
The most recent prefixes adopted wereronna,quetta,ronto, andquecto in 2022, after a proposal from British metrologist Richard J. C. Brown (since before 2022, Q/q and R/r were the only Latin letters available for abbreviations, all other Latin letters are either already used for other prefixes (a,c,d,E,f,G,h,k,M,m,n,P,p,T,Y,y,Z,z) or already used forSI 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) or easily confused with mathematical operators (I and l are easily confused with 1, O and o are easily confused with 0, X and x are easily confused with ×)). The large prefixesronna andquetta 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.[7]
The prefixes frompeta toquetta 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 letterh has been removed from some of these stems and the initial lettersz,y,r, andq have been added, ascending in reverse alphabetical order, to avoid confusion with other metric prefixes.
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 forkilo is k.) Except for the early prefixes ofkilo,hecto, anddeca, the symbols for the prefixes for multiples are uppercase letters, and those for the prefixes for submultiples are lowercase letters.[8]
All of the metric prefix symbols are made from upper- and lower-caseLatin letters except for the symbol formicro, which is uniquely a Greek letterμ.[a]
The prefix symbols are always prepended to the symbol for the unit without any intervening space or punctuation.[9] 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 prefixesdeci- andcenti-, and less frequentlyhecto anddeca, 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".[10] These prefixes are also commonly used to create metric units corresponding to older conventional units, for examplehectares andhectopascals.
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 befactored 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.
Whenmega andmicro were adopted in 1873, three prefixes existed starting with "m". It was necessary to use a symbol other than upper and lowercase 'm'. Eventually the Greek letter "μ" was adopted.
With the lack of a "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.[c] BecauseASCII,EBCDIC, and other common encodings lacked code-points for "μ", this tradition remained even as computers replaced typewriters.
WhenISO 8859-1 was created, it included the "μ" symbol formicro at codepoint0xB5; later, the whole of ISO 8859-1 was incorporated into the initial version ofUnicode. Many fonts that support both characters render them identically, but because the micro sign and the Greek lower-case letter have different applications (normally, a Greek letter would be used with other Greek letters, but the micro sign is never used like that), some fonts render them differently, e.g.Linux Libertine andSegoe UI.[citation needed]
Most English-language keyboards do not have a "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.
arbitrary Unicode codepoints can be entered in decimal with:Alt sustained,0181, and releasingAlt. A leading "0" is required (this registers as the corresponding Unicode hexadecimal code-point, 0xB5 = 181.), or
arbitrary Unicode codepoints can be entered in hexadecimal as:Alt++b5 (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:Alt+230 (the leading zero must be omitted);
under X11, when aCompose key has been enabled:Composemu
under X11, withibus version 1.5.19 (or higher) active, and a non-composing input method selected: The default keybinding for starting codepoint input isCtrl+⇧ Shift+u. The key sequenceCtrl+⇧ Shift+ub5space then produces U+00B5, the micro sign.
on the VGA console's virtual terminals like tty1: arbitrary Unicode codepoints can be entered in decimal as:Alt sustained,181, and releasingAlt. A leading "0" is not required.
TheLaTeX typesetting system features anSIunitx package in which the units of measurement are spelled out, for example, \qty{3}{\tera\hertz} formats as "3 THz".[13]
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.[citation needed] 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.[14]
The unitskilogram,gram,milligram, microgram, and smaller are commonly used for measurement ofmass. However, megagram, gigagram, and larger are rarely used;tonnes (and kilotonnes, megatonnes, etc.) orscientific notation are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".
Thelitre (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, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres).[citation needed]
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 purposes, the cubic metre is usually used.[citation needed]
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 namemicron, which is officially deprecated. In some fields, such aschemistry, theångström (0.1 nm) has been used commonly instead of the nanometre. Thefemtometre, used mainly in particle physics, is sometimes called afermi. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as thesolar radius,astronomical units,light years, andparsecs; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.[citation needed]
Prefixes for the SI standard unitsecond are most commonly encountered for quantities less than one second. For larger quantities, the system ofminutes (60 seconds),hours (60 minutes) anddays (24 hours) isaccepted for use with the SI and more commonly used. When speaking of spans of time, the length of the day is usually standardised to86400 seconds so as not to create issues with the irregularleap second.[citation needed]
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 inastronomy, theJulian year orannum (a) is a standardised variant of theyear, equal to exactly31557600 seconds (365+ 1 /4 days). The unit is so named because it was the average length of a year in theJulian calendar. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum (Ma) orgigaannum (Ga).[citation needed]
Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states:[17] "Prefix symbols may be used with the unit symbol °C and prefix names may be used with the unit namedegree Celsius. For example, 12 m°C (12 millidegrees Celsius) is acceptable." In practice, it is more common for prefixes to be used with thekelvin 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).[citation needed]
In use thejoule and kilojoule are common, with larger multiples seen in limited contexts. In addition, thekilowatt-hour, a composite unit formed from thekilowatt and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour).[citation needed]
Several definitions exist for the non-SI unitcalorie. Distinguished are gram calories and kilogram calories. One kilogram calorie, which equals one thousand gram calories, often appears capitalized and without a prefix (i.e.Cal) when referring to "dietary calories" in food.[18] It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1 kcal = 1000 cal = 1 Cal.
Metric prefixes are widely used outside the metric SI system. Common examples include themegabyte and thedecibel. Metric prefixes rarely appear withimperial orUS 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, theyear, with symbol 'a' (from the Latinannus), is commonly used with metric prefixes:ka, Ma, and Ga.[19]
Official policies about the use of SI prefixes with non-SI units vary slightly between theInternational Bureau of Weights and Measures (BIPM) and the AmericanNational 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)",[17] whereas the BIPM adds information about the use of prefixes with the symbolas 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."[20]
Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.[21][22][23] The decimal prefix for ten thousand,myria- (sometimes speltmyrio-), and the earlybinary prefixesdouble- (2×) anddemi- (1/2×) were parts of the original metric system adopted by France in 1795,[24][d]but were not retained when the SI prefixes were internationally adopted by the 11thCGPM conference in 1960.
Other metric prefixes used historically includehebdo- (107) andmicri- (10−14).
Double prefixes have been used in the past, such asmicromillimetres ormillimicrons (nownanometres),micromicrofarads (μμF; nowpicofarads, pF),kilomegatonnes (nowgigatonnes),hectokilometres (now 100 kilometres) and the derived adjectivehectokilometric (typically used for qualifying the fuel consumption measures).[25] These are not compatible with the SI.
Other obsolete double prefixes included "decimilli-" (10−4), which was contracted to "dimi-"[26] 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 1033) if a driver for prefixes at such scales ever materialises, with a restriction that the last prefix must always bequetta- orquecto-. This usage has not been approved by the BIPM.[27][28]
In written English, the symbolK is often used informally to indicate a multiple of thousand in many contexts. For example, one may talk of a40K salary (40000), or call theYear 2000 problem theY2K 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[29] and population.[30]
For nearly a century[clarification needed], engineers used the abbreviationMCM to designate a "thousandcircular mils" in specifying the cross-sectional area of largeelectrical cables. Since the mid-1990s,kcmil has been adopted as the official designation of a thousand circular mils, but the designationMCM still remains in wide use. A similar system is used in natural gas sales in the United States:m (orM) for thousands andmm (orMM) for millions (thousand thousands) ofBritish thermal units ortherms, and in the oil industry,[31] whereMMbbl is the symbol for "millions of barrels". These usages of the capital letterM for "thousand" in MCM is fromRoman numerals, in whichM means 1000.[32][31]
Binary prefix – Symbol placed before units of digital information to indicate multiplication by a power of two
CJK Compatibility – Unicode block (U+3300-33FF) containing various characters composed into ideographic squares, and embedding Japanese Katakana words, or digits and ideographs for dates, or symbols with Latin letters for measurement unitsPages displaying wikidata descriptions as a fallback
E1 series (preferred numbers) – Series of preferred values for passive electrical componentsPages displaying short descriptions of redirect targets
Engineering notation – A version of scientific notation in which the exponent of ten reflects powers of a thousand
International vocabulary of metrology – Committee under the chairmanship of the director of the BIPMPages displaying wikidata descriptions as a fallback
ISO/IEC 80000 – International standard on physical quantities and units of measurement
Numeral prefix – Prefix derived from numerals or other numbers
^ForASCII compatibility in general text usage,μ is frequently substituted with the Latin letteru.
^me = 9.1093837139(28)×10−31 kg[11]. Converting to grams gives9.1093837015×10−28 g. Rounding to the nearest power of ten gives1×10−27 g, or 1 rg.
^Sometimes the symbol 'u' is marked by adding a downstroke using a pen or pencil, or a slash '/u'.
^"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.
^Conn, Carole; Kravitz, Len."Remarkable Calorie". University of New Mexico. Retrieved2017-05-22.
^Gargaud, Muriel; Amils, Ricardo; Cleaves, Henderson James (26 May 2011). "Ga".Encyclopedia of Astrobiology. Springer Science & Business Media. p. 621.ISBN978-3-642-11271-3.
^Brewster, David (1830).The Edinburgh Encyclopædia. Vol. 12. Edinburgh, UK: William Blackwood, John Waugh, John Murray, Baldwin & Cradock, J.M. Richardson. p. 494. Retrieved2015-10-09.
^Brewster, David (1832).The Edinburgh Encyclopaedia. Vol. 12 (1st American ed.). Joseph and Edward Parker. Retrieved2015-10-09.
^"La loi du 18 Germinal an 3".L'histoire du mètre [The History of the Metre] (in French).Archived from the original on 2022-11-26. Retrieved2015-10-12 – via histoire.du.metre.free.fr.Décision de tracer le mètre, unité fondamentale, sur une règle de platine. Nomenclature des « mesures républicaines ». Reprise de la triangulation [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]
^Brown, Richard J.C. (2022-04-27). "Reply to "Facing a shortage of the Latin letters for the prospective new SI symbols: Alternative proposal for the new SI prefixes"".Accreditation and Quality Assurance.27 (3):143–144.doi:10.1007/s00769-022-01499-7.S2CID248397680.
