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 decadic. 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 multiplication by one thousand: one kilogram is equal to one thousand grams. The prefix milli, likewise, may be added to metre to indicate division by one thousand; 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 nonmetric 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.^{[1]}^{[2]} 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).
The BIPM specifies twentyfour prefixes for the International System of Units (SI).
Prefix  Base 10  Decimal  Adoption ^{[nb 1]}  

Name  Symbol  
quetta  Q  10^{30}  1000000000000000000000000000000  2022^{[3]} 
ronna  R  10^{27}  1000000000000000000000000000  2022 
yotta  Y  10^{24}  1000000000000000000000000  1991 
zetta  Z  10^{21}  1000000000000000000000  1991 
exa  E  10^{18}  1000000000000000000  1975 
peta  P  10^{15}  1000000000000000  1975 
tera  T  10^{12}  1000000000000  1960 
giga  G  10^{9}  1000000000  1960 
mega  M  10^{6}  1000000  1873 
kilo  k  10^{3}  1000  1795 
hecto  h  10^{2}  100  1795 
deca  da  10^{1}  10  1795 
10^{0}  1  —  
deci  d  10^{−1}  0.1  1795 
centi  c  10^{−2}  0.01  1795 
milli  m  10^{−3}  0.001  1795 
micro  μ  10^{−6}  0.000001  1873 
nano  n  10^{−9}  0.000000001  1960 
pico  p  10^{−12}  0.000000000001  1960 
femto  f  10^{−15}  0.000000000000001  1964 
atto  a  10^{−18}  0.000000000000000001  1964 
zepto  z  10^{−21}  0.000000000000000000001  1991 
yocto  y  10^{−24}  0.000000000000000000000001  1991 
ronto  r  10^{−27}  0.000000000000000000000000001  2022 
quecto  q  10^{−30}  0.000000000000000000000000000001  2022 

First uses of prefixes in SI date back to definition of kilogram after the French Revolution at the end of the 18th century. Several more prefixes came into use by the 1947 IUPAC 14th International Conference of Chemistry^{[4]} before being officially adopted for the first time in 1960.^{[5]}
The most recent prefixes adopted were ronna, quetta, ronto, and quecto in 2022, after a proposal from British metrologist Richard J. C. Brown. The large prefixes ronna and quetta were adopted in anticipation of needs from data science, and because unofficial prefixes that did not meet SI requirements were already circulating. The small prefixes were added as well even without such a driver in order to maintain symmetry.^{[6]}
When mega and micro were adopted in 1873, there were then three prefixes starting with "m", so it was necessary to use some other symbol besides upper and lowercase 'm'. Eventually the Greek letter "µ" was adopted.
However various other abbreviations remained common, including "mic", "mm", and "u", in part motivated by the lack of a "µ" key on most typewriters.
From about 1960 onwards, "u" prevailed in typewritten documents.^{[11]} Because ASCII, EBCDIC, and other common encodings lacked codepoints for "µ", this tradition remained even as computers replaced typewriters.
When ISO 88591 was created, it included the "µ" symbol for micro at codepoint 0xB5
.
The whole of ISO 88591 was incorporated into the initial version of Unicode, but subsequently Unicode version 6 deprecated the micro symbol on codepoint U+00b5
in favour of the Greek letter "μ" on codepoint U+03bc
.
Most keyboards do not have a "µ" key, so it is necessary to use a keychord; this varies depending on the operating system, physical keyboard layout, and user's language.
The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example,
\qty{3}{\tera\hertz}
formats as "3 THz".^{[12]}
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 nonmetric units, but not, for example, with the nonSI units of time.^{[13]}
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".^{[citation needed]}
The kilogram is the only coherent unit of the International System of Units that includes a metric prefix.^{[14]}^{: 144 }
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, 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 nonSI name micron. 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 nonmetric units are used, such as the solar radius, astronomical units, light years, and parsecs; the astronomical unit is mentioned in the SI standards as an accepted nonSI unit.^{[citation needed]}
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 86400 seconds so as not to create issues with the irregular leap 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 longscale scientific work, particularly in astronomy, the Julian year or annum is a standardised variant of the year, equal to exactly 31557600 seconds (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 or gigaannum.^{[citation needed]}
The SI unit of angle is the radian, but degrees, as well as arcminutes and arcseconds, see some scientific use.^{[citation needed]}
Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states:^{[15]} "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 in units of MK (megakelvins), and molecular cooling may be described in mK (millikelvins).^{[citation needed]}
In use the joule and kilojoule are common, with larger multiples seen in limited contexts. In addition, the kilowatthour, 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. terawatthour).^{[citation needed]}
There exist a number of definitions for the nonSI unit, the calorie. There 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.^{[16]} 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 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.^{[17]}
Official policies about the use of SI prefixes with nonSI 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 timerelated unit symbols (names) min (minute), h (hour), d (day); nor with the anglerelated symbols (names) ° (degree), ′ (minute), and ″ (second),^{[15]} 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."^{[18]}
Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.^{[19]}^{[20]}^{[21]} The decimal prefix for ten thousand, myria (sometimes spelled myrio), and the early binary prefixes double (2×) and demi (1/2×) were parts of the original metric system adopted by France in 1795,^{[22]} 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 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).^{[23]} These are not compatible with the SI.
Other obsolete double prefixes included "decimilli" (10^{−4}), which was contracted to "dimi"^{[24]} 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 is not currently approved by the BIPM.^{[25]}^{[26]}
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 (40000), 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" or "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 (10^{9}) and trillion (10^{12}), respectively, for large quantities, typically currency^{[27]} and population.^{[28]}
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, engineers used the abbreviation MCM to designate a "thousand circular mils" in specifying the crosssectional area of large electrical cables. Since the mid1990s, 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 of British thermal units or therms, and in the oil industry,^{[29]} where MMbbl is the symbol for "millions of barrels". This usage of the capital letter M for "thousand" is from Roman numerals, in which M means 1000.^{[30]}
The original metric system adopted by France in 1795 included the two binary prefixes double (2×) and demi (1/2×).^{[22]} However, they were not retained when the SI prefixes were internationally adopted by the 11th CGPM conference in 1960.
In some fields of information technology, it has been common to designate nondecimal multiples based on powers of 1024, rather than 1000, for some SI prefixes (kilo, mega, giga), contrary to the definitions in the International System of Units (SI). The SI does not permit the metric prefixes to be used in this conflicting sense.^{[31]} This practice was once sanctioned by some industry associations, including JEDEC. The International Electrotechnical Commission (IEC) standardised the system of binary prefixes (kibi, mebi, gibi, etc.) for this purpose.^{[32]}^{[b]}
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 doublelitre et le demilitre, le doublehectogramme et le demihectogramme, 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 doublelitre and the halflitre, the doublehectogram and the halfhectogram, and so on.]