A metric prefix is a unit prefix that precedes a basic unit of measure to indicate a multiple or fraction of the unit. While all metric prefixes in common use today are decadic, historically there have been a number of binary metric prefixes as well.^{[1]} Each prefix has a unique symbol that is prepended to the 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 standardized for use in the International System of Units (SI) by the International Bureau of Weights and Measures (BIPM) in resolutions dating from 1960 to 1991.^{[2]} Since 2009, they have formed part of the International System of Quantities.
The BIPM specifies twenty prefixes for the International System of Units (SI).

Each prefix name has a symbol that is used in combination with the symbols for units of measure. For example, the symbol for 'kilo' is 'k', and is used to produce 'km', 'kg', and 'kW', which are the SI symbols for kilometre, kilogram, and kilowatt, respectively. Where the Greek letter 'μ' is unavailable, the symbol for micro 'µ' may be used. Where both variants are unavailable, the micro prefix is commonly written as the lowercase Latin letter 'u'.
Prefixes corresponding to an integer power of one thousand are generally preferred. Hence '100 m' is preferred over '1 hm' (hectometre) or '10 dam' (decametres). The prefixes hecto, deca, deci, and centi are commonly used for everyday purposes, and the centimetre (cm) is especially common. However, 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".^{[3]}
Prefixes may not be used in combination. This also applies to mass, for which the SI base unit (kilogram) already contains a prefix. For example, milligram (mg) is used instead of microkilogram (μkg).
In the arithmetic of measurements having units, the units are treated as multiplicative factors to values. If they have prefixes, all but one of the prefixes must be expanded to their numeric multiplier, except when combining values with identical units. Hence,
When powers of units occur, for example, squared or cubed, the multiplication prefix must be considered part of the unit, and thus included in the exponentiation.
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. The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidynes and milligauss). Metric prefixes may also be used with nonmetric units.
The choice of prefixes with a given unit is usually dictated by convenience of use. Unit prefixes for amounts that are much larger or smaller than those actually encountered are seldom used.
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. Megagram and teragram are occasionally used to disambiguate the metric tonne from other units with the name 'ton'.
The kilogram is the only base unit of the International System of Units that includes a metric prefix.
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 packaged products such as wine and the decilitre is less frequently. The latter two items include prefixes corresponding to an exponent that is not divisible by three.
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.
The kilometre, metre, centimetre, millimetre, and smaller are common. (However, the decimetre is rarely used.) The micrometre is often referred to by the nonSI term micron. In some fields, such as chemistry, the ångström (equal to 0.1 nm) historically competed with 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, nonmetric units are used, such as astronomical units, light years, and parsecs; the astronomical unit is mentioned in the SI standards as an accepted nonSI unit.
The second, millisecond, microsecond, and shorter are common. The kilosecond and megasecond also have some use, though for these and longer times one usually uses either scientific notation or minutes, hours, and so on.
The SI unit of angle is the radian, but degrees, minutes and seconds see some scientific use.
Official policy also varies from common practice for the degree Celsius (°C). NIST states:^{[4]} "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).
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).
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 capitalized and without a prefix (i.e. 'Cal') when referring to "dietary calories" in food.^{[5]} 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 system of metric units. 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 or 'kip'). They are also used with other specialized units used in particular fields (e.g., megaelectronvolt, gigaparsec, millibarn). They are also occasionally used with currency units (e.g., gigadollar), mainly by people who are familiar with the prefixes from scientific usage. In geology and paleontology, the year, with symbol a (from the Latin annus), is commonly used with metric prefixes: ka, Ma, and Ga.
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)",^{[4]} 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."^{[6]}
When an SI prefix is affixed to a root word, the prefix carries the stress, while the root drops its stress but retains a full vowel in the syllable that is stressed when the root word stands alone. For example, kilobyte is /ˈkɪlɒbaɪt/, with stress on the first syllable. However, words in common use outside the scientific community may follow idiosyncratic stress rules. In English speaking countries, kilometre is often pronounced /kɪˈlɒmɪtər/, with reduced vowels on both syllables of metre.
The prefix giga is usually pronounced in English as /ˈɡɪɡə/, with hard ⟨g⟩ as in 'get', but sometimes /ˈdʒɪɡə/, with soft ⟨g⟩ as in 'gin'.
The LaTeX typesetting system features an SIunitx package in which the units of measurement are spelled out, for example, \SI{3}{\tera\hertz}
formats as "3 THz".
Some of the prefixes formerly used in the metric system have fallen into disuse and were not adopted into the SI.^{[7]}^{[8]}^{[9]} The decimal prefix myria (sometimes also written as myrio) (ten thousand) as well as the binary prefixes double and demi, denoting a factor of 2 and 1/2 (one half), respectively, were parts of the original metric system adopted by France in 1795.^{[1]} These 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 (now picofarads), kilomegatons (now gigatons), hectokilometres (now 100 kilometres) and the derived adjective hectokilometric (typically used for qualifying the fuel consumption measures).^{[10]} These are not compatible with the SI.
Other obsolete double prefixes included "decimilli" (10^{−4}), which was contracted to "dimi"^{[11]} and standardized in France up to 1961.
Although the yotta prefix is large, the field of computer data is likely to approach values reaching or exceeding yottabytes in the future. Thus, various proposals for a prefix beyond yotta have been put forth.
In 2010, UC Davis student Austin Sendek started a petition to designate "hella" as the SI prefix for one octillion (short scale; long scale: quadrilliard; 10^{27}).^{[12]} The petition gathered over 60,000 supporters by circulating through Facebook and media coverage.^{[13]} It was also adopted by Wolfram Alpha.^{[14]}
Brian C. Lacki^{[15]} follows Z and Y with the adopted prefixes X, W and V to mean 10^{27}, 10^{30} and 10^{33} respectively, thus continuing the inverse alphabetical order.
A proposal made to the BIPM is ronna (R) for 10^{27}, quecca (Q) for 10^{30}, ronto (r) for 10^{−27} and quecto (q) for 10^{−30}.^{[16]}
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", but never "kilo" (despite that being the origin of the letter).
The financial and general news media mostly use m/M, b/B and t/T as abbreviations for million, billion (10^{9}) and trillion (10^{12}), respectively, for large quantities, typically currency^{[17]} and population.^{[18]}
The medical and automotive fields in the United States use the abbreviations "cc" or "ccm" for cubic centimetres. 1 cubic centimetre is equivalent to 1 millilitre.
For nearly a century, the electrical construction industry used the abbreviation "MCM" to designate a "thousand circular mils" in specifying thicknesses 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,^{[19]} 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 1,000.^{[20]}
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). This practice was once sanctioned by some industry associations, including JEDEC. The International Electrotechnical Commission (IEC) standardized the system of binary prefixes (kibi, mebi, gibi, etc.) for this purpose.^{[21]}^{[Note 1]}
This article is based on material taken from the Free Online Dictionary of Computing prior to 1 November 2008 and incorporated under the "relicensing" terms of the GFDL, version 1.3 or later.
Atto (symbol a) is a unit prefix in the metric system denoting a factor of 10−18 or 0.000000000000000001.
The unit multiple was adopted at the 12th General Conference on Weights and Measures (CGPM) in Resolution 8. It is derived from the Danish word atten, meaning "eighteen".
Examples of its use:
Weight of the HIV1 virus is about 1×10−18 kg or 1×10−15 g, which can be written as 1 fg or 1000 ag.
More examples available.
CentiCenti (symbol c) is a unit prefix in the metric system denoting a factor of one hundredth. Proposed in 1793 and adopted in 1795, the prefix comes from the Latin centum, meaning "hundred". Since 1960, the prefix is part of the International System of Units (SI). It is mainly used in combination with metre to form centimetre, a common unit of length.
Example:
A honey bee is about 1.3 centimetres long.
DeciDeci (symbol d) is a decimal unit prefix in the metric system denoting a factor of one tenth. Proposed in 1793 and adopted in 1795, the prefix comes from the Latin decimus, meaning "tenth". Since 1960, the prefix is part of the International System of Units (SI). Its most frequent use, however, is in a nonSI unit, the decibel, used to measure sound intensity (relative to a reference) and many other ratios.
Deciliter or dl is common in recipes. Many European homes have a deciliter measure for flour, water etc.
Example:
A compact disc is about 12 centimeters or 1.2 decimeters.
ExaExa is a decimal unit prefix in the metric system denoting 1018 or 1000000000000000000. It was added as an SI prefix to the International System of Units (SI) in 1975, and has the unit symbol E.
Exa comes from the Ancient Greek ἕξ héx, used as a prefix ἑξά hexá, meaning six (like hexa), because it is equal to 10006.
Examples:
The total storage needed by Google Mail as of April 2012, ignoring backups and compression, is more than an exabyte (10,240 megabytes of storage per user multiplied by an estimated 260 million users).
1 EeV = 1018 electronvolts = 0.1602 joule
United States electric energy consumption is about 15 exajoule per year.
1 exasecond is approximately 32 billion years
1 exametre is approximately 110 light years
0.43 Es ≈ the approximate age of the Universe
1.6 Em—172 ± 12.5 light years—Diameter of Omega Centauri (one of the largest known globular clusters, perhaps containing over a million stars)
23.6 exahashes/s is the calculation rate of the Bitcoin network ≈ 23600000000000000000 hashes per second (Mar 2018)
FemtoFemto (symbol f) is a unit prefix in the metric system denoting a factor of 10−15 or 0.000000000000001. Adopted by the 11th General Conference on Weights and Measures, it was added in 1964 to the SI. It is derived from the Danish word femten, meaning "fifteen".
Examples of use:
The HIV1 virus has the mass of about 1 x 10−15 g or 1 fg. Orders of magnitude (mass)
a proton has a diameter of about 1.6 to 1.7 femtometres.
More examples available.The femtometre shares the unit symbol (fm) with the older nonSI unit fermi, to which it is equivalent. The fermi, named in honour of Enrico Fermi, is often encountered in nuclear physics.
GigaGiga ( or ) is a unit prefix in the metric system denoting a factor of a (shortform) billion (109 or 1000000000). It has the symbol G.
Giga is derived from the Greek word γίγας, meaning "giant". The Oxford English Dictionary reports the earliest written use of giga in this sense to be in the Reports of the IUPAC 14th Conference in 1947: "The following prefixes to abbreviations for the names of units should be used: G giga 109×".When referring to information units in computing, such as gigabyte, giga may sometimes mean 1073741824 (230), although such use is inconsistent, contrary to standards and has been discouraged by the standards organizations. The inconsistency is that gigabit is never (or very rarely) used with the binary interpretation of the prefix, while gigabyte is sometimes used this way. The binary prefix gibi has been adopted for 230, while reserving giga exclusively for the metric definition.
HectoHecto (symbol: h) is a decimal unit prefix in the metric system denoting a factor of one hundred. It was adopted as a multiplier in 1795, and comes from the Greek ἑκατόν hekaton, meaning "hundred". In 19th century English it was sometimes spelled hecato, in line with a puristic opinion by Thomas Young. Its unit symbol as an SI prefix in the International System of Units (SI) is the lower case letter h.
It is rarely used except in certain specific applications:
hectopascal (hPa), in meteorology, for atmospheric pressure, the modern equivalent of the traditional millibar.
hectolitre (hl or hL), in agriculture, for liquids (notably wine and milk) and bulk commodities (e.g., grain).
hectogram (hg), in agronomy, for quantities of animal feed (hectogram/animal) and for measures of agricultural productivity (hectogram/hectare); also used in Italy abbreviated as etto, and in Canada, New Zealand and Sweden simply as 100 g, for retail sale of cold cuts and meat.
hectometre (hm), in radio astronomy, occasionally used to indicate a radio band by wavelength
hectare (ha, or 100 ares), in surveying, as a measure of land area equal to one square hectometre, 1 (hm)2 = 10,000 m2
KiloKilo is a decimal unit prefix in the metric system denoting multiplication by one thousand (103). It is used in the International System of Units where it has the unit symbol k, in lower case.
The prefix kilo is derived from the Greek word χίλιοι (chilioi), meaning "thousand". It was originally adopted by Antoine Lavoisier's research group in 1795, and introduced into the metric system in France with its establishment in 1799.
In 19th century English it was sometimes spelled chilio, in line with a puristic opinion by Thomas Young
MegaMega is a unit prefix in metric systems of units denoting a factor of one million (106 or 1000000). It has the unit symbol M. It was confirmed for use in the International System of Units (SI) in 1960. Mega comes from Ancient Greek: μέγας, translit. megas, lit. 'great'.
MicroMicro (Greek letter μ or legacy micro symbol µ) is a unit prefix in the metric system denoting a factor of 10−6 (one millionth). Confirmed in 1960, the prefix comes from the Greek μικρός (mikrós), meaning "small".
The symbol for the prefix comes from the Greek letter μ (mu). It is the only SI prefix which uses a character not from the Latin alphabet. "mc" is commonly used as a prefix when the character "μ" is not available; for example, "mcg" commonly denotes a microgram. Also the letter u instead of μ is allowed by one of the ISO documents.
Examples:
Typical bacteria are 1 to 10 micrometres in diameter.
Eukaryotic cells are typically 10 to 100 micrometres in diameter.
MilliMilli (symbol m) is a unit prefix in the metric system denoting a factor of one thousandth (10−3). Proposed in 1793 and adopted in 1795, the prefix comes from the Latin mille, meaning "one thousand" (the Latin plural is milia). Since 1960, the prefix is part of the International System of Units (SI).
NanoNano (symbol n) is a unit prefix meaning "one billionth". Used primarily with the metric system, this prefix denotes a factor of 10−9 or 0.000000001. It is frequently encountered in science and electronics for prefixing units of time and length.
Examples:
One nanometer is about the length that a fingernail grows in one second.
Three gold atoms lined up are about one nanometer long.
If a toy marble were scaled down to one nanometer wide, Earth would scale to about one meter (3.3 feet) wide.
One nanosecond is about the time required for light to travel 30 cm in air, or 20 cm in an optical fiber.The prefix derives from the Greek νᾶνος (Latin nanus), meaning "dwarf". The General Conference on Weights and Measures (CGPM) officially endorsed the usage of nano as a standard prefix in 1960.
When used as a prefix for something other than a unit of measure (as for example in words like "nanoscience"), nano refers to nanotechnology, or means "on a scale of nanometres". See nanoscopic scale.
PetaPeta () is a decimal unit prefix in the metric system denoting multiplication by 1015 (1000000000000000). It was adopted as an SI prefix in the International System of Units in 1975, and has the symbol P.
Peta is derived from the Greek πέντε, meaning "five". It denotes the fifth power of 1000 (10005). It is similar to the prefix penta ("five"), but without the letter n (on the analogy of the prefix tera [from the Greek for "monster"] for 10004 looking like tetra ["four"] with a letter missing).
Examples:
1 petametre = 1015 metres
1 petasecond = 1015 seconds (31.7 million years)
1 petahertz = 1015 cycle per second. Visible light is around 0.5 PHz.
1 petabyte = 1015 bytes
the mass–energy equivalence is 89.9 PJ/kg
1 lightyear = 9.461 Pm
PicoPico (symbol p) is a unit prefix in the metric system denoting one trillionth, a factor of 10−12 (0.000000000001).
Derived from the Spanish pico, meaning peak, beak, bit, this was one of the original 12 prefixes defined in 1960 when the International System of Units was established.The radius of atoms range from 25 picometers (hydrogen) to 260 picometers (caesium). One picolightyear is about nine kilometers (six miles).
TeraTera is a unit prefix in the metric system denoting multiplication by 1012 or 1000000000000 (one trillion short scale; one billion long scale). It has the symbol T. Tera is derived from Greek word τέρας teras, meaning "monster". The unit prefix was confirmed for use in the International System of Units (SI) in 1960.
Examples of its use:
terahertz radiation: electromagnetic waves within the band of frequencies from 0.3 to 3 THz. Visible light is around 500 THz.
terabit and terabyte, units used in data storage.
teragram: equal to 109 kg. The Great Pyramid of Giza has a mass of about 6 Tg.
terasecond: approximately 31,558 years
teralitre: equal to 109 m3. Lake Zurich contains about 4 TL of water.
terawatt: used to measure total human energy consumption. In 2010 it was 16 TW (TJ/s).
terametre (= 1,000,000,000 km): Light travels 1.079 Tm in one hour.
YottaYotta is the largest decimal unit prefix in the metric system, denoting a factor of 1024 or 1000000000000000000000000; that is, one million million million million, or one septillion. It has the unit symbol Y. The prefix name is derived from the Ancient Greek οκτώ (októ), meaning "eight", because it is equal to 1,0008. It was added as an SI prefix to the International System of Units (SI) in 1991.Usage examples:
The mass of the Earth is 5,972.6 Yg.
The mass of the oceans is about 1.4 Yg.
The total power output of the Sun is approximately 385 YW.
The observable universe is estimated to be 880 Ym in diameter.
One yottabyte (YB) is a unit of digital information or information storage capacity that contains one septillion bytes or 1,000 zettabytes. The yobibyte (YiB) is a related unit that uses a binary prefix, and means 1,0248 bytes, which is approximately 1.2 septillion bytes.
ZeptoZepto (unit symbol z) is a unit prefix in the metric system denoting a factor of 10−21 or 0.000000000000000000001.
Adopted in 1991, it comes from the Latin septem, meaning "seven", since it is equal to 1000−7.
Examples of its use:
The electric charge on one electron: 160.217657 zeptocoulombs (zC).
One zeptomole (zmol) of substance contains 602 particles.
The human insulin molecule has a mass of roughly 10 zeptograms.
ZettaZetta is a decimal unit prefix in the metric system denoting a factor of 1021 or 1000000000000000000000. The prefix was added as an SI prefix to the International System of Units (SI) in 1991 and has the symbol Z.
Zetta is evocative of the Latin numeral septem, meaning "seven". Therefore it was used to denote the seventh power of 1000.A prefix of the same value, hepta, was informally introduced a few years before the promulgation of zetta. It was formed from the Greek ἑπτά (hepta), also meaning "seven". Hepta never received official sanction and is obsolete. Zeta (ζήτα), although the 6th letter of the modern and classical Greek alphabet, denotes a value of seven (7) when used as a numeral.
Examples:
The mass of Earth's atmosphere is approximately 5 zettagrams (Zg).
The volume of seawater in the Earth's oceans is approximately 1.369 zettalitres (ZL).
The Avogadro constant is 602.2 Zmol−1.
Estimates of the diameter of the Milky Way Galaxy are between 0.9 and 1.7 zettametres (Zm).
A zettabyte (ZB) is a quantity of information or information storage capacity equal to 1021 bytes, 10007 bytes, 1,000 exabytes or 1 sextillion bytes. The zebibyte (ZiB) is a related unit that uses a binary prefix, and means 10247 bytes.
Base units  

Derived units with special names  
Other accepted units  
See also 
 

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