This article needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2008)

An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol) to form a decimal multiple or submultiple. The abbreviation SI is from the French language name Système International d’Unités (also known as International System of Units). SI prefixes are used to reduce the quantity of zeroes in numerical equivalencies. For example, one-billionth of an ampere (a small electrical current) can be written as 0.000 000 001 ampere. In symbol form, this is written as 0.000 000 001 A. Using an SI prefix, these are equivalent to 1 nanoampere or 1 nA. The SI prefixes are governed by the Bureau International des Poids et Mesures (BIPM, also known as the International Bureau of Weights and Measures) and are the product of four resolutions dating from 1960 to 1991. Image File history File links Question_book-3. ... For other uses, see Decimal (disambiguation). ... A multiple of a number is the product of that number with any integer. ... “SI” redirects here. ... In linguistics, a prefix is a type of affix that precedes the morphemes to which it can attach. ... The International Bureau of Weights and Measures is the English name of the Bureau international des poids et mesures (BIPM, often written in English Bureau International des Poids et Mesures), a standards organisation, one of the three organizations established to maintain the International System of Units (SI) under the terms...

List of SI prefixes

The twenty SI prefixes are shown in the chart below.

The long and short scales are two different numerical systems used throughout the world: Short scale is the English translation of the French term échelle courte. ... The long and short scales are two different numerical systems used throughout the world: Short scale is the English translation of the French term échelle courte. ... For other uses, see Decimal (disambiguation). ... Look up si, Si, SI in Wiktionary, the free dictionary. ... Yotta (symbol Y) is a SI prefix in the SI system of units denoting 1024 or 1 000 000 000 000 000 000 000 000. ... Main article: Names of large numbers A septillion is a number written as either: a 1 followed by 24 zeros (10 to the 24th power, as used in the short scale system of numeration. ... This list compares various sizes of positive numbers, including counts of things, dimensionless numbers and probabilities. ... Zetta (symbol Z) is a SI prefix in the SI system of units denoting 1021 or 1 000 000 000 000 000 000 000. ... Main article: Names of large numbers A sextillion is a number written as either: a 1 followed by 21 zeros (10 to the 21st power, as used in the short scale system of numeration. ... In long scale usage: one trilliard = 1,000,000,000,000,000,000,000 = 1021 one thousand trilliards =one (long scale) quadrillion This word is not found in standard English dictionaries. ... Exa (symbol E) is a prefix in the SI system of units denoting 1018 or 1 000 000 000 000 000 000. ... Main article: Names of large numbers A quintillion is a number written as either: a 1 followed by 18 zeros (10 to the 18th power, as used in the short scale system of numeration. ... Trillion may mean either of the two numbers (see long and short scales for more detail): 1,000,000,000,000 (one million million; ) - increasingly common meaning in English language usage. ... This article describes the SI prefix peta. ... This list compares various sizes of positive numbers, including counts of things, dimensionless numbers and probabilities. ... In some countries and languages using the long scale: one billiard = = one short scale quadrillion. ... Tera (symbol: T) is a prefix in the SI system of units denoting 1012, or 1 000 000 000 000. ... Trillion may mean either of the two numbers (see long and short scales for more detail): 1,000,000,000,000 (one million million; ) - increasingly common meaning in English language usage. ... Look up billion in Wiktionary, the free dictionary. ... Look up giga- in Wiktionary, the free dictionary. ... Look up billion in Wiktionary, the free dictionary. ... Milliard is a French-derived word meaning the number 1,000,000,000 (109; one thousand million; SI prefix giga). ... Mega (symbol M) is a SI prefix in the SI system of units denoting a factor of 106, i. ... One million (1,000,000), or one thousand thousand, is the natural number following 999,999 and preceding 1,000,001. ... Kilo (symbol: k) is a prefix in the SI system denoting 103 or 1,000. ... Look up one thousand in Wiktionary, the free dictionary. ... Hecto (symbol h) is a SI prefix in the SI system of units denoting a factor of 102 (100). ... 100 (one hundred) (the Roman numeral is C for centum) is the natural number following 99 and preceding 101. ... This article is about the SI prefix. ... This article is about the number 10. ... This article is about the number one. ... Deci- is a Latin numerical prefix meaning 10. ... Centi (symbol c) is a SI prefix in the SI system of units denoting a factor of 10-2, or 1/100. ... Milli (symbol m) is a SI prefix in the SI system of units denoting a factor of 10-3, or 1/1,000. ... Micro is a SI prefix in the SI system of units denoting a factor of 10−6 (one millionth). ... This article describes the SI prefix. ... For other meanings of Pico see Pico (disambiguation) Pico (symbol p) is a prefix in the SI system of units denoting a factor of 10-12. ... Femto is a prefix (see all prefixes) to a unit and means that it is 10-15 times this unit, or, one quindecillionth (European) or one quadrillionth (American). ... Atto- (symbol a) is an SI prefix to a unit and means that it is 10-18 times this unit. ... Zepto (symbol z) is a prefix in the SI system of units denoting a factor of 10-21. ... Yocto (symbol y) is a SI prefix in the SI system of units denoting a factor of 10-24. ... The Conférence générale des poids et mesures (General Conference on Weights and Measures or CGPM) is one of the three organizations established to maintain the SI system under the terms of the Metre Convention (1875). ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ...

Usage

General use of prefix names and symbols

Twenty SI prefixes are available to combine with units of measure. For example, the prefix name kilo denotes a multiple of one thousand, so 1 kilometre equals 1000 metres, 1 kilogram equals 1000 grams, 1 kilowatt equals 1000 watts, and so on. Each SI prefix name has an associated symbol which can be used in combination with the symbols for units of measure. Thus, the “kilo” symbol, k, can be used to produce km, kg, and kW, (kilometre, kilogram, and kilowatt). SI prefixes are internationally recognized and also exist outside the SI (many of them long pre-date SI, going back to the original introduction of the metric system); prefixes may also be used in combination with non-SI units; for example: milligauss (mG), kilofoot (kft) and microinch (µin). “km” redirects here. ... This article is about the unit of length. ... Kg redirects here. ... BIC pen cap, about 1 gram. ... For other uses, see Watt (disambiguation). ...

Prohibition of multiple prefixes

The kilogram is the only SI base unit that has an SI prefix as part of its unit name and symbol. Because multiple prefixes may not be used (such as microkilogram or µkg), the prefixes are used with the unit gram and its symbol g (e.g. milligram or mg).

SI prefixes with symbols for time and angles

Officially endorsed policies of the BIPM and the [American] National Institute of Standards and Technology (NIST) vary slightly with respect to the use of the SI prefixes—both between their respective agencies and from real-world practice. For instance, the NIST advises that “…to avoid confusion, prefix symbols (and prefixes) 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).” The BIPM’s position on the use of SI prefixes with units of time larger than the second is the same as that of the NIST but their position with regard to angles differs: they state “However astronomers use milliarcsecond, which they denote mas, and microarcsecond, µas, which they use as units for measuring very small angles.” Motto: (traditional) In God We Trust (official, 1956–present) Anthem: The Star-Spangled Banner Capital Washington, D.C. Largest city New York City Official language(s) None at the federal level; English de facto Government Federal Republic  - President George W. Bush (R)  - Vice President Dick Cheney (R) Independence - Declared - Recognized... NIST logo The National Institute of Standards and Technology (NIST, formerly known as The National Bureau of Standards) is a non-regulatory agency of the United States Department of Commerce’s Technology Administration. ...

SI prefixes with °C

A similar difference between officially endorsed policy and actual practice exists with regard to the symbol for degree Celsius (°C). The NIST states “Prefix symbols may be used with the unit symbol °C and prefixes may be used with the unit name ‘degree Celsius.’ For example, 12 m°C (12 millidegrees Celsius) is acceptable.” Notwithstanding this official endorsement, the practice of using prefixed forms of “°C” (such as “µ°C”) has not been well-adopted in science and engineering; prefixed forms of the kelvin are usually used instead. For other uses, see Kelvin (disambiguation). ...

Details

See also: Non-SI unit prefixes

Examples: There exist several unit prefixes used like the SI prefixes, but that are not part of the SI system. ...

• 5 cm = 5 × 10⁻² m = 5 × 0.01 m = 0.05 m
• 3 MW = 3 × 10⁶ W = 3 × 1 000 000 W = 3 000 000 W

The prefix always takes precedence over any exponentiation; thus "km²" means square kilometre and not kilo–square metre. For example, 3 km² is equal to 3 000 000 m² and not to 3000 m² (nor to 9 000 000 m²). Thus the SI prefixes provide steps of a factor one million instead of one thousand in the case of an exponent 2, of a thousand million in the case of an exponent 3, etc. As a result large numbers may be needed, even if the prefixes are fully used, or intermediate units (like the litre) are introduced. The litre or liter (see spelling differences) is a unit of volume. ...

Prefixes where the exponent is divisible by three are often recommended. Hence "100 m" rather than "1 hm". In mathematics, exponentiation is a process generalized from repeated multiplication, in much the same way that multiplication is a process generalized from repeated addition. ...

The obsolete prefixes such as myrio- and myria- were dropped before SI was adopted in 1960, probably because they did not fit this pattern, no one-letter symbol was available (M, m, and µ already being used; the two-letter symbols mo and ma were used instead) and were rarely used anyway. There exist several unit prefixes used like the SI prefixes, but that are not part of the SI system. ... Myria is an obsolete prefix in the SI system of units that denoted a factor of 10+4 (ten thousand, 10,000). ...

The prefix kilo derives from the Greek word χίλια (khilia or chilia) = thousand.

Double prefixes such as those formerly used in micromicrofarads (picofarads), hectokilometres (100 kilometres), and millimicrons or micromillimetres (both nanometres) were also dropped with the introduction of the SI. Examples of various types of capacitors. ...

Though in principle legal, many combinations of prefixes with quantities are rarely used. In most contexts only a few, i.e. the most common, standard combination are established:

• Mass: hectogram, gram, milligram, microgram, and smaller are common. However, megagram or larger are rarely used; tonnes or scientific notation are used instead. Megagram is sometimes used to disambiguate the (metric) tonne from the various (non-metric) tons.
• Volume in litres: litre, decilitre, centilitre, millilitre, microlitre, and smaller are common. Larger volumes are sometimes denoted in hectolitres; otherwise in cubic metres or cubic kilometres. In Australia, large quantities of water are measured in kilolitres, megalitres and gigalitres.
• Length: kilometre, metre, decimetre, centimetre, millimetre, and smaller are common. The micrometre is often referred to by the non-SI term micron. In some fields such as chemistry, the angstrom (equal to 0.1 nm) competes with the nanometre. The femtometre, used mainly in particle physics, is usually called a fermi. At large scales, megametre, gigametre, and larger are rarely used. Often used are astronomical units, light years, and parsecs; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.
• Time: 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 United Kingdom, Ireland, Australia and New Zealand previously used the long scale number name conventions, but have now at least partly switched to the short scale usage. In particular, above a million and below a millionth, the same name has different values in the two naming systems, so billion and trillion (for example) have unfortunately become potentially ambiguous terms internationally. Using the SI prefixes can circumvent this problem. Scientific notation, also known as standard form, is a notation for writing numbers that is often used by scientists and mathematicians to make it easier to write large and small numbers. ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ... For other uses, see Chemistry (disambiguation). ... An angstrom, angström, or Ã¥ngström (symbol Ã…) is a unit of length. ... Femtometre (American spelling: femtometer) is an SI measure of length that is equal to 10−15 (femto) of a metre. ... The fermi is a non-SI unit of length that is internationally recognised and equivalent to the SI-recognised femtometre. ... The astronomical unit (AU or au or a. ... A light-year or lightyear (symbol: ly) is a unit of measurement of length, specifically the distance light travels in vacuum in one year. ... A parsec is the distance from the Earth to an astronomical object which has a parallax angle of one arcsecond. ... The long and short scales are two different numerical systems used throughout the world: Short scale is the English translation of the French term échelle courte. ... The long and short scales are two different numerical systems used throughout the world: Short scale is the English translation of the French term échelle courte. ...

Pronunciation

There are two accepted pronunciations for the prefix giga-: [ˈgɪgə] and [ˈdʒɪgə]. According to the American writer Self, in the 1920s a German committee member of the International Electrotechnical Commission proposed giga- as a prefix for 10⁹, drawing on a verse by the humorous poet Christian Morgenstern that appeared in the third (1908) edition of Galgenlieder (Gallows Songs). This suggests a hard German g was originally intended as the pronunciation. Self was unable to ascertain at what point the /dʒ/ (soft g) pronunciation became accepted, but as of 1995 current practice had returned to /g/ (hard g).^{[1] [2]} The International Electrotechnical Commission (IEC) is an international standards organization dealing with electrical, electronic and related technologies. ...

When any SI prefix is affixed to a root word, the prefix carries the primary stress, and the root word carries a secondary stress on the same syllable that is stressed when the root word stands alone. For example, the pronunciation and stress of gigabyte is /ˈɡɪɡəbaɪt/. However, when a word with an SI prefix is also commonly used outside the scientific community, it may adopt other pronunciations that do not follow this rule. For example, kilometre (or in the USA, kilometer) may also be pronounced /kɨˈlɑmɨtɚ/. In linguistics, stress is the relative emphasis that may be given to certain syllables in a word. ... This article is about the unit of measurement. ...

Use outside SI

The symbol "K" is often used informally to mean a multiple of (a) thousand, so one may talk of "a 40K salary" (40 000), or the Y2K problem. In these cases an uppercase K is often used, although using an uppercase K is never correct when writing under the rules of the SI. Also, it is often used as a prefix to designate the binary prefix kilo = 2¹⁰ = 1024, although this is now non-standard. This article is about the millennial computer glitch. ... // In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten (such as computer memory sizes). ...

Non-SI units

• Prefixes go back to the introduction of the metric system in the 1790s, long before the SI was introduced in 1960. The prefixes (including those introduced after the introduction of SI) are used with any metric units, SI or not (e.g. millidynes).
• SI prefixes rarely appear coupled with imperial units or English units except in some specialised cases (e.g. microinches, kilofeet, kilopound or 'kip').
• They are also used with other specialized units used in particular fields (e.g. megaelectronvolts, gigaparsecs).
• They are also occasionally used with currency units (e.g., gigadollar), mainly by people who are familiar with the prefixes from scientific usage.

The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ... This article is about post-1824 imperial units, see also English unit, U.S. customary units or Avoirdupois. ... English unit is the American name for a unit in one of a number of systems of units of measurement, some obsolete, and some still in use. ...

Computing

Main article: Binary prefix

The prefixes k and greater are common in computing, where they are applied to information and storage units like the bit and the byte. Since 2¹⁰ = 1024, and 10³ = 1000, this led to the SI prefix letters being used to denote "binary" powers. Although these are incorrect usages according to the SI standards it seems common to apply base 10 prefixes, when relating to computer memory, as follows: // In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten (such as computer memory sizes). ... For the formal concept of computation, see computation. ... This article is about the unit of information. ... For the computer industry magazine, see Byte (magazine). ...

k

= 2¹⁰ = 1 024

M

= 2²⁰ = 1 048 576

G

= 2³⁰ = 1 073 741 824

T

= 2⁴⁰ = 1 099 511 627 776

P

= 2⁵⁰ = 1 125 899 906 842 624

These prefixes, however, usually retain their powers-of-1000 meanings when used to describe either disk storage or rates of data transmission (bit rates): 10 Mbit/s Ethernet runs at 10,000,000 bit/s, not 10,485,760 bit/s. The confusion is compounded by the fact that the units of information (the bit and the byte) are not part of SI, where the bit, byte, octet, baud or symbol rate would rather be given in hertz. Although some use "bit" for the bit and "b" for the byte, "b" is often used for bit and "B" for byte instead. It is recommended by several standards bodies to use bit and B to keep the units very distinct, as in kbit or MiB.^{[citations needed]} French-speakers often use "o" for "octet", nowadays a synonym for byte. In telecommunications and computing, bit rate (sometimes written bitrate) is the frequency at which bits are passing a given (physical or metaphorical) point. It is quantified using the bit per second (bit/s) unit. ... A megabit per second (mbps or mbit/s) is a unit of data transmission equal to 1,000 kilobits per second or 1,000,000 bits per second. ... Ethernet is a large, diverse family of frame-based computer networking technologies that operate at many speeds for local area networks (LANs). ... This article is about the unit of information. ... For the computer industry magazine, see Byte (magazine). ... In computing, an octet is a grouping of eight bits. ... For the town in France, see Baud, Morbihan. ... In digital communications, the symbol rate is the bit rate divided by the number of bits transmitted in each symbol. ... This article is about the SI unit of frequency. ... This article is about the unit of information. ... For the computer industry magazine, see Byte (magazine). ...

Consequently, the International Electrotechnical Commission (IEC) adopted new binary prefixes in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol plus 'i'. So now, one kilobyte (1 kB) equals 1000 bytes, whereas one kibibyte (1 KiB) equals 2¹⁰ = 1024 bytes. Likewise mebi (Mi; 2²⁰), gibi (Gi; 2³⁰), tebi (Ti; 2⁴⁰), pebi (Pi; 2⁵⁰), and exbi (Ei; 2⁶⁰). Although the IEC standard does not mention them, the sequence can be readily extended to zebi (Zi; 2⁷⁰) and yobi (Yi; 2⁸⁰). The practical use of these binary prefixes is growing only very slowly and is largely limited to expert literature. They remain mostly ignored by marketing literature. The International Electrotechnical Commission (IEC) is an international standards organization dealing with electrical, electronic and related technologies. ... // In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten (such as computer memory sizes). ... A kilobyte (derived from the SI prefix kilo-, meaning 1,000) is a unit of information or computer storage equal to either 1,000 bytes or 1,024 bytes (210), depending on context. ... A kibibyte (a contraction of kilo binary byte) is a unit of information or computer storage, commonly abbreviated KiB (never kiB). 1 kibibyte = 210 bytes = 1,024 bytes The kibibyte is closely related to the kilobyte, which can be used either as a synonym for kibibyte or to refer to... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ... In computing, binary prefixes are often used to quantify large numbers where powers of two are more useful than powers of ten. ...

Proposed changes

Extension

Continuing backwards in the alphabet, after zetta and yotta, proposals for the next large number include xenta and xona (among others), the latter as an alteration of the Latin-derived numerical prefix nona-, and the next small number would also start with an ‘x’.^{[citation needed]} For other uses, see Latin (disambiguation). ... A numerical prefix is a prefix that denotes a number, which is usually a multiplier for the thing being prefixed. ...

One proposed extension is, after zetta and yotta, xona, weka, vunda, uda, and treda.^[3]

Preserving the rule on abbreviating the prefixes (a Latin capital for the large number and a lower-case letter for the small number), even without consensus on the full name the following prefix symbols could be used without ambiguity: ‘X’, ‘W’, ‘V’, ‘x’, ‘w’, ‘v’. The logically next small prefix symbol, ‘u’, was formerly the accepted substitution for ‘µ’ (now withdrawn), the symbol for “micro”.^[4][5] Abcdefghijklmnopqrstuvwxyz redirects here. ...

Another proposal for xenta/xona is novetta, from the Italian nove.^{[citation needed]} This does not have the convenience of backward alphabetic order. Also, the symbol 'n' is already in use for "nano".

Harmonisation

There are also proposals for further harmonisation of the capitalisation. Therefore the symbols for kilo, hecto, and deka would be changed from ‘k’ to ‘K’, from ‘h’ to ‘H’, and from ‘da’ to ‘D’. Likewise some lobby for the removal of prefixes that do not fit the 10^±3n scheme, namely hecto, deka, deci, and centi. The CGPM has postponed its decision on both matters for now. The Conférence générale des poids et mesures (General Conference on Weights and Measures or CGPM) is one of the three organizations established to maintain the SI system under the terms of the Metre Convention (1875). ...

An unsolved (and maybe unsolvable) issue is the application of prefixes to units with exponents other than ±1. The prefix is always applied before the exponent. This eventually led to the introduction of special units for area and volume without exponents in the original metric system: The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ...

• 1 are (a) = 100 m² (10 m × 10 m = 1 dam × 1 dam = 1 dam²)
  • ⇒ 1 ca = 1 m² (1 m × 1 m)
  • ⇒ 1 ha = 10 000 m² (100 m × 100 m = 1 hm × 1 hm = 1 hm²)
• 1 stere (st) = 1 m³
• 1 litre (l or L) = 1 dm³ = 1 mst = 0.001 m³

Of these the litre and the hectare are the most ubiquitous in common use: Litre designations are sometimes used to differentiate a volume of liquid (as opposed to a gas, or solid which are usually designated as cubic volumes). Hectares are widely used as a metric alternative to the acre (approximately 2.5 acres to the hectare). An are (symbol a) is a unit of area, equal to 100 square meters (10 m × 10 m), used for measuring land area. ... The cubic metre (symbol m³) is the SI derived unit of volume. ... The litre or liter (see spelling differences) is a unit of volume. ...

See also

// In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten (such as computer memory sizes). ... Engineering notation is scientific notation in which the powers of ten are limited to those where the exponent is a multiple of three, i. ... The International System of Units (symbol: SI) (for the French phrase Syst me International dUnit s) is the most widely used system of units. ... There exist several unit prefixes used like the SI prefixes, but that are not part of the SI system. ... Different cultures have different traditional numeral systems used for writing numbers and for naming large numbers. ... The following is a table of the numbers 0 through 10 in a sample of the languages and writings of the world. ... An order of magnitude is the class of scale or magnitude of any amount, where each class contains values of a fixed ratio to the class preceding it. ... Scientific notation, also known as standard form, is a notation for writing numbers that is often used by scientists and mathematicians to make it easier to write large and small numbers. ... The SI system of units defines seven SI base units: physical units defined by an operational definition. ... SI derived units are part of the SI system of measurement units and are derived from the seven SI base units. ...

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. This article does not cite any references or sources. ... “GFDL” redirects here. ...

Year 1983 (MCMLXXXIII) was a common year starting on Saturday (link displays the 1983 Gregorian calendar). ... is the 135th day of the year (136th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 207th day of the year (208th in leap years) in the Gregorian calendar. ...

External links

Standards organisations

• The International Bureau of Weights and Measures (BIPM): SI prefixes
• US NIST "Definitions of the SI units: The twenty SI prefixes"
• US NIST "Definitions of the SI units: The binary prefixes"
• International Standard ISO 2955: "Information processing - Representation of SI and other units in Systems with limited Character sets"

Other proposals

• Proposal for an extension of the SI-prefix system to even larger and smaller units