SI prefix 1

SI prefix

SI prefixes in everyday use

Text Symbol Factor

tera T 1,000,000,000,000

giga G 1,000,000,000

mega M 1,000,000

kilo k 1,000

hecto h 100

(none) (none) 1

centi c 0.01

milli m 0.001

micro μ 0.00000 1

The International System of Units (SI) specifies a set of unit prefixes known as SI prefixes or metric prefixes. AnSI prefix is a name that precedes a basic unit of measure to indicate a decadic multiple or fraction of the unit. Eachprefix has a unique symbol that is prepended to the unit symbol. The SI prefixes are standardized by the InternationalBureau of Weights and Measures in resolutions dating from 1960 to 1991.[1] Their usage is not limited to SI unitsand many of these date back to the introduction of the metric system in the 1790s.SI prefixes are used to reduce the number of zeros shown in numerical quantities before or after a decimal point. Forexample, an electrical current of 0.000000001ampere, or one-billionth (short scale) of an ampere, is written by usingthe SI-prefix nano as 1nanoampere or 1nA.

List of SI prefixesThe International System of Units specifies twenty SI prefixes:

SI prefixes

Prefix Symbol 1000m 10n Decimal Short scale Long scale Since[2]

yotta Y 10008 1024 1000000000000000000000000 Septillion Quadrillion 1991

zetta Z 10007 1021 1000000000000000000000 Sextillion Trilliard 1991

exa E 10006 1018 1000000000000000000 Quintillion Trillion 1975

peta P 10005 1015 1000000000000000 Quadrillion Billiard 1975

tera T 10004 1012 1000000000000 Trillion Billion 1960

giga G 10003 109 1000000000 Billion Milliard 1960

mega M 10002 106 1000000 Million 1960

kilo k 10001 103 1000 Thousand 1795

hecto h 10002/3 102 100 Hundred 1795

SI prefix 2

deca da 10001/3 101 10 Ten 1795

10000 100 1 One –

deci d 1000−1/3 10−1 0.1 Tenth 1795

centi c 1000−2/3 10−2 0.01 Hundredth 1795

milli m 1000−1 10−3 0.001 Thousandth 1795

micro μ 1000−2 10−6 0.000001 Millionth 1960

nano n 1000−3 10−9 0.000000001 Billionth Milliardth 1960

pico p 1000−4 10−12 0.000000000001 Trillionth Billionth 1960

femto f 1000−5 10−15 0.000000000000001 Quadrillionth Billiardth 1964

atto a 1000−6 10−18 0.000000000000000001 Quintillionth Trillionth 1964

zepto z 1000−7 10−21 0.000000000000000000001 Sextillionth Trilliardth 1991

yocto y 1000−8 10−24 0.000000000000000000000001 Septillionth Quadrillionth 1991

Examples• 5 cm = 5×10−2 m = 5×0.01m = 0.05m• 3 MW = 3×106 W = 3×1000000W = 3000000W

General use of prefix names and symbolsTwenty SI prefixes are available to combine with units of measure. For example, the prefix kilo- denotes a multipleof 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 forunits of measure. Thus, the "kilo-" symbol, k, can be used to produce km, kg, and kW, (kilometre, kilogram, andkilowatt). 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-SIunits; for example: milligauss (mG), kilofoot (kft) and microinch (µin).Prefixes may not be used in combination. This even applies for mass, for which the SI base unit (which is thekilogram, not the gram) already contains a prefix. So milligram (mg) is used instead of microkilogram (µkg), forexample.Prefixed values cannot be multiplied or divided together, and they have to be converted into non-prefixed standardform for such calculations. For example, 5 mV × 5 mA ≠ 25 mW. The correct calculation is: 5 mV × 5 mA = 5 ×10−3 V × 5 × 10−3 A = 25 x 10−6 W = 25 µW = 0.025 mW.Prefixes corresponding to an exponent that is divisible by three are often recommended. Hence "100 m" rather than"1 hm" (hectometre) or "10 dam" (decametres). The "non-three" prefixes (hecto-, deca-, deci-, and centi-) arehowever more commonly used for everyday purposes than in science.

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SI prefixes with symbols for time and anglesOfficial policies about the use of these prefixes vary slightly between the Bureau International des Poids et Mesures(BIPM) and the American National Institute of Standards and Technology (NIST); and some of the policies of bothbodies are at variance with everyday practice. For instance, the NIST advises that "…to avoid confusion, prefixsymbols (and prefixes) are not used with the time-related unit symbols (names) min (minute), h (hour), d (day); norwith the angle-related symbols (names) ° (degree), ′ (minute), and ″ (second)." [1] The BIPM’s position on the use ofSI prefixes with units of time larger than the second is the same as that of the NIST but their position with regard toangles differs: they state "However astronomers use milliarcsecond, which they denote mas, and microarcsecond,µas, which they use as units for measuring very small angles." [2]

SI prefixes for temperature in °COfficial policy also varies from common practice for the degree Celsius (°C). NIST states (http:/ / physics. nist. gov/Pubs/ SP811/ sec06. html); "Prefix symbols may be used with the unit symbol °C and prefixes may be used with theunit name 'degree Celsius'. For example, 12 m°C (12 millidegrees Celsius) is acceptable."

Exponentiation of symbolsWhen units occur in exponentiation, for example, in square and cubic forms, any size prefix is considered part of theunit, and thus included in the exponentiation.• 1km2 means one square kilometre or the size of a square of 1000 m by 1000 m and not 1000 square metres.• 2Mm3 means two cubic megametre or the size of two cubes of 1000000m by 1000000m by 1000000m or 2×1018

m3, and not 2000000cubic metres (2×106 m3).

PronunciationThere are two accepted pronunciations for the prefix giga-: /ˈɡɪɡə/ and /ˈdʒɪɡə/. According to the American writerKevin Self, in the 1920s a German committee member of the International Electrotechnical Commission proposedgiga- 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 thepronunciation. Self was unable to ascertain at what point the /dʒ/ (soft g) pronunciation became accepted, but as of1995 current practice had returned to /ɡ/ (hard g). [3] [4]

When an SI prefix is affixed to a root word, the prefix carries the stress, while the root drops its stress but retains afull vowel in the syllable that is stressed when the root word stands alone. For example, gigabyte is /ˈɡɪɡəbaɪt/, withstress on the first syllable. However, words in common use outside the scientific community may followidiosyncratic stress rules. Kilometre is commonly pronounced /kɨˈlɒmɨtər/, with reduced vowels on both syllables ofmetre.

Disallowed and obsolete prefixesThe prefix myria- 'ten thousand' [5] [6] denoting a factor of 10000, originated from the Greek μύριοι (mýrioi) for tenthousand, and the prefixes demi and double, denoting a factors of 1/2 and 2, respectively,[7] were parts of the originalmetric system adopted by France in 1795. These were not retained when the SI prefixes were internationally adoptedby the 11th CGPM conference in 1960. The binary prefixes were dropped because they were neither decimal norsymmetrical.Double prefixes such as those formerly used in micromicrofarads (picofarads), hectokilometres (100 kilometres), and millimicrons or micromillimetres (both nanometres) were disallowed with the introduction of the SI. The choice of commonly used prefixes with a given unit is usually dictated by convenience of use, unit prefixes that are much

SI prefix 4

larger or smaller than encountered in practice, are seldom used, albeit valid combinations. In most contexts only afew, the most common, standard combination are established:• Mass: kilogram, hectogram, gram, milligram, microgram, and smaller are common. However, megagram or larger

are rarely used; tonnes (and kilotonnes etc.) or scientific notation are used instead. Megagram is occasionally usedto disambiguate the (metric) tonne from the various (non-metric) tons.

• Volume in litres: litre, decilitre, centilitre, millilitre, microlitre, and smaller are common. Larger volumes aresometimes denoted in hectolitres; otherwise in cubic metres or cubic kilometres. In Australia, large quantities ofwater are measured in kilolitres, megalitres and gigalitres.

• Length: kilometre, metre, decimetre, centimetre, millimetre, and smaller are common. The micrometre is oftenreferred to by the non-SI term micron. In some fields such as chemistry, the angstrom (equal to 0.1 nm) competeswith the nanometre. The femtometre, used mainly in particle physics, is usually called a fermi. For large scales,megametre, gigametre, and larger are rarely used. Often used are astronomical units, light years, and parsecs; theastronomical 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 havesome use, though for these and longer times one usually uses either scientific notation or minutes, hours, and soon.

Non-SI units• The use of prefixes can be traced 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 the SI, are used withany metric units, whether officially included in the SI or not (e.g., millidynes).

• SI prefixes rarely appear with imperial units or English units except in some special 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.

Similar symbols in abbreviationsThe symbol K is often used informally to mean a multiple of thousand in many contexts. For example, one may talkof a 40K salary (40 000), or call the Year 2000 problem as Y2K problem. In these cases an uppercase K is often used,although the uppercase K is the official symbol of the kelvin.In other financial and business contexts, the letter M is often used to denote multiplication by 1000, in recognition ofthe Latin term mille (meaning one thousand), also used in roman numerals. In these situations one million is oftenwritten as 1 MM. In other financial situations, one M can mean million, such as £2M or £2m which is equal to £2000 000. Similar usage of M occurs in the term CPM (Cost per mille) used in advertising.For nearly a century, the electrical construction industry used the acronym "MCM" to designate a "thousand circularmils" in specifying thicknesses of large electrical cables. Since the mid-1990s, the term "kcmil" has been adopted asthe "official" designation of a thousand circular mils, but the designation "MCM" still remains in wide use. A similarsystem is used in Natural Gas sales in the United States: m (or M) for thousands and mm (or MM) for millions ofBtus or Therms.

SI prefix 5

Units used in computing and telecommunicationsThe International System of Units does not define units of information, such as the storage size units bit and byte.This has allowed ambiguities to emerge with respect to the symbols in use, as well as their usage and meaning incombination with the SI prefixes. The bit is often given the symbol bit or b, while byte is usually written as byte, B,and occasionally as b. Thus, kb/s often means kilobits per second, but may sometimes refer to kilobytes per second.The National Institute of Standards and Technology in the United States has suggested the use of bit for bits and Bfor bytes.[8] The use of 'b' for 'bytes' is ambiguous and should be strongly discouraged.In non-standard use, K is often used as a symbol prefix to the units bit and byte to designate the binary prefix kibi =210 = 1024.

Binary prefixesThe prefixes kilo, mega, giga and greater are often used in combination with the storage size units bit and byte.The binary multiple 210 = 1024 is close to the value 1000 of the prefix kilo, therefore computer professionals havehistorically used the unit kilobyte to refer to 1024 bytes of computer memory, in non-conformance with the SIdefinition of the prefix kilo. Likewise, 220 = 1048576, which is close to 1000000 has been expressed with the megaprefix. This has led to some confusion, because megabyte is commonly used to refer to 1000000bytes in thespecifications of hard disk drive capacities and network transmission bit rates. Although it is common to size diskdrives in MB or GB(106 or 109) and refer to capacity as "megabytes" or "gigabytes", this has led to a number oflawsuits from purchasers who were expecting 220 or 230 and considered themselves shortchanged by the seller. Toprotect themselves, some sellers actually write out the full term as "1,000,000" or "1,000,000,000".To eliminate this ambiguity the International Electrotechnical Commission (IEC) adopted new binary prefixes in1998 (IEC 80000-13:2008 formerly subclauses 3.8 and 3.9 of IEC 60027-2:2005). Each binary prefix is formed fromthe first syllable of the decimal prefix with the similar value, and the syllable 'bi' (pronounced 'bee'). The symbols arethe decimal symbol, always capitalized, followed by the letter 'i'.According to this standard one kilobyte (1 kB) is 103 or 1000 bytes, whereas one kibibyte (1 KiB) is 210 or 1024bytes. Likewise mebi (Mi; 220 or 1048576), gibi (Gi; 230 or 1073741824), tebi (Ti; 240), pebi (Pi; 250), exbi (Ei; 260),zebi (Zi; 270) and yobi (Yi; 280).The use of these new binary prefixes is increasing,[9] but is largely limited to technical literature and new computersoftware.

Related proposalsIn 2010, an online petition sought to establish hella as the SI prefix for 1027.[10] The prefix, which has since appearedin the San Francisco Chronicle, Daily Telegraph, Wired and some other geek magazines, was recognized by Googlein May 2010.[11] [12] [13] Ian Mills, president of the Consultative Committee on Units, considers the chances ofofficial adoption to be remote.[14]

ReferencesThis article was originally based on material from the Free On-line Dictionary of Computing, which is licensedunder the GFDL.[1] http:/ / physics. nist. gov/ Pubs/ SP811/ sec06. html[2] http:/ / www. bipm. fr/ en/ si/ si_brochure/ chapter3/ prefixes. html[3] Self, Kevin (October 1994). "Technically speaking". Spectrum (IEEE): 18.[4] Self, Kevin (April 1995). "Technically speaking". Spectrum (IEEE): 16.[5] 29th Congress of the United States, Session 1 (13 May 1866). "H.R. 596, An Act to authorize the use of the metric system of weights and

measures" (http:/ / lamar. colostate. edu/ ~hillger/ laws/ metric-act-bill. html). .

SI prefix 6

[6] D. Brewster (1830). The Edinburgh Encyclopaedia (http:/ / books. google. com/ books?id=h6miHpDMjXEC& pg=PA494). p. 494. .[7] histoire.du.metre.free.fr (http:/ / histoire. du. metre. free. fr/ fr/ Pages/ Sommaire/ 06. htm)[8] Ambler Thompson, Barry N. Taylor. (2008). Guide for the Use of the International System of Units (SI) (http:/ / physics. nist. gov/ cuu/ pdf/

sp811. pdf). Gaithersburg, MD: National Institute of Standards and Technology p. 74. This source recommends B as a symbol for byte, but issilent concerning bits.

[9] Sadat Karim, neowin.net (27 March 2010). "Ubuntu implements units policy, will switch to base-10 units in future release" (http:/ / www.neowin. net/ news/ ubuntu-implements-units-policy-will-switch-to-base-10-units-in-future-release). .

[10] Moore, Matthew (2 March 2010). "Hella number: scientists call for new word for 1,000,000,000,000,000,000,000,000,000" (http:/ / www.telegraph. co. uk/ science/ science-news/ 7352204/ Hella-number-scientists-call-for-new-word-for-1000000000000000000000000000. html).The Telegraph. . Retrieved 4 June 2010. "More than 20,000 scientists, students and members of the public have signed an online petitionbacking the new quantity, which would be used for figures with 27 zeros after the first digit."

[11] "Jargon Watch". Wired 18 (6). June 2010. "...a proposed metric prefix...useful for describing mega-measurements like Earth's mass (6Hellagrams). A Facebook petition garnered 30,000 signatures".

[12] "The Official Petition to Establish "Hella-" as the SI Prefix for 10^27" (http:/ / www. facebook. com/ pages/The-Official-Petition-to-Establish-Hella-as-the-SI-Prefix-for-1027/ 277479937276?v=info). Facebook. . Retrieved 4 June 2010.

[13] Kim, Ryan (24 May 2010). "Google gets behind 'hella' campaign" (http:/ / www. sfgate. com/ cgi-bin/ blogs/ techchron/ detail?&entry_id=64240). The San Francisco Chronicle. . Retrieved 4 June 2010.

[14] Chawkins, Steve (6 June 2010). "Physics major has a name for a really big number" (http:/ / articles. latimes. com/ 2010/ jul/ 06/ local/la-me-hella-20100706/ 2). Los Angeles Times. p. 2. .

External links• Bureau International des Poids et Mesures (http:/ / www. bipm. fr/ en/ home/ ) (BIPM)• SI prefixes at BIPM (http:/ / www. bipm. fr/ en/ si/ prefixes. html)• US NIST Definitions of the SI units: The twenty SI prefixes (http:/ / physics. nist. gov/ cuu/ Units/ prefixes. html)• US NIST Definitions of the SI units: The binary prefixes (http:/ / physics. nist. gov/ cuu/ Units/ binary. html)• International Standard ISO 2955: "Information processing - Representation of SI and other units in Systems with

limited Character sets" (http:/ / isotc. iso. org/ livelink/ livelink/ 4289384/ISO_2955-1983E_repr_of_SI_units_with_limited_char_sets. pdf?func=doc. Fetch&nodeid=4289384)[Withdrawn]

• ISO 1000:2004 "SI Units and recommendations for the use of their multiples and of certain other units"• Proposal for an extension of the SI-prefix system (http:/ / jimvb. home. mindspring. com/ unitsystem. htm) to

even larger and smaller units, by a computer scientist (not an official proposal by an organization).

Article Sources and Contributors 7

Article Sources and ContributorsSI prefix  Source: http://en.wikipedia.org/w/index.php?oldid=449014474  Contributors: -

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