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	Topics SI prefix Topic Home 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 decimalDecimal The decimal numeral system has ten as its base.... multiple or submultipleMultiple (mathematics) A multiple of a number is the product of that number with any integer. ... . The abbreviation SI is from the French language name Système International d’Unités (also known as International System of UnitsFacts About International System of Units The International System of Units is the modern form of the metric system.... ). SI prefixPrefix (linguistics) In linguistics, a prefix is a type of affix that precedes the morphemes to which it can attach.... es 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 (BIPM, also known as the International Bureau of Weights and MeasuresInternational Bureau of Weights and Measures The International Bureau of Weights and Measures is the English name of the Bureau international des poids et mesures ... ) and are the product of dating from 1960 to 1991. List of SI prefixes The twenty SI prefixes are shown in the chart below. Usage 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 TechnologyNational Institute of Standards and Technology The National Institute of Standards and Technology is a non-regulatory agency of the United States Department of Commerces ... (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 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 “However astronomers use milliarcsecond, which they denote mas, and microarcsecond, µas, which they use as units for measuring very small angles.” 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 “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 kelvinKelvin The Kelvin scale is a temperature scale where absolute zero—the coldest possible temperature where there is no heat en... are usually used instead. Details Examples: 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 3,000 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 litreLitre The litre or liter is a unit of volume.... ) are introduced. Prefixes where the exponent is divisible by three are often recommended. Hence "100 m" rather than "1 hm". The obsolete prefixesNon-SI unit prefix There exist several unit prefixes used like the SI prefixes, but that are not part of the SI system.... 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. The prefix kilo derives from the GreekGreek language Summary Greek has a documented history of 3,500 years, the longest of any single language within the Indo-European family.... word ????a (khilia or chilia) = thousand. Double prefixes such as those formerly used in micromicrofaradsFarad The farad is the SI unit of capacitance. ... (picofarads), hectokilometres (100 kilometres), and millimicrons or micromillimetres (both nanometres) were also dropped with the introduction of the SI. 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 notationScientific notation Scientific notation is a notation for writing numbers that is often used by scientists and mathematicians to make it easier ... 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 micronMicrometre A micrometre is an SI unit of length equal to one millionth of a metre, or about a tenth of the size of a droplet of mist o... . In some fields such as chemistryChemistry Chemistry is the science of matter at the atomic to molecular scale, dealing primarily with collections of atoms .... , the angstrom (equal to 0.1 nm) competes with the nanometre. The femtometre, used mainly in particle physics, is usually called a fermiFacts About Fermi (unit) The fermi is a non-SI unit of length that is internationally recognised and equivalent to the SI-recognised femtometre.... . At large scales, megametre, gigametre, and larger are rarely used. Often used are astronomical unitAstronomical unit The astronomical unit is a unit of length.... s, light years, and parsecParsec Overview The parsec is a unit of length used in astronomy.... s; 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 scaleLong and short scales Short scale is the English translation of the French term chelle courte, which designates a system of numeric names in w... number name conventions, but have now at least partly switched to the short scaleLong and short scales Short scale is the English translation of the French term chelle courte, which designates a system of numeric names in w... 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. Pronunciation There are two accepted pronunciations for the prefix giga-: and . According to the American writer Self, in the 1920s a German committee member of the International Electrotechnical CommissionInternational Electrotechnical Commission Overview The International Electrotechnical Commission is an international standards organization dealing with electrical, electroni... 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 (soft g) pronunciation became accepted, but as of 1995 current practice had returned to (hard g). When any SI prefix is affixed to a root word, the prefix carries the primary stressStress (linguistics) In linguistics, stress is the relative emphasis given to certain syllables in a word.... , 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 gigabyteGigabyte A gigabyte is a unit of information or computer storage equal to one billion bytes.... is . 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 . 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 problemYear 2000 problem The Year 2000 problem was the result of a practice in computer program design that caused some date-related processing to o... . 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 prefixBinary prefix In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten.... kilo = 2¹⁰ = 1024, although this is now non-standard. Non-SI units Prefixes go back to the introduction of the metric systemMetric system The metric system is a decimalized system of measurement based on the metre and the gram.... 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 unitImperial unit The Imperial units or the Imperial system is a collection of English units, first defined in the Weights and Measures ... s or English unitEnglish unit English unit is an American term that refers to a unit in one of a number of systems of units of measurement, some obsolete,... s 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. Computing The prefixes k and greater are common in computingComputing Originally, the word *computing* was synonymous with counting and calculating, and a science and technology that deals wit... , where they are applied to information and storage units like the bitBit A bit refers to a digit in the binary numeral system .... and the byteByte A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored.... . 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: kilo (k) : ˜ kibi = Ki = 2¹⁰ = 1024¹ = 1 024; mega (M) : ˜ mebi = Mi = 2²⁰ = 1024² = 1 048 576; giga (G) : ˜ gibi = Gi = 2³⁰ = 1024³ = 1 073 741 824; tera (T) : ˜ tebi = Ti = 2⁴⁰ = 1024⁴ = 1 099 511 627 776 (= 1 024 Gi); peta (P) : ˜ pebi = Pi = 2⁵⁰ = 1024⁵ = 1 125 899 906 842 624 (= 1 048 576 Gi); exa (E) : ˜ exbi = Ei = 2⁶⁰ = 1024⁶ = 1 152 921 504 606 846 976 (= 1 073 741 824 Gi). These prefixes, however, usually retain their powers-of-1000 meanings when used to describe either disk storage or rates of data transmission: 10 Mbit/s EthernetEthernet Ethernet is a large and diverse family of frame-based computer networking technologies for local area networks .... runs at 10,000,000 bit/s, not 10,485,760 bit/s. This creates confusion when the two usages in memory or filesystems and in transmission or disk storage are colliding, notably for monitoring fle transmission rates. The confusion is compounded by the fact that the units of information (the bitBit A bit refers to a digit in the binary numeral system .... and the byteByte A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored.... ) are not part of SI, where the bit, byte, octetOctet (computing) In computer technology and networking, an octet is a group of 8 bits.... , baudBaud In telecommunications and electronics, baud is a measure of the symbol rate, that is the number of distinct symbolic change... or symbol rateSymbol rate In digital communications, the symbol rate is the bit rate divided by the number of bits transmitted in each symbol.... would rather be given in hertzHertz The hertz is the SI unit of frequency.... . Although some use "bit" for the bitBit A bit refers to a digit in the binary numeral system .... and "b" for the byteByte A byte is commonly used as a unit of storage measurement in computers, regardless of the type of data being stored.... , "b" is often used for bit and "B" for byte instead. The baud symbol is normally "Bd" but it is often confused with bits per second and abbreviated with just its initial, in either letter case, even though it does not directly measure the effective binary data rate but the way this data is encoded and modulated over a transmission link. It is recommended by several standards bodies to not abbreviate bit and to use B for byte, in order to keep the units very distinct, as in kbit or MiB. French-speakers often use "o" for "octet", today a near synonym for the byte. Consequently, the International Electrotechnical CommissionInternational Electrotechnical Commission The International Electrotechnical Commission is an international standards organization dealing with electrical, electroni... (IEC) adopted new binary prefixBinary prefix In computing, binary prefixes can be used to quantify large numbers where powers of two are more useful than powers of ten.... es in 1998, formed from the first syllable of the decimal prefix plus 'bi' (pronounced 'bee'). The symbol is the decimal symbol (converted to capital for the first prefix) plus 'i'. So now, one kilobyteKilobyte A kilobyte is a unit of information or computer storage equal to either 1024 or 1000 bytes.... (1 kB) equals 1000 bytes, whereas one kibibyteKibibyte A kibibyte is a unit of information or computer storage, abbreviated KiB.... (1 KiB) equals 2¹⁰ = 1024 bytes. Likewise mebi (Mi; 2²⁰), gibi (Gi; 2³⁰), tebi (Ti; 2⁴⁰), pebi (Pi; 2⁵⁰), exbi (Ei; 2⁶⁰), zebi (Zi; 2⁷⁰) and yobiYobi Yobi may refer to the following:**Yobi* or *Yeoubi, a fictional character in the movie Yobi, the Five Tailed Fox... (Yi; 2⁸⁰). The practical use of these binary prefixes is growing slowly and is largely limited to expert literature. Their use in marketing literature is rare. Proposed changes 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. 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 systemMetric system The metric system* is a decimalized system of measurement based on the metre and the gram.... : 1 areAre An are is a unit of area, equal to 100 square meters , used for measuring land area.... (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 stereStère The st?re is an old measurement unit for volume and equals one cubic metre.... (st) = 1 m³ 1 litreLitre Overview The litre or liter is a unit of volume.... (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). See also External links Standards organisations Other proposals to even larger and smaller units, by a computer scientist (not an official proposal by an organisation).