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LIST OF SI PREFIXES The twenty SI prefixes are shown in the chart below. 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 Metre s, 1 Kilogram equals 1000 Gram s, 1 kilowatt equals 1000 Watt s, 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 may also be used in combination with non-SI units; for example: milligauss (mG), kilofoot (kft) and microinch (µin). 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 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.” 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. DETAILS See Also: Non-SI unit prefixes Examples:
The prefix always takes precedence over any exponentiation; thus "km&2" means ''square kilometre'' and not ''kilo–square metre''. For example, 3 km&2 is equal to 3,000,000 m&2 and ''not'' to 3000 m&2 (''nor'' to 9,000,000 m&2). 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. Prefixes where the Exponent is divisible by three are often recommended. Hence "100 m" rather than "1 hm". 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, while two-letter mo and ma symbols were used instead) and were rarely used anyway. 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. 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:
† 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. PRONUNCIATION |
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