Editing Metric prefix (section)

== Application to units of measurement ==
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.{{cn|date=November 2022}} The prefixes, including those introduced after 1960, are used with any metric unit, whether officially included in the SI or not (e.g., millidyne and milligauss). Metric prefixes may also be used with some non-metric units, but not, for example, with the non-SI units of time.<ref>{{SIbrochure9th|page=145}}</ref>

=== Metric units ===

==== Mass ====
The units [[kilogram]], [[gram]], [[milligram]], microgram, and smaller are commonly used for measurement of [[mass]]. However, megagram, gigagram, and larger are rarely used; [[tonne]]s (and kilotonnes, megatonnes, etc.) or [[scientific notation]] are used instead. The megagram does not share the risk of confusion that the tonne has with other units with the name "ton".

The kilogram is the only coherent unit of the [[International System of Units]] that includes a metric prefix.{{refn|{{SIbrochure9th}}}}{{rp|p=144}}

==== Volume ====
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 liquids, and the decilitre is used less frequently. Bulk agricultural products, such as grain, beer and wine, often use the hectolitre (100 litres).{{cn|date=November 2022}}

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.{{cn|date=November 2022}}

==== Length ====
The kilometre, metre, centimetre, millimetre, and smaller units are common. The decimetre is rarely used. The micrometre is often referred to by the older non-SI name ''[[Micrometre#SI standardization|micron]]'', which is officially deprecated. In some fields, such as [[chemistry]], the [[Angstrom|ångström]] (0.1&nbsp;nm) has been used commonly instead of the nanometre. The [[femtometre]], used mainly in particle physics, is sometimes called a [[fermi (unit)|fermi]]. For large scales, megametre, gigametre, and larger are rarely used. Instead, ad hoc non-metric units are used, such as the [[solar radius]], [[astronomical unit]]s, [[light year]]s, and [[parsec]]s; the astronomical unit is mentioned in the SI standards as an accepted non-SI unit.{{cn|date=November 2022}}

==== Time ====
{{see also|Metric time|Orders of magnitude (time)}}

Prefixes for the SI standard unit [[second]] are most commonly encountered for quantities less than one second. For larger quantities, the system of [[minute]]s (60&nbsp;seconds), [[hour]]s (60&nbsp;minutes) and [[day]]s (24&nbsp;hours) is [[Non-SI units mentioned in the SI|accepted for use with the SI]] and more commonly used. When speaking of spans of time, the length of the day is usually standardised to {{val|86,400}}&nbsp;seconds so as not to create issues with the irregular [[leap second]].{{cn|date=November 2022}}

Larger multiples of the second such as kiloseconds and megaseconds are occasionally encountered in scientific contexts, but are seldom used in common parlance. For long-scale scientific work, particularly in [[astronomy]], the [[Julian year (astronomy)|Julian year]] or ''annum'' (a) is a standardised variant of the [[year]], equal to exactly {{val|31557600}}&nbsp;seconds ({{sfrac|365| 1 |4}}&nbsp;days). The unit is so named because it was the average length of a year in the [[Julian calendar]]. Long time periods are then expressed by using metric prefixes with the annum, such as megaannum (Ma) or [[gigaannum]] (Ga).{{cn|date=November 2022}}

==== Angle ====
The SI unit of angle is the [[radian]], but [[Degree (angle)|degrees]], as well as [[Minutes and seconds of arc|arc-minutes and arc-seconds]], see some scientific use in fields such as astronomy.<ref>Barbieri, C., & Bertini, I. (2020). Fundamentals of Astronomy (2nd ed.), p. 1-2. CRC Press. https://doi.org/10.1201/9780429287305</ref>

==== Temperature ====
Common practice does not typically use the flexibility allowed by official policy in the case of the degree Celsius (°C). NIST states:<ref name="Special Publication 811"/> "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&nbsp;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 with the unit of MK (megakelvin), and molecular cooling may be given with the unit mK (millikelvin).{{cn|date=November 2022}}

==== Energy ====
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 [[Watt#Kilowatt|kilowatt]] and hour, is often used for electrical energy; other multiples can be formed by modifying the prefix of watt (e.g. terawatt-hour).{{cn|date=November 2022}}

Several definitions exist for the non-SI unit [[calorie]]. Distinguished 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 "[[Calorie|dietary calories]]" in food.<ref>{{Cite web
| url = http://www.unm.edu/~lkravitz/Article%20folder/remarkablecalorie.html
| title = Remarkable Calorie
| first1 = Carole | last1 = Conn
| first2 = Len    | last2 = Kravitz
| publisher = University of New Mexico
| access-date = 22 May 2017
}}</ref> It is common to apply metric prefixes to the gram calorie, but not to the kilogram calorie: thus, 1&nbsp;kcal = 1000&nbsp;cal = 1&nbsp;Cal.

=== Non-metric units ===
Metric prefixes are widely used outside the metric SI system. Common examples include the [[megabyte]] and the [[decibel]]. Metric prefixes rarely appear with [[imperial units|imperial]] or [[United States customary units|US]] units except in some special cases (e.g., microinch, kilofoot, [[kilopound]]). They are also used with other specialised units used in particular fields (e.g., [[megaelectronvolt]], [[gigaparsec]], [[millibarn]], [[kilodalton]]). In astronomy, geology, and palaeontology, the [[Year#Symbols|year]], with symbol 'a' (from the Latin ''annus''), is commonly used with metric prefixes: [[Kiloannus|ka]], Ma, and Ga.<ref>{{Cite encyclopedia
| last1 = Gargaud | first1 = Muriel
| last2 = Amils   | first2 = Ricardo
| last3 = Cleaves | first3 = Henderson James
| date = 2011-05-26
| df = dmy-all
| title = Ga
| encyclopedia = Encyclopedia of Astrobiology
| publisher = Springer Science & Business Media
| isbn = 978-3-642-11271-3
| lang = en
| page = 621
}}</ref>

Official policies about the use of SI prefixes with non-SI 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 time-related unit symbols (names) min (minute), h (hour), d (day); nor with the angle-related symbols (names) °&nbsp;(degree), ′&nbsp;(minute), and ″&nbsp;(second)",<ref name="Special Publication 811">{{Cite report
| last1 = Thompson | first1 = Ambler
| last2 = Taylor   | first2 = Barry N.
| date = March 2008
| title = Special Publication&nbsp;811
| edition = 2008
| publisher = [[National Institute of Standards and Technology]]
| url = http://physics.nist.gov/Pubs/SP811/sec06.html
| access-date = 21 June 2018
| via = nist.gov
| lang = en
}}</ref> 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."<ref>{{Cite report
| title = The International System of Units (SI)
| series = SI Brochure
| publisher = International Bureau of Weights and Measures
| url = http://www.bipm.org/en/publications/si-brochure/chapter3.html
| access-date = 5 March 2017
}}</ref>