Editing International System of Units (section)

=== Prefixes ===

{{Main|Metric prefix}}

Like all metric systems, the SI uses [[metric prefix]]es to systematically construct, for the same physical quantity, a set of units that are decimal multiples of each other over a wide range. For example, driving distances are normally given in [[kilometre]]s (symbol {{val|u=km}}) rather than in metres. Here the metric prefix '[[kilo-]]' (symbol 'k') stands for a factor of 1000; thus, {{val|1|u=km}} = {{val|1000|u=m}}.

The SI provides twenty-four metric prefixes that signify decimal powers ranging from 10<sup>−30</sup> to 10<sup>30</sup>, the most recent being adopted in 2022.<ref name="SIBrochure9thEd" />{{rp|pages=143–144}}<ref>{{cite web |first1=Daniel|last1=Lawler|title=Earth now weighs six ronnagrams: New metric prefixes voted in |url=https://phys.org/news/2022-11-earth-ronnagrams-metric-prefixes-voted.html |publisher=phys.org |date=18 November 2022}}</ref><ref>{{cite web |date=18 November 2022 |title=List of Resolutions for the 27th meeting of the General Conference on Weights and Measures |url=https://www.bipm.org/documents/20126/64811223/Resolutions-2022.pdf |publisher=BIPM|url-status=dead |access-date=18 November 2022 |archive-date=18 November 2022 |archive-url=https://web.archive.org/web/20221118153958/https://www.bipm.org/documents/20126/64811223/Resolutions-2022.pdf}}</ref><ref>{{cite web |title=New prefixes for the SI adopted by the General Conference on Weights and Measures |url=https://www.bipm.org/en/-/2022-12-19-si-prefixes |access-date=11 January 2023 |website=BIPM }}</ref> Most prefixes correspond to integer powers of 1000; the only ones that do not are those for 10, 1/10, 100, and 1/100.
The conversion between different SI units for one and the same physical quantity is always through a power of ten. This is why the SI (and metric systems more generally) are called ''decimal systems of measurement units''.<ref name="DecimalSystem">{{cite web |url=https://usma.org/decimal-nature-of-the-metric-system |title=Decimal Nature of the Metric System |publisher=[[US Metric Association]] | date=2015 |access-date=15 April 2020 |archive-url=https://web.archive.org/web/20200415225727/https://usma.org/decimal-nature-of-the-metric-system/ |archive-date=15 April 2020 |url-status=live}}</ref>

{{Anchor|Compound unit}}The grouping formed by a prefix symbol attached to a unit symbol (e.g. '{{val|u=km}}', '{{val|u=cm}}') constitutes a new inseparable unit symbol. This new symbol can be raised to a positive or negative power. It can also be combined with other unit symbols to form ''compound unit'' symbols.<ref name="SIBrochure9thEd" />{{rp|page=143}} For example, {{val|u=g/cm3}} is an SI unit of [[density]], where {{val|u=cm3}} is to be interpreted as ({{val|u=cm}}){{sup|3}}.

Prefixes are added to unit names to produce multiples and [[submultiple]]s of the original unit. All of these are integer powers of ten, and above a hundred or below a hundredth all are integer powers of a thousand. For example, ''kilo-'' denotes a multiple of a thousand and ''milli-'' denotes a multiple of a thousandth, so there are one thousand millimetres to the metre and one thousand metres to the kilometre. The prefixes are never combined, so for example a millionth of a metre is a ''micrometre'', not a ''millimillimetre''. Multiples of the kilogram are named as if the gram were the base unit, so a millionth of a kilogram is a ''milligram'', not a ''microkilogram''.<ref name="SIBrochure">{{SIbrochure8th}}</ref>{{rp|page=122}}<ref name="NIST811"/>{{rp|page=14}}

The BIPM specifies 24 prefixes for the International System of Units (SI):

{{SI prefixes (infobox)}}