Editing International System of Units (section)

== Related units ==

=== Non-SI units accepted for use with SI ===

[[File:CubeLitre.svg|right|thumb|upright=1.1|While not an SI-unit, the litre may be used with SI units. It is equivalent to {{nowrap|1=({{val|10|u=cm}}){{sup|3}} = ({{val|1|u=dm}}){{sup|3}} = {{val|e=-3|u=m3}}}}.]]
Many non-SI units continue to be used in the scientific, technical, and commercial literature. Some units are deeply embedded in history and culture, and their use has not been entirely replaced by their SI alternatives. The CIPM recognised and acknowledged such traditions by compiling a list of non-SI units accepted for use with SI,<ref name="SIBrochure"/> including the hour, minute, degree of angle, litre, and decibel.

This is a list of units that are not defined as part of the [[International System of Units]] ([[SI]]) but are otherwise mentioned in the SI Brochure,<ref name="bipm">[[Bureau international des poids et mesures]], "Non-SI units that are accepted for use with the SI", in: [https://www.bipm.org/documents/20126/41483022/SI-Brochure-9.pdf Le Système international d'unités (SI) / The International System of Units (SI), 9th ed.] (Sèvres: 2019), {{ISBN|9789282222720|}}, c. 4, pp. 145–146.</ref> listed as being accepted for use alongside SI units, or for explanatory purposes.

{| class="wikitable sortable"
|-
 !scope="col"| Name
 !scope="col"| Symbol
 !scope="col"| Quantity
 !scope="col"| Value in SI units
|-
 | [[minute]]
 |style="text-align:center"| min
 | rowspan="3" | [[time]]
 | {{val|1|u=min}} = {{val|60|ul=s}}
|-
 | [[hour]]
 |style="text-align:center"| h
 | {{val|1|u=h}} = {{val|60|u=min}} = {{val|3600|u=s}}
|-
 | [[day]]
 |style="text-align:center"| d
 | {{val|1|u=d}} = {{val|24|u=h}} = {{val|1440|u=min}} = {{val|86400|u=s}}
|-
 | [[astronomical unit]]
 |style="text-align:center"|au
 | [[length]]
 | {{val|1|u=au}} = {{val|149597870700|u=m}}
|-
 | [[degree (angle)|degree]]
 |style="text-align:center"| °
 |rowspan="3"| [[angle|plane angle]] and [[phase (waves)|phase angle]]
 | {{val|1|u=°}} = {{nowrap|(π / 180) rad}}
|-
 | [[minute of arc|arcminute]]
 |style="text-align:center"| ′
 | {{val|1|u=′}} = {{nowrap|(1 / 60)°}} = {{nowrap|(π / {{val|10800}}) rad}}
|-
 | [[second of arc|arcsecond]]
 |style="text-align:center"| ″
 | {{val|1|u=″}} = {{nowrap|(1 / 60)′}} = {{nowrap|(1 / 3600)°}} = {{nowrap|(π / {{val|648000}}) rad}}
|-
 | [[hectare]]
 |style="text-align:center"| ha
 | [[area]]
 | {{val|1|u=ha}} = {{val|1|u=hm2}} = {{val|10000|u=m2}}
|-
 | [[litre]]
 |style="text-align:center"| l, L
 | [[volume]]
 | {{val|1|u=L}} = {{val|1|u=dm3}} = {{val|1000|u=cm3}} = {{val|0.001|u=m3}}
|-
 | [[tonne]]
 |style="text-align:center"| t
 | rowspan="2" | [[mass]]
 | {{val|1|u=t}} = {{val|1|u=Mg}} = {{val|1000|u=kg}}
|-
 | [[Dalton (unit)|dalton]]
 |style="text-align:center"| Da
 | {{val|1|u=Da}} = {{physconst|mu}}{{efn|A footnote in the 9th SI Brochure gives an exact definition of the dalton.}}
|-
 | [[electronvolt]]
 |style="text-align:center"| eV
 | [[energy]]
 | {{val|1|u=eV}} = {{physconst|eV}}
|-
 | [[neper]]
 |style="text-align:center"| Np
 |rowspan="2"| [[logarithmic ratio quantity]]
 | {{n/a}}
|-
 | bel, [[decibel]]
 |style="text-align:center"| B, dB
 | {{n/a}}
|-
|}
The SI prefixes can be used with several of these units, but not, for example, with the non-SI units of time. 
Others, in order to be converted to the corresponding SI unit, require conversion factors that are not powers of ten. Some common examples of such units are the customary units of time, namely the minute (conversion factor of {{val|60|u=s/min}}, since {{nowrap|1={{val|1|u=min}} = {{val|60|u=s}}}}), the hour ({{val|3600|u=s}}), and the day ({{val|86400|u=s}}); the degree (for measuring plane angles, {{nowrap|1={{val|1|u=deg}} = {{val|p=(π /180)&nbsp;|u=rad}});}} and the [[electronvolt]] (a unit of energy, {{nowrap|1={{val|1|u=eV}} = {{val|1.602176634e-19|u=J}}}}).<ref name="bipm"/>

=== Metric units not recognised by SI ===

{{Main|List of metric units}}
Although the term ''metric system'' is often used as an informal alternative name for the International System of Units,<ref name="MetricSystemAsNameForSI">{{cite journal |last=Olthoff|first=Jim|url=https://www.nist.gov/blogs/taking-measure/all-times-all-peoples-how-replacing-kilogram-empowers-industry |title=For All Times, For All Peoples: How Replacing the Kilogram Empowers Industry |journal=[[NIST]] | date=2018 |access-date=14 April 2020 |archive-url=https://web.archive.org/web/20200316195625/https://www.nist.gov/blogs/taking-measure/all-times-all-peoples-how-replacing-kilogram-empowers-industry |archive-date=16 March 2020 |url-status=live |quote=... the International System of Units (SI), popularly known as the metric system.}}</ref> other metric systems exist, some of which were in widespread use in the past or are even still used in particular areas. There are also individual [[metric units]] such as the [[sverdrup]] and the [[darcy (unit)|darcy]] that exist outside of any system of units. Most of the units of the other metric systems are not recognised by the SI.

=== Unacceptable uses ===

Sometimes, SI unit name variations are introduced, mixing information about the corresponding physical quantity or the conditions of its measurement; however, this practice is unacceptable with the SI. "Unacceptability of mixing information with units: When one gives the value of a quantity, any information concerning the quantity or its conditions of measurement must be presented in such a way as not to be associated with the unit."<ref name="SIBrochure"/>
Instances include: "[[watt-peak]]" and "[[watt RMS]]"; "[[geopotential metre]]" and "[[vertical metre]]"; "[[standard cubic metre]]"; "[[atomic second]]", "[[ephemeris second]]", and "[[sidereal second]]".