Editing International System of Units (section)

=== SI base units ===

{{Main|SI base unit}}

The SI selects seven units to serve as [[SI base unit|base unit]]s, corresponding to seven base physical quantities. They are the [[second]], with the symbol {{val|u=s}}, which is the SI unit of the physical quantity of [[time]]; the [[metre]], symbol {{val|u=m}}, the SI unit of [[length]]; [[kilogram]] ({{val|u=kg}}, the unit of [[mass]]); [[ampere]] ({{val|u=A}}, [[electric current]]); [[kelvin]] ({{val|u=K}}, [[thermodynamic temperature]]); [[Mole (unit)|mole]] ({{val|u=mol}}, [[amount of substance]]); and [[candela]] ({{val|u=cd}}, [[luminous intensity]]).<ref name="SIBrochure9thEd" />
The base units are defined in terms of the defining constants.  For example, the kilogram is defined by taking the Planck constant {{math|''h''}} to be {{val|6.62607015|e=-34|u=J.s}}, giving the expression in terms of the defining constants<ref name="SIBrochure9thEd" />{{rp|page=131}}
: {{nowrap|1={{val|1|u=kg}} = {{sfrac|({{val|299792458}}){{sup|2}}|({{val|6.62607015|e=-34}})({{val|9192631770}})}}{{sfrac|{{math|''h''}}{{thin space}}{{math|Δ''ν''{{sub|Cs}}}}|{{math|''c''}}{{sup|2}}}}.}}
All units in the SI can be expressed in terms of the base units, and the base units serve as a preferred set for expressing or analysing the relationships between units. The choice of which and even how many quantities to use as base quantities is not fundamental or even unique – it is a matter of convention.<ref name="SIBrochure9thEd" />{{rp|page=126|quote=[...] the choice of the base units was never unique, but grew historically and became familiar to users of the SI}}

{| class="wikitable" style="margin:1em auto 1em auto"
|+ <big>SI base units</big><ref name="SIBrochure9thEd"/>{{rp|page=136}}
|-
 ! scope="col" | Unit name
 ! scope="col" | Unit symbol
 ! scope="col" | [[Dimensional analysis#Definition|Dimension symbol]]
 ! scope="col" | [[Physical quantity|Quantity name]]
 ! scope="col" width=80 |Typical symbols
 ! scope="col" | Definition
|-
 ! scope="row" | [[second]]
 |style="text-align:center" |s
 |style="text-align:center" |<math>\mathsf{T}</math>
 | [[time]]
 |<math>t</math>
 |The duration of {{val|9192631770}} periods of the radiation corresponding to the transition between the two [[hyperfine structure|hyperfine]] levels of the [[ground state]] of the [[caesium-133]] atom.
|-
 ! scope="row" | [[metre]]
 |style="text-align:center" |m
 |style="text-align:center" |<math>\mathsf{L}</math>
 |[[length]]
 |<math>l</math>, <math>x</math>, <math>r</math>, etc.
 |The distance travelled by light in vacuum in {{sfrac|{{val|299792458}}}} second.
|-
 ! scope="row" | [[kilogram]]{{br}}<ref group="n">Despite the prefix "kilo-", the kilogram is the coherent base unit of mass, and is used in the definitions of derived units. Nonetheless, prefixes for the unit of mass are determined as if the gram were the base unit.</ref>
 |style="text-align:center" |kg
 |style="text-align:center" |<math>\mathsf{M}</math>
 |[[mass]]
 |<math>m</math>
 |The kilogram is defined by setting the [[Planck constant]] {{math|''h''}} to {{val|6.62607015|e=-34|u=J.s}} ({{nowrap|1=J = kg⋅m{{sup|2}}⋅s{{sup|−2}}}}), given the definitions of the metre and the second.<ref name="NIST 2018-11">{{cite news|url=https://www.nist.gov/news-events/news/2018/11/historic-vote-ties-kilogram-and-other-units-natural-constants|title=Historic Vote Ties Kilogram and Other Units to Natural Constants|last=Materese|first=Robin|date=16 November 2018|work=NIST|access-date=16 November 2018}}</ref>
|-
 ! scope="row" | [[ampere]]
 |style="text-align:center" |A
 |style="text-align:center" |<math>\mathsf{I}</math>
 |[[electric current]]
 |<math>I,\; i</math>
 |The flow of {{sfrac|1|{{val|1.602176634|e=-19}}}} times the [[elementary charge]] {{math|''e''}} per second, which is approximately {{val|6.2415090744|e=18}} elementary charges per second.
|-
 ! scope="row" | [[kelvin]]
 |style="text-align:center" |K
 |style="text-align:center" |<math>\mathsf{\Theta}</math>
 |[[thermodynamic temperature|thermodynamic{{br}}temperature]]
 |<math>T</math>
 |The kelvin is defined by setting the fixed numerical value of the [[Boltzmann constant]] {{math|''k''}} to {{val|1.380649|e=-23|u=J.K-1}}, ({{nowrap|1=J = kg⋅m<sup>2</sup>⋅s<sup>−2</sup>}}), given the definition of the kilogram, the metre, and the second.
|-
 ! scope="row" | [[mole (unit)|mole]]
 |style="text-align:center" |mol
 |style="text-align:center" |<math>\mathsf{N}</math>
 |[[amount of substance]]
 |<math>n</math>
 |The amount of substance of {{val|6.02214076|e=23}} elementary entities.<ref group="n">When the mole is used, the elementary entities must be specified and may be [[atom]]s, [[molecule]]s, [[ion]]s, [[electron]]s, other particles, or specified groups of such particles.</ref> This number is the fixed numerical value of the [[Avogadro constant]], {{math|''N''<sub>A</sub>}}, when expressed in the unit mol<sup>−1</sup>.
|-
 ! scope="row" | [[candela]]
 |style="text-align:center" |cd
 |style="text-align:center" |<math>\mathsf{J}</math>
 |[[luminous intensity]]
 |<math>I_{\rm v}</math>
 |The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency {{val|5.4|e=14|u=hertz}} and that has a radiant intensity in that direction of {{sfrac|1|683}} watt per [[steradian]].
|-
 | colspan="6" |
; Notes
{{reflist|group="n"}}
|}