Editing Measurement (section)

==== International System of Units ====
The [[International System of Units]] (abbreviated as SI from the [[French language]] name ''Système International d'Unités'') is the modern revision of the [[metric system]]. It is the world's most widely used [[system of units]], both in everyday [[commerce]] and in [[science]]. The SI was developed in 1960 from the [[metre–kilogram–second]] (MKS) system, rather than the [[centimetre–gram–second]] (CGS) system, which, in turn, had many variants. The SI units for the seven base physical quantities are:<ref>{{SIbrochure9th}}</ref>
{| class="wikitable"
|-
! Base quantity
! Base unit
! Symbol
! Defining constant
|-
| [[time]]
| [[second]]
| s
| hyperfine splitting in [[caesium-133]]
|-
| [[length]]
| metre
| m
| [[speed of light]], ''c''
|-
| [[mass]]
| [[kilogram]]
| kg
| [[Planck constant]], ''h''
|-
| [[electric current]]
| [[ampere]]
| A
| [[elementary charge]], ''e''
|-
| [[temperature]]
| [[kelvin]]
| K
| [[Boltzmann constant]], ''k''
|-
| [[amount of substance]]
| [[mole (unit)|mol]]
| mol
| [[Avogadro constant]], ''N''<sub>A</sub>
|-
| [[luminous intensity]]
| [[candela]]
| cd
| [[luminous efficacy]] of a 540&nbsp;THz source, ''K''<sub>cd</sub>
|}

In the SI, base units are the simple measurements for time, length, mass, temperature, amount of substance, electric current and light intensity. Derived units are constructed from the base units: for example, the [[watt]], i.e. the unit for power, is defined from the base units as m<sup>2</sup>·kg·s<sup>−3</sup>. Other physical properties may be measured in compound units, such as material density, measured in kg·m<sup>−3</sup>.

===== Converting prefixes =====
The SI allows easy multiplication when switching among units having the same base but different prefixes. To convert from metres to centimetres it is only necessary to multiply the number of metres by 100, since there are 100 centimetres in a metre. Inversely, to switch from centimetres to metres one multiplies the number of centimetres by 0.01 or divides the number of centimetres by 100.