{{short description|Physical system of measurement that uses the centimetercentimetre, gram, and second as base units}}
{{redirect|CGS}}
{{outline|Outline of the metric system}}
The '''centimeter–gram–secondcentimetre–gram–second system of units''' (abbreviated '''CGS''' or '''cgs''') is a variant of the [[metric system]] based on the [[centimetercentimetre]] as the unit of [[length]], the [[gram]] as the unit of [[mass]], and the [[second]] as the unit of [[time]]. All CGS [[mechanics|mechanical]] units are unambiguously derived from these three base units, but there are several different ways in which the CGS system was extended to cover [[electromagnetism]].<ref>{{Cite encyclopedia|url=https://www.britannica.com/science/centimetre-gram-second-system|title=Centimetre-gram-second system {{!}} physics|encyclopedia=Encyclopedia Britannica|access-date=2018-03-27|language=en}}{{failed verification|date=April 2018|reason=article is about viscosity, not electromagnetism}}</ref><ref>{{Cite web|url=https://www.maplesoft.com/support/help/maple/view.aspx?path=Units/CGS|title=The Centimeter-Gram-Second (CGS) System of Units – Maple Programming Help|website=www.maplesoft.com|access-date=2018-03-27}}</ref><ref>{{cite arXiv |title=Babel of units: The evolution of units systems in classical electromagnetism |last=Carron |first=Neal J. |date=21 May 2015 |class=physics.hist-ph |eprint=1506.01951}}</ref>
The CGS system has been largely supplanted by the [[MKS system of units|MKS system]] based on the [[metermetre]], [[kilogram]], and second, which was in turn extended and replaced by the [[International System of Units]] (SI). In many fields of science and engineering, SI is the only system of units in use, but there remain certain subfields where CGS is prevalent.
In measurements of purely mechanical systems (involving units of length, mass, [[force]], [[energy]], [[pressure]], and so on), the differences between CGS and SI are straightforward and rather trivial; the [[Unit conversion|unit-conversion factors]] are all [[Exponentiation#Powers of ten|powers of 10]] as {{nowrap|1=100 cm = 1 m}} and {{nowrap|1=1000 g = 1 kg}}. For example, the CGS unit of force is the [[dyne]], which is defined as {{val|1|u=g⋅cm/s<sup>2</sup>}}, so the SI unit of force, the [[newton (unit)|newton]] ({{val|1|u=kg⋅m/s<sup>2</sup>}}), is equal to {{val|100000|u=dynes}}.