Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Metric system
open-in-new
<p>The metric system is a <a href="/facts/System_of_measurement/xIsWmFWW">system of measurement</a> that <a href="/facts/Standardization/vS9AhNMS">standardizes</a> a set of base units and a <a href="/facts/Nomenclature/YhTgjjyY">nomenclature</a> for describing relatively large and small quantities via <a href="/facts/Decimal/7omfVNIM">decimal</a>-based multiplicative <a href="/facts/Unit_prefix/CTotM82r">unit prefixes</a>. Though the rules governing the metric system have changed over time, the modern definition, the <a href="/facts/International_System_of_Units/S7YLglNz">International System of Units</a> (SI), defines the <a href="/facts/Metric_prefix/Y2MzkaeL">metric prefixes</a> and seven base units: <a href="/facts/Metre/hPhzc1CV">metre</a> (m), <a href="/facts/Kilogram/lU2zbPk8">kilogram</a> (kg), <a href="/facts/Second/jtFxnJXx">second</a> (s), <a href="/facts/Ampere/rcKdfC2R">ampere</a> (A), <a href="/facts/Kelvin/E3trzC4C">kelvin</a> (K), <a href="/facts/Mole_(unit)/kCMFwCc0">mole</a> (mol), and <a href="/facts/Candela/grEDstnw">candela</a> (cd). </p><p>An <a href="/facts/SI_derived_unit/RLu52lWy">SI derived unit</a> is a named combination of base units such as <a href="/facts/Hertz/6Wshj5qm">hertz</a> (cycles per second), <a href="/facts/Newton_(unit)/Ral9XgNl">newton</a> (kg⋅m/s2), and <a href="/facts/Tesla_(unit)/K6I5Stn9">tesla</a> (1 kg⋅s−2⋅A−1) and in the case of <a href="/facts/Celsius/okAEu1CA">Celsius</a> a shifted scale from Kelvin. Certain units have been <a href="/facts/Non-SI_units_mentioned_in_the_SI/uBgEpKMP">officially accepted for use with the SI</a>. Some of these are decimalised, like the <a href="/facts/Litre/QKwbQz8I">litre</a> and <a href="/facts/Electronvolt/YMT1fqX9">electronvolt</a>, and are considered "metric". Others, like the <a href="/facts/Astronomical_unit/gjsbu5xU">astronomical unit</a> are not. Ancient non-metric but SI-accepted multiples of time, <a href="/facts/Minute/WTRsLrWy">minute</a> and <a href="/facts/Hour/rBje78F6">hour</a>, are base 60 (<a href="/facts/Sexagesimal/bspNRWdR">sexagesimal</a>). Similarly, the angular measure <a href="/facts/Degree_(angle)/f48Ns8SX">degree</a> and submultiples, <a href="/facts/Arcminute/tybs6YGC">arcminute</a>, and <a href="/facts/Arcsecond/tybs6YGC">arcsecond</a>, are also sexagesimal and SI-accepted. </p><p>The SI system derives from the older <a href="/facts/MKS_units/fMpV1vDw">metre, kilogram, second</a> (MKS) system of units, though the definition of the base units has evolved over time. Today, all base units are defined by <a href="/facts/Physical_constant/uI2QvAHw">physical constants</a>; not by example as physical objects as they were in the past. </p><p>Other metric system variants include the <a href="/facts/Centimetre%25E2%2580%2593gram%25E2%2580%2593second_system_of_units/qbHxv78q">centimetre–gram–second system of units</a>, the <a href="/facts/Metre%25E2%2580%2593tonne%25E2%2580%2593second_system_of_units/kvvWcMkY">metre–tonne–second system of units</a>, and the <a href="/facts/Gravitational_metric_system/PW9C82yy">gravitational metric system</a>. Each has <a href="/facts/List_of_metric_units/uf8Lbdel">unaffiliated metric units</a>. Some of these systems are still used in limited contexts. </p>