International System of Units (SI) | Units, Facts, & Definition

International System of Units (SI), international decimal system of weights and measures derived from and extending the metric system of units. InternationalSystemofUnits Sections&Media Article IntroductionWidelyusedunitsintheSIsystemMetricconversions FastFacts 2-MinSummary Facts&RelatedContent Media Videos AdditionalInfo MoreArticlesOnThisTopic Contributors ArticleHistory Home Technology Engineering MechanicalEngineering InternationalSystemofUnits measurement Alternatetitles:SI,SISystem,SystèmeInternationald’Unités Print print Print Pleaseselectwhichsectionsyouwouldliketoprint: TableOfContents Cite verifiedCite Whileeveryefforthasbeenmadetofollowcitationstylerules,theremaybesomediscrepancies. Pleaserefertotheappropriatestylemanualorothersourcesifyouhaveanyquestions. SelectCitationStyle MLA APA ChicagoManualofStyle CopyCitation Share Share Sharetosocialmedia Facebook Twitter URL https://www.britannica.com/science/International-System-of-Units More GiveFeedback ExternalWebsites Feedback Corrections?Updates?Omissions?Letusknowifyouhavesuggestionstoimprovethisarticle(requireslogin). FeedbackType Selectatype(Required) FactualCorrection Spelling/GrammarCorrection LinkCorrection AdditionalInformation Other YourFeedback SubmitFeedback Thankyouforyourfeedback Oureditorswillreviewwhatyou’vesubmittedanddeterminewhethertorevisethearticle. JoinBritannica'sPublishingPartnerProgramandourcommunityofexpertstogainaglobalaudienceforyourwork! ExternalWebsites NationalCenterforBiotechnologyInformation-PubMedCentral-Theinternationalsystemofunits(SI)inhistoricalperspective. BritannicaWebsites ArticlesfromBritannicaEncyclopediasforelementaryandhighschoolstudents. internationalsystemofunits(SI)-StudentEncyclopedia(Ages11andup) By TheEditorsofEncyclopaediaBritannica | ViewEditHistory FastFacts 2-MinSummary RelatedTopics: mole kilogram second metre kelvin ...(Showmore) Seeallrelatedcontent→ UnderstandtheInternationalSystemofUnitsanditssevenbasicSIUnitsOverviewoftheInternationalSystemofUnits.EncyclopædiaBritannica,Inc.SeeallvideosforthisarticleInternationalSystemofUnits(SI),FrenchSystèmeInternationald’Unités,internationaldecimalsystemofweightsandmeasuresderivedfromandextendingthemetricsystemofunits.Adoptedbythe11thGeneralConferenceonWeightsandMeasures(CGPM)in1960,itisabbreviatedSIinalllanguages.Rapidadvancesinscienceandtechnologyinthe19thand20thcenturiesfosteredthedevelopmentofseveraloverlappingsystemsofunitsofmeasurementsasscientistsimprovisedtomeetthepracticalneedsoftheirdisciplines.Theearlyinternationalsystemdevisedtorectifythissituationwascalledthemetre-kilogram-second(MKS)system.TheCGPMaddedthreenewunits(amongothers)in1948:aunitofforce(thenewton),definedasthatforcewhichgivestoamassofonekilogramanaccelerationofonemetrepersecondpersecond;aunitofenergy(thejoule),definedastheworkdonewhenthepointofapplicationofanewtonisdisplacedonemetreinthedirectionoftheforce;andaunitofpower(thewatt),whichisthepowerthatinonesecondgivesrisetoenergyofonejoule.Allthreeunitsarenamedforeminentscientists. BritannicaQuiz Science:FactorFiction? Doyougetfiredupaboutphysics?Giddyaboutgeology?Sortoutsciencefactfromfictionwiththesequestions. UnderstandtheconceptofderivedunitsofmeasurementOverviewofunitsderivedfromthesevenbasicunitsoftheInternationalSystemofUnits.EncyclopædiaBritannica,Inc.SeeallvideosforthisarticleThe1960InternationalSystembuildsontheMKSsystem.Itssevenbasicunits,fromwhichotherunitsarederived,weredefinedasfollows:forlength,themetre,definedasthedistancetraveledbylightinavacuumin1/299,792,458second;formass,thekilogram,whichequaled1,000gramsasdefinedbytheinternationalprototypekilogramofplatinum-iridiuminthekeepingoftheInternationalBureauofWeightsandMeasuresinSèvres,France;fortime,thesecond,thedurationof9,192,631,770periodsofradiationassociatedwithaspecifiedtransitionofthecesium-133atom;forelectriccurrent,theampere,whichwasthecurrentthat,ifmaintainedintwowiresplacedonemetreapartinavacuum,wouldproduceaforceof2×10−7newtonpermetreoflength;forluminousintensity,thecandela,definedastheintensityinagivendirectionofasourceemittingradiationoffrequency540×1012hertzandthathasaradiantintensityinthatdirectionof1/683wattpersteradian;foramountofsubstance,themole,definedascontainingasmanyelementaryentitiesofasubstanceasthereareatomsin0.012kgofcarbon-12;andforthermodynamictemperature,thekelvin.OnMay20,2019,theCGPMredefinedthekilogram,theampere,themole,andthekelvinintermsoffundamentalphysicalconstants.Forthekilogram,theconstantchosenwasPlanck’sconstant,whichisdefinedasequalto6.62607015×10−34joulesecond.Onejouleisequaltoonekilogramtimesmetresquaredpersecondsquared.Sincethesecondandthemetrewerealreadydefined,thekilogramwouldthenbedeterminedbyaccuratemeasurementsofPlanck’sconstant.Theamperewasredefinedsuchthattheelementarychargeisequalto1.602176634×10−19coulomb.ThekelvinwasredefinedsuchthattheBoltzmannconstantisequalto1.380649×10−23jouleperkelvin,andthemolewasredefinedsuchthattheAvogadroconstantisequalto6.02214076×1023permole.WidelyusedunitsintheSIsystemAlistofthewidelyusedunitsintheSIsystemisprovidedinthetable. InternationalSystemofUnits(SI) unit abbreviation physicalquantity Baseunits metre m length second s time kilogram kg mass ampere A electriccurrent kelvin K thermodynamictemperature candela cd luminousintensity mole mol amountofsubstance unit abbreviation numberofmetres approximateU.S.equivalent Length kilometre km 1,000 0.62mile centimetre cm 0.01 0.39inch millimetre mm 0.001 0.039inch micrometre μm 0.000001 0.000039inch nanometre nm 0.000000001 0.000000039inch unit abbreviation numberofsquaremetres approximateU.S.equivalent Area squarekilometre sqkm,orkm2 1,000,000 0.3861squaremile hectare ha 10,000 2.47acres are a 100 119.60squareyards squarecentimetre sqcm,orcm2 0.0001 0.155squareinch unit abbreviation numberofcubicmetres approximateU.S.equivalent Volume cubicmetre m3 1 1.307cubicyards cubiccentimetre cucm,cm3,orcc 0.000001 0.061cubicinch unit abbreviation numberoflitres approximateU.S.equivalent Capacity kilolitre kl 1,000 1.31cubicyards litre l 1 61.02cubicinches centilitre cl 0.01 0.61cubicinch millilitre ml 0.001 0.061cubicinch microlitre μl 0.000001 0.000061cubicinch unit abbreviation numberofgrams approximateU.S.equivalent Massandweight metricton t 1,000,000 1.102shorttons gram g 1 0.035ounce centigram cg 0.01 0.154grain milligram mg 0.001 0.015grain microgram μg 0.000001 0.000015grain unit symbol physicalquantity expressedinbaseunits Energy hertz Hz frequency 1/s newton N force,weight (m×kg)/s2 joule J work,energy,quantityofheat (m2×kg)/s2 pascal Pa pressure,stress kg/(m×s2) watt W power (m2×kg)/s3 coulomb C electriccharge s×A volt V electricpotentialdifference (m2×kg)/(s3×A) farad F electriccapacitance (s2×s2×A2)/(m2×kg) ohm Ω electricresistance,reactance (m2×kg)/(s3×A2) siemens S electricconductance (s3×A2)/(m2×kg) weber Wb magneticflux (m2×kg)/(s2×A) tesla T magneticinduction kg/(s2×A) henry H inductance (m2×kg)/(s2×A2) lumen lm luminousflux cd×sr lux lx illuminance (cd×sr)/m2 MetricconversionsAlistofmetricconversionsisprovidedinthetable. CommonequivalentsandconversionfactorsforU.S.CustomaryandSIsystems approximatecommonequivalents *CommontermnotusedinSI. **Exact. Source:NationalBureauofStandardsWallChart. 1inch =25millimetres 1foot =0.3metre 1yard =0.9metre 1mile =1.6kilometres 1squareinch =6.5squarecentimetres 1squarefoot =0.09squaremetre 1squareyard =0.8squaremetre 1acre =0.4hectare* 1cubicinch =16cubiccentimetres 1cubicfoot =0.03cubicmetre 1cubicyard =0.8cubicmetre 1quart(liq) =1litre* 1gallon =0.004cubicmetre 1ounce(avdp) =28grams 1pound(avdp) =0.45kilogram 1horsepower =0.75kilowatt 1millimetre =0.04inch 1metre =3.3feet 1metre =1.1yards 1kilometre =0.6mile(statute) 1squarecentimetre =0.16squareinch 1squaremetre =11squarefeet 1squaremetre =1.2squareyards 1hectare* =2.5acres 1cubiccentimetre =0.06cubicinch 1cubicmetre =35cubicfeet 1cubicmetre =1.3cubicyards 1litre* =1quart(liq) 1cubicmetre =264gallons 1gram =0.035ounce(avdp) 1kilogram =2.2pounds(avdp) 1kilowatt =1.3horsepower conversionsaccuratewithin10partspermillion inches×25.4** =millimetres feet×0.3048** =metres yards×0.9144** =metres miles×1.60934 =kilometres squareinches×6.4516** =squarecentimetres squarefeet×0.0929030 =squaremetres squareyards×0.836127 =squaremetres acres×0.404686 =hectares cubicinches×16.3871 =cubiccentimetres cubicfeet×0.0283168 =cubicmetres cubicyards×0.764555 =cubicmetres quarts(liq)×0.946353 =litres gallons×0.00378541 =cubicmetres ounces(avdp)×28.3495 =grams pounds(avdp)×0.453592 =kilograms horsepower×0.745700 =kilowatts millimetres×0.0393701 =inches metres×3.28084 =feet metres×1.09361 =yards kilometres×0.621371 =miles(statute) squarecentimetres×0.155000 =squareinches squaremetres×10.7639 =squarefeet squaremetres×1.19599 =squareyards hectares×2.47105 =acres cubiccentimetres×0.0610237 =cubicinches cubicmetres×35.3147 =cubicfeet cubicmetres×1.30795 =cubicyards litres×1.05669 =quarts(liq) cubicmetres×264.172 =gallons grams×0.0352740 =ounces(avdp) kilograms×2.20462 =pounds(avdp) kilowatts×1.34102 =horsepower TheEditorsofEncyclopaediaBritannicaThisarticlewasmostrecentlyrevisedandupdatedbyErikGregersen.