SI Units | NIST

The SI is made up of 7 base units that define the 22 derived units with special names and symbols, which are illustrated in NIST SP 1247, SI Base Units ... Skiptomaincontent Officialwebsitesuse.gov A.govwebsitebelongstoanofficialgovernmentorganizationintheUnitedStates. Secure.govwebsitesuseHTTPS Alock( Lock Alockedpadlock )orhttps://meansyou’vesafelyconnectedtothe.govwebsite.Sharesensitiveinformationonlyonofficial,securewebsites. https://www.nist.gov/pml/weights-and-measures/metric-si/si-units PhysicalMeasurementLaboratory WeightsandMeasures LaboratoryHomepage DivisionHomepage AboutOWM ExpandorCollapse CalendarofEvents ResourcesforContinuingEducationandTraining StaffandTechnicalExpertsDirectory Programs ExpandorCollapse InternationalLegalMetrology ExpandorCollapse DocumentaryStandards GlossaryofAcronyms ILMPOrganizationalPrimer OIMLResources LaboratoryMetrology ExpandorCollapse CalibrationProcedures StandardOperatingProcedures GoodLaboratoryPractices GoodMeasurementPractices ProficiencyTesting StateLabProgramResources LabMetrologyTraining LawsandRegulations ExpandorCollapse FuelsandLubricantsSubcommittee L&RResources PackagingandLabeling PreciousMetalsConversion PriceVerification RetailandUnitPricingLaws LegalMetrologyDevices ExpandorCollapse Hydrogen:U.S.NationalWorkGroup Taximeters:U.S.NationalWorkGroup ElectricVehicleFuelingandSubmetering NTEP TrainingMaterials Metric(SI) ExpandorCollapse SIUnits UnderstandingMetric EverydaywithMetric SIEducation&Training UnitConversion MetricPolicy Resources ExpandorCollapse OWMContactsSystems ExpandorCollapse HowtoResetPasswordUsingtheContactsSystem(PDF) QuickGuideforOWMContactsSystem(PDF) HowtoRequestPublicationsusingOWMContactsSystem(PDF) HowtoRequestTraining&TranscriptUsingtheContactsSystem(PDF) NIST/OWMSecurityandPrivacyPolicies StateDirectorsA-C ExpandorCollapse StateDirectorsD-H StateDirectorsI-L StateDirectorsM StateDirectorsN StateDirectorsO-S StateDirectorsT-V StateDirectorsW-Z StateLaboratoriesA-C ExpandorCollapse StateLaboratoriesD-H StateLaboratoriesI-L StateLaboratoriesM StateLaboratoriesN StateLaboratoriesO-S StateLaboratoriesT-V StateLaboratoriesW-Z TheNationalConferenceonWeightsandMeasures(NCWM) U.S.W&MandMetrologistDirectory Publications ExpandorCollapse MeasurementMattersNewsletter NISTHandbooks ExpandorCollapse Handbook44CurrentEdition Handbook130CurrentEdition Handbook133CurrentEdition NISTSpecialPublications InternalReports(NISTIR) MetricPublications NCWMAnnualReports SIUnits Share Facebook Linkedin Twitter Email TheSIrestsonafoundationofseven(7)definingconstants:thecesiumhyperfinesplittingfrequency,thespeedoflightinvacuum,thePlanckconstant,theelementarycharge(i.e.thechargeonaproton),theBoltzmannconstant,theAvogadroconstant,andtheluminousefficacyofaspecifiedmonochromaticsource.Definitionsofallseven(7)SIbaseunitsareexpressedusinganexplicit-constantformulationandexperimentallyrealizedusingaspecificmisesenpratique(practicaltechnique). ThesevenSIbaseunits,whicharecomprisedof: Length-meter(m) Time-second(s) Amountofsubstance-mole(mole) Electriccurrent-ampere(A) Temperature-kelvin(K) Luminousintensity-candela(cd) Mass-kilogram(kg)   TheInternationalSystemofUnits(SI),commonlyknownasthemetricsystem,istheinternationalstandardformeasurement.TheInternationalTreatyoftheMeterwassignedinParisonMay20,1875byseventeencountries,includingtheUnitedStatesandisnowcelebratedaroundtheglobeasWorldMetrologyDay.NISTprovidesofficialU.S.representationinthevariousinternationalbodiesestablishedbytheMeterConvention:CGPM-GeneralConferenceonWeightsandMeasures;CIPM -InternationalCommitteeforWeightsandMeasures;andBIPM-TheInternationalBureauofWeightsandMeasures. TheSIismadeupof7baseunitsthatdefinethe22derivedunitswithspecialnamesandsymbols,whichareillustratedinNISTSP1247,SIBaseUnitsRelationshipPoster. TheSIplaysanessentialroleininternationalcommerceandiscommonlyusedinscientificandtechnologicalresearchanddevelopment.LearnmoreabouttheSIinNISTSP330andSP811. TableofSIbaseunits. Credit: NIST ResourcesforStudentsandTeachers RedefiningtheSI –InNovember2018,theworld’smeasurementexpertsvotedandunanimouslyapprovedarevisionoftheSIthatestablishesameasurementsystementirelybasedonphysicalconstantsofnature.ThechangesbecameeffectiveonWorldMetrologyDay,May20,2019.(NIST) LearnmoreabouttheRoadtotheRevisedSI.LearnmoreabouttheSIRedefinition.(NIST) TheLastArtifactdocumentaryandcompanionGrade5to12educationalresources,whichdocumenttheworkthatwentonbehindthescenestomodernizetheInternationalSystemofUnits(SI).(MontanaPBS) NISTSP1247SIBaseUnitsRelationshipsPoster–acolorfulposterillustratingtherelationshipsoftheInternationalSystemofUnits(SI)derivedunitswithspecialnamesandsymbolsandtheseventraditionalbaseunits.(NIST) MetricTriviaQuiz–Howmuchdoyouknowaboutthemetricsystem(SI)?TrytheNISTMetricTriviaQuizonlineorusetheAlexaskilltotestyourknowledgeandbeonyourwaytothinkingmetric!(NIST) SIEducationandTraining–ExploreNISTmetricsystemeducationresourcesthatbuildSImeasurementsystemfamiliarityandfluency.(NIST) KidsEducationResources.(NIST) BustingMythsabouttheMetricSystem.(NISTTakingMeasureBlog) Resources BecomingFamiliarwithSI EverydayEstimation MetricationFAQs Prefixes SIPublications UnderstandingMetric WritingwithMetricUnits NationalMetricWeek Metric Contacts MetricProgram [email protected] CreatedApril12,2010,UpdatedDecember6,2021