The *IfcMeasureResource* schema specifies units and defined measure types that may be assigned to quantities.

NOTE The fundamental unit types used in this schema are based on the SI system defined in ISO 1000+A1, 1992, 1998. Units in measurement systems other than SI may be derived using this schema. Many definitions declared in theIfcMeasureResourceschema is adapted from ISO 10303-41

NOTE In the definitions of the unit exponents the use of superscript font has been omitted. Therefore, m2 means square metre, m3 means cubic meter.

In different types of units there are five basic cases:

- Basic SI-units, which cover a number of fundamental units of mainly physical quantities defined by ISO-1000 such as meter or millimeter as unit for length measure or square meter as a unit for area measure. The unit may have a scaling prefix (for example: milli, kilo).
- Derived SI-units, which are defined as a derivation of the basic SI-units, for example, Newton (kg m / s2) as a unit of force. Both basic and derived SI-units are in the IFCs represented by IfcSIUnit.
- Conversion based units, which can be derived from SI-units by a scaling factor; e.g. inch which can be defined using SI-unit for length measure, i.e. an inch is 25.4 millimeters.
- Derived units, which can be defined as a derivation or combination of a number of basic units. In a derived unit each of the basic unit "component" has a dimensional exponent in defining the derived unit; e.g. kg / m2, where kilogram (kg) has dimensional exponent 1 and meter (m) has exponent -2.
- Context dependent units, which cannot be defined as conversion based unit using SI-units.

With regard to the usage of the measure defined types (for example, IfcLengthMeasure, IfcTimeMeasure) as attribute datatypes in this specification, there are three basic cases:

- The datatype of an attribute of an entity type is a measure defined type as such without possibility on an instance level to define the unit of the measure value:

In this case, it is the global unit assignment for the corresponding unit for the measure type that defines the unit for all the usages of this defined measure type (except for cases 2 and 3 below).`ENTITY IfcBoundingBox`

SUBTYPE OF(IfcGeometricRepresentationItem);

Corner : IfcCartesianPoint;

XDim : IfcPositiveLengthMeasure;

YDim : IfcPositiveLengthMeasure;

ZDim : IfcPositiveLengthMeasure;

END_ENTITY;

- The datatype of an attribute is IfcMeasureWithUnit, which allows for definition of unit per instance of that entity type, independent and possibly overriding the global unit assignment:

In this case the relevant measure defined type (from the IfcMeasureWithUnit.ValueComponent : IfcValue select list) is not exactly defined by the schema, but implied by the context.`ENTITY IfcConversionBasedUnit`

SUPERTYPE OF (ONEOF

(IfcConversionBasedUnitWithOffset))

SUBTYPE OF (IfcNamedUnit);

Name : IfcLabel;

ConversionFactor : IfcMeasureWithUnit;

END_ENTITY;

- The entity type has a separate "unit" attribute which allows for defining the unit for another attribute of the entity type for representing the actual value:
`ENTITY IfcPropertySingleValue`

SUBTYPE OF (IfcSimpleProperty);

NominalValue : IfcValue;

Unit : OPTIONAL IfcUnit;

END_ENTITY;

Although in the cases 2 and 3 different units could be used for different instances of the same entity type or for the same measure type in attributes of different entity types, it is recommended not to mix different units for same measure defined types, if it can be avoided. Below some examples of each of the above basic cases are given.

NOTE In the example instantiations in the form of IFC data exchange files, mainly the measure and unit -relevant attributes are given the values; the other attributes are given no values (in the form of $-sign) independent of whether they should actually have values because of being nonoptional attributes.

Table 684 indicates measures, units, and corresponding data types.

MeasureTypeUnitSymbolDerivationIfcSiUnitEnumIfcUnitEnumIfcDerivedUnitEnumDefined value typesAbsorbed dose, specific energy

impact, kerma, absorbed dose indexSI / Derived gray Gy J / kg GRAY ABSORBEDDOSEUNIT IfcAbsorbedDoseMeasure Acceleration Derived m / s2 ACCELERATIONUNIT IfcAccelerationMeasure Acidity (pH) Derived pH pH mol / l PHUNIT IfcPHMeasure Activity (of radionuclide) SI / Derived becquerel Bq 1 / s BECQUEREL RADIOACTIVITYUNIT IfcRadioActivityMeasure Amount of substance SI/Basic mole mol MOLE AMOUNTOFSUBSTANCEUNIT IfcAmountOfSubstanceMeasure Angular velocity Derived rad / s ANGULARVELOCITYUNIT IfcAngularVelocityMeasure Area SI/Derived square metre m2 m2 SQUARE_METRE AREAUNIT IfcAreaMeasure Area density Derived kg / m2 AREADENSITYUNIT IfcAreaDensityMeasure - IfcBoolean - IfcComplexNumber Compound plane angle Compound degree, min, s COMPOUNDPLANEANGLEUNIT IfcCompoundPlaneAngleMeasure - IfcContextDependentMeasure - IfcCountMeasure - IfcDescriptiveMeasure Capacitance SI / Derived farad F C / V FARAD ELECTRICCAPACITANCEUNIT IfcElectricCapacitanceMeasure Celsius temperature SI / Basic degree Celsius ºC 1 ºC = 1 K DEGREE_CELSIUS THERMODYNAMICTEMPERATUREUNIT IfcThermodynamicTemperatureMeasure Curvatue Derived rad / m CURVATUREUNIT IfcCurvatureMeasure - IfcDate

(lexical representation according to ISO 8601)- IfcDateTime

(lexical representation according to ISO 8601)Dose equivalent, dose equivalent

indexSI / Derived sievert Sv J / kg SIEVERT DOSEEQUIVALENTUNIT IfcDoseEquivalentMeasure - IfcDuration

(lexical representation according to ISO 8601)Dynamic viscosity Derived Pa · s DYNAMICVISCOSITYUNIT IfcDynamicViscosityMeasure Electric charge,

quantity of electricitySI / Derived coulomb C A · s COULOMB ELECTRICCHARGEUNIT IfcElectricChargeMeasure Electric conductance SI / Derived siemens S 1 / W SIEMENS ELECTRICCONDUCTANCEUNIT IfcElectricConductanceMeasure Electric current SI / Basic ampere A AMPERE ELECTRICCURRENTUNIT IfcElectricCurrentMeasure Electric potential, potential

difference, tension, electromotive forceSI / Derived volt V W / A VOLT ELECTRICVOLTAGEUNIT IfcElectricVoltageMeasure Electric resistance SI / Derived ohm W V / A OHM ELECTRICRESISTANCEUNIT IfcElectricResistanceMeasure Energy, work, quantity of heat SI / Derived joule J N · m JOULE ENERGYUNIT IfcEnergyMeasure Force SI / Derived newton N kg · m / s2 NEWTON FORCEUNIT IfcForceMeasure Frequency SI / Derived hertz Hz 1 / s HERTZ FREQUENCYUNIT IfcFrequencyMeasure Heat flux density Derived W / m2 HEATFLUXDENSITYUNIT IfcHeatFluxDensityMeasure Heating value Derived J / kg HEATINGVALUEUNIT IfcHeatingValueMeasure - IfcIdentifier Illuminance SI / Derived lux lx lm / m2 LUX ILLUMINANCEUNIT IfcIlluminanceMeasure Inductance SI / Derived henry H Wb / A HENRY INDUCTANCEUNIT IfcInductanceMeasure - IfcInteger (Integer) Count rate Derived 1 / s INTEGERCOUNTRATEUNIT IfcIntegerCountRateMeasure Ion concentration Derived g / l IONCONCENTRATIONUNIT IfcIonConcentrationMeasure Isothermal moisture capacity Derived m3 / kg ISOTHERMALMOISTURECAPACITYUNIT IfcIsothermalMoistureCapacityMeasure Kinematic viscosity Derived m2 / s KINEMATICVISCOSITYUNIT IfcKinematicViscosityMeasure Length SI / Basic metre m METRE LENGTHUNIT IfcLengthMeasure - IfcLabel Linear force Derived N / m LINEARFORCEUNIT IfcLinearForceMeasure Linear moment Derived N · m / m LINEARMOMENTUNIT IfcLinearMomentMeasure Linear stiffness Derived N / m LINEARSTIFFNESSUNIT IfcLinearStiffnessMeasure Linear velocity Derived m / s LINEARVELOCITYUNIT IfcLinearVelocityMeasure - IfcLogical Luminous flux SI / Derived lumen lm cd · sr LUMEN LUMINOUSFLUXUNIT IfcLuminousFluxMeasure Luminous intensity SI / Basic candela cd CANDELA LUMINOUSINTENSITYUNIT IfcLuminousIntensityMeasure Luminous intensity distribution Derived cd / lm LUMINOUSINTENSITYDISTRIBUTIONUNIT IfcLuminousIntensityDistributionMeasure Magnetic flux SI / Derived weber Wb V · s WEBER MAGNETICFLUXUNIT IfcMagneticFluxMeasure Magnetic flux density SI / Derived tesla T Wb / m2 TESLA MAGNETICFLUXDENSITYUNIT IfcMagneticFluxDensityMeasure Mass SI / Basic gram g (kg) GRAM MASSUNIT IfcMassMeasure Mass density Derived kg / m3 MASSDENSITYUNIT IfcMassDensityMeasure Mass flow rate Derived kg / s MASSFLOWRATEUNIT IfcMassFlowRateMeasure Mass per length Derived kg / m MASSPERLENGTHUNIT IfcMassPerLengthMeasure Modulus of elasticity Derived N / m2 MODULUSOFELASTICITYUNIT IfcModulusOfElasticityMeasure Modulus of linear subgrade reaction Derived N / m2 MODULUSOFLINEARSUBGRADEREACTIONUNIT IfcModulusOfLinearSubgradeReactionMeasure Modulus of rotational subgrade reaction Derived N · m / m · rad MODULUSOFROTATIONALSUBGRADEREACTIONUNIT IfcModulusOfRotationalSubgradeReactionMeasure Modulus of subgrade reaction Derived N / m3 MODULUSOFSUBGRADEREACTIONUNIT IfcModulusOfSubgradeReactionMeasure Moisture diffusivity Derived m3 / s MOISTUREDIFFUSIVITYUNIT IfcMoistureDiffusivityMeasure Molecular weight Derived g / mol MOLECULARWEIGHTUNIT IfcMolecularWeightMeasure Moment of inertia Derived m4 MOMENTOFINERTIAUNIT IfcMomentOfInertiaMeasure - IfcMonetaryMeasure (Non negative length) m LENGTHUNIT IfcNonNegativeLengthMeasure - IfcNormalisedRatioMeasure - IfcNumericMeasure - IfcParameterValue Planar force Derived N / m2 PLANARFORCEUNIT IfcPlanarForceMeasure Plane angle SI / Derived radian rad m / m = 1 RADIAN PLANEANGLEUNIT IfcPlaneAngleMeasure (Positive length) m LENGTHUNIT IfcPositiveLengthMeasure (Positive plane angle) rad PLANEANGLEUNIT IfcPositivePlaneAngleMeasure - IfcPositiveRatioMeasure Power SI / Derived watt W J / s WATT POWERUNIT IfcPowerMeasure Pressure, stress SI / Derived pascal Pa N / m2 PASCAL PRESSUREUNIT IfcPressureMeasure - IfcRatioMeasure - IfcReal Rotational frequency Derived cycles / s ROTATIONALFREQUENCYUNIT IfcRotationalFrequencyMeasure Rotational mass Derived kg · m2 ROTATIONALMASSUNIT IfcRotationalMassMeasure Rotational stiffness Derived N · m / rad ROTATIONALSTIFFNESSUNIT IfcRotationalStiffnessMeasure Sectional area integral Derived m5 SECTIONALAREAINTEGRALUNIT IfcSectionalAreaIntegralMeasure Section modulus Derived m3 SECTIONMODULUSUNIT IfcSectionModulusMeasure Shear modulus Derived N / m2 SHEARMODULUSUNIT IfcShearModulusMeasure Solid angle SI / Derived steradin sr m2 / m2 = 1 STERADIAN SOLIDANGLEUNIT IfcSolidAngleMeasure Sound power watt W W SOUNDPOWERUNIT IfcSoundPowerMeasure Sound power level decibel db W / W SOUNDPOWERLEVELUNIT IfcSoundPowerLevelMeasure Sound pressure pascal Pa Pa SOUNDPRESSUREUNIT IfcSoundPressureMeasure Sound pressure level decibel db Pa / Pa SOUNDPRESSURELEVELUNIT IfcSoundPressureLevelMeasure Specific heat capacity Derived J / kg· K SPECIFICHEATCAPACITYUNIT IfcSpecificHeatCapacityMeasure Temperature gradient K / m TEMPERATUREGRADIENTUNIT IfcTemperatureGradientMeasure Temperature change K / s TEMPERATURERATEOFCHANGEUNIT IfcTemperatureRateOfChangeMeasure - IfcText Thermal admittance Derived W / m2 · K THERMALADMITTANCEUNIT IfcThermalAdmittanceMeasure Thermal conductivity Derived W / m · K THERMALCONDUCTIVITYUNIT IfcThermalConductivityMeasure Thermal expansion coefficient 1 / K THERMALEXPANSIONCOEFFICIENTUNIT IfcThermalExpansionCoefficientMeasure Thermal resistance Derived m2 · K / W THERMALRESISTANCEUNIT IfcThermalResistanceMeasure Thermal transmittance Derived W / m2 · K THERMALTRANSMITTANCEUNIT IfcThermalTransmittanceMeasure Thermodynamic temperature SI / Basic kelvin K KELVIN THERMODYNAMICTEMPERATUREUNIT IfcThermodynamicTemperatureMeasure - IfcTime

(lexical representation according to ISO 8601)Time SI / Basic second s SECOND TIMEUNIT IfcTimeMeasure - IfcTimeStamp Torque Derived N· m TORQUEUNIT IfcTorqueMeasure Vapor permeability Derived kg / s · m · Pa VAPORPERMEABILITYUNIT IfcVaporPermeabilityMeasure Volume SI / Derived cubic metre m3 m3 CUBIC_METRE VOLUMEUNIT IfcVolumeMeasure Volumetric flow rate Derived m3 / s VOLUMETRICFLOWRATEUNIT IfcVolumetricFlowRateMeasure Warping constant m6 WARPINGCONSTANTUNIT IfcWarpingConstantMeasure Warping moment Derived N · m2 WARPINGMOMENTUNIT IfcWarpingMomentMeasure

Table 684 — Measures and units