# HKUnit A class for managing the units of measure within HealthKit. ``` class HKUnit ``` ## Overview The unit class supports most standard SI units (meters, seconds, and grams), SI units with prefixes (centimeters, milliseconds and kilograms) and equivalent non-SI units (feet, minutes, and pounds). HealthKit also supports creating complex units by mathematically combining existing units. You use units when working with HealthKit quantities. Quantities store both the value (as a `double` data type) and its corresponding unit. You can then request the value from the quantity in any compatible units. For more information on working with quantities, see [`HKQuantity`](/documentation/HealthKit/HKQuantity). > Number formatters that use units (for example, , , and ) use a custom enumeration to specify their units. For example, the class uses the enum. The ``doc://com.apple.healthkit/documentation/HealthKit/HKUnit`` class provides several methods to translate between the formatter enumerations and the HealthKit units. For more information, see Working with formatter units. ### Using Units As with many HealthKit classes, don’t subclass the [`HKUnit`](/documentation/HealthKit/HKUnit) class. The `HKUnit` class is implemented using a facade design pattern. It uses custom subclasses to represent instances of the different unit types. For example, the [`second()`](/documentation/HealthKit/HKUnit/second()) convenience method actually returns an instance of the private `HKTimeUnit` subclass. Additionally, the unit class uses a single unit instance to represent all copies of the same unit in your app, wherever possible. For example, two calls to the [`second()`](/documentation/HealthKit/HKUnit/second()) method return the same unit object. This helps reduce the amount of memory used by unit instances. ## Topics ### Working with units [`+ unitFromString:`](/documentation/HealthKit/HKUnit/init(from:)-9qont) Returns the unit instance described by the provided string. [`unitString`](/documentation/HealthKit/HKUnit/unitString) A string representation of the unit object. [`- isNull`](/documentation/HealthKit/HKUnit/isNull()) Returns a Boolean value indicating whether the unit is null. ### Working with formatter units [`+ energyFormatterUnitFromUnit:`](/documentation/HealthKit/HKUnit/energyFormatterUnit(from:)) Converts a HealthKit unit object into a corresponding energy formatter enumeration value. [`+ unitFromEnergyFormatterUnit:`](/documentation/HealthKit/HKUnit/init(from:)-1j1pq) Converts an energy formatter enumeration value into a corresponding HealthKit unit object. [`+ lengthFormatterUnitFromUnit:`](/documentation/HealthKit/HKUnit/lengthFormatterUnit(from:)) Converts a HealthKit unit object into a corresponding length formatter enumeration value. [`+ unitFromLengthFormatterUnit:`](/documentation/HealthKit/HKUnit/init(from:)-55e1u) Converts a length formatter enumeration value into a corresponding HealthKit object. [`+ massFormatterUnitFromUnit:`](/documentation/HealthKit/HKUnit/massFormatterUnit(from:)) Converts a HealthKit unit object into a corresponding mass formatter enumeration value. [`+ unitFromMassFormatterUnit:`](/documentation/HealthKit/HKUnit/init(from:)-7h2li) Converts a mass formatter enumeration value into a corresponding HealthKit unit object. ### Constructing mass units [`+ gramUnit`](/documentation/HealthKit/HKUnit/gram()) Returns a HealthKit unit for measuring mass in grams. [`+ gramUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/gramUnit(with:)) Returns a HealthKit unit for measuring mass, using gram units with the provided prefix. [`+ ounceUnit`](/documentation/HealthKit/HKUnit/ounce()) Returns a HealthKit unit for measuring mass in ounces. [`+ poundUnit`](/documentation/HealthKit/HKUnit/pound()) Returns a HealthKit unit for measuring mass in pounds. [`+ stoneUnit`](/documentation/HealthKit/HKUnit/stone()) Returns a HealthKit unit for measuring mass in stones. [`+ moleUnitWithMolarMass:`](/documentation/HealthKit/HKUnit/moleUnit(withMolarMass:)) Returns a HealthKit unit for measuring mass in moles for a given molar mass. [`+ moleUnitWithMetricPrefix:molarMass:`](/documentation/HealthKit/HKUnit/moleUnit(with:molarMass:)) Returns a HealthKit unit for measuring mass in moles, with the given prefix and molar mass. [`HKUnitMolarMassBloodGlucose`](/documentation/HealthKit/HKUnitMolarMassBloodGlucose) The molecular mass of blood glucose, typically used to create mole units for blood glucose. ### Constructing length units [`+ meterUnit`](/documentation/HealthKit/HKUnit/meter()) Returns a HealthKit unit for measuring length in meters. [`+ meterUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/meterUnit(with:)) Returns a HealthKit unit for measuring length, using meter units with the provided prefix. [`+ inchUnit`](/documentation/HealthKit/HKUnit/inch()) Returns a HealthKit unit for measuring length in inches. [`+ footUnit`](/documentation/HealthKit/HKUnit/foot()) Returns a HealthKit unit for measuring length in feet. [`+ yardUnit`](/documentation/HealthKit/HKUnit/yard()) Returns a HealthKit unit for measuring length in yards. [`+ mileUnit`](/documentation/HealthKit/HKUnit/mile()) Returns a HealthKit unit for measuring length in miles. ### Constructing volume units [`+ literUnit`](/documentation/HealthKit/HKUnit/liter()) Returns a HealthKit unit for measuring volume in liters. [`+ literUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/literUnit(with:)) Returns a HealthKit unit for measuring volume, using liter units with the provided prefix. [`+ fluidOunceUSUnit`](/documentation/HealthKit/HKUnit/fluidOunceUS()) Returns a HealthKit unit for measuring volume in US fluid ounces. [`+ fluidOunceImperialUnit`](/documentation/HealthKit/HKUnit/fluidOunceImperial()) Returns a HealthKit unit for measuring volume in imperial fluid ounces. [`+ cupUSUnit`](/documentation/HealthKit/HKUnit/cupUS()) Returns a HealthKit unit for measuring volume in US cups. [`+ cupImperialUnit`](/documentation/HealthKit/HKUnit/cupImperial()) Returns a HealthKit unit for measuring volume in imperial cups. [`+ pintUSUnit`](/documentation/HealthKit/HKUnit/pintUS()) Returns a HealthKit unit for measuring volume in US pints. [`+ pintImperialUnit`](/documentation/HealthKit/HKUnit/pintImperial()) Returns a HealthKit unit for measuring volume in imperial pints. ### Constructing pressure units [`+ pascalUnit`](/documentation/HealthKit/HKUnit/pascal()) Returns a HealthKit unit for measuring pressure in pascals. [`+ pascalUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/pascalUnit(with:)) Returns a HealthKit unit for measuring pressure, using pascal units with the provided prefix. [`+ millimeterOfMercuryUnit`](/documentation/HealthKit/HKUnit/millimeterOfMercury()) Returns a HealthKit unit for measuring pressure in millimeters of mercury. [`+ inchesOfMercuryUnit`](/documentation/HealthKit/HKUnit/inchesOfMercury()) Returns a HealthKit unit for measuring pressure in inches of mercury. [`+ centimeterOfWaterUnit`](/documentation/HealthKit/HKUnit/centimeterOfWater()) Returns a HealthKit unit for measuring pressure in centimeters of water. [`+ atmosphereUnit`](/documentation/HealthKit/HKUnit/atmosphere()) Returns a HealthKit unit for measuring pressure in atmospheres. [`+ decibelAWeightedSoundPressureLevelUnit`](/documentation/HealthKit/HKUnit/decibelAWeightedSoundPressureLevel()) Returns a HealthKit unit for measuring the difference between the local pressure and the ambient atmospheric pressure caused by sound. ### Constructing time units [`+ secondUnit`](/documentation/HealthKit/HKUnit/second()) Returns a HealthKit unit for measuring time in seconds. [`+ secondUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/secondUnit(with:)) Returns a HealthKit unit for measuring time, using second units with the provided prefix. [`+ minuteUnit`](/documentation/HealthKit/HKUnit/minute()) Returns a HealthKit unit for measuring time in minutes. [`+ hourUnit`](/documentation/HealthKit/HKUnit/hour()) Returns a HealthKit unit for measuring time in hours. [`+ dayUnit`](/documentation/HealthKit/HKUnit/day()) Returns a HealthKit unit for measuring time in days. ### Constructing energy units [`+ jouleUnit`](/documentation/HealthKit/HKUnit/joule()) Returns a HealthKit unit for measuring energy in joules. [`+ jouleUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/jouleUnit(with:)) Returns a HealthKit unit for measuring energy, using joule units with the provided prefix. [`+ kilocalorieUnit`](/documentation/HealthKit/HKUnit/kilocalorie()) Returns a HealthKit unit for measuring energy in kilocalories. [`+ largeCalorieUnit`](/documentation/HealthKit/HKUnit/largeCalorie()) Returns a HealthKit unit for measuring energy in large calories (Cal). [`+ smallCalorieUnit`](/documentation/HealthKit/HKUnit/smallCalorie()) Returns a HealthKit unit for measuring energy in small calories (cal). [`+ calorieUnit`](/documentation/HealthKit/HKUnit/calorie()) Returns a HealthKit unit for measuring energy in calories. ### Constructing power units [`+ wattUnit`](/documentation/HealthKit/HKUnit/watt()) Returns a HealthKit unit for measuring power in watts. [`+ wattUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/wattUnit(with:)) Returns a HealthKit unit for measuring power, using watt units with the provided prefix. ### Constructing temperature units [`+ degreeCelsiusUnit`](/documentation/HealthKit/HKUnit/degreeCelsius()) Returns a HealthKit unit for measuring temperature in degrees Celsius. [`+ degreeFahrenheitUnit`](/documentation/HealthKit/HKUnit/degreeFahrenheit()) Returns a HealthKit unit for measuring temperature in degrees Fahrenheit. [`+ kelvinUnit`](/documentation/HealthKit/HKUnit/kelvin()) Returns a HealthKit unit for measuring temperature in kelvins. ### Constructing hearing sensitivity units [`+ decibelHearingLevelUnit`](/documentation/HealthKit/HKUnit/decibelHearingLevel()) Returns a HealthKit unit for measuring the intensity of a sound. ### Constructing frequency units [`+ hertzUnit`](/documentation/HealthKit/HKUnit/hertz()) Returns a HealthKit unit for measuring frequency in hertz. [`+ hertzUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/hertzUnit(with:)) Returns a HealthKit unit for measuring frequency in hertz with the provided prefix. ### Constructing vision units [`+ diopterUnit`](/documentation/HealthKit/HKUnit/diopter()) Returns a HealthKit unit for measuring the optical power of a lens using diopter units. [`+ prismDiopterUnit`](/documentation/HealthKit/HKUnit/prismDiopter()) Returns a HealthKit unit for measuring the prismatic deviation of a lens using prism diopter units. ### Constructing angle units [`+ degreeAngleUnit`](/documentation/HealthKit/HKUnit/degreeAngle()) Returns a HealthKit unit for measuring angles using degrees. [`+ radianAngleUnit`](/documentation/HealthKit/HKUnit/radianAngle()) Returns a HealthKit unit for measuring angles using radians. [`+ radianAngleUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/radianAngleUnit(with:)) Returns a HealthKit unit for measuring angles, using radian units with the provided prefix. ### Constructing electrical conductance units [`+ siemenUnit`](/documentation/HealthKit/HKUnit/siemen()) Returns a HealthKit unit for measuring electrical conductance in siemens. [`+ siemenUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/siemenUnit(with:)) Returns a HealthKit unit for measuring electrical conductance, using siemen units with the provided prefix. ### Electrical potential difference [`+ voltUnit`](/documentation/HealthKit/HKUnit/volt()) Returns a HealthKit unit for measuring the difference in electrical potential using volts. [`+ voltUnitWithMetricPrefix:`](/documentation/HealthKit/HKUnit/voltUnit(with:)) Returns a HealthKit unit for measuring the electrical potential difference in volts with the provided prefix. ### Constructing pharmacology units [`+ internationalUnit`](/documentation/HealthKit/HKUnit/internationalUnit()) Returns a HealthKit unit that measures the amount of a biologically active substance in international units (IU). ### Constructing scalar units [`+ countUnit`](/documentation/HealthKit/HKUnit/count()) Returns a HealthKit unit for measuring counts. [`+ percentUnit`](/documentation/HealthKit/HKUnit/percent()) Returns a HealthKit unit for measuring percentages. ### Performing unit math [`- unitMultipliedByUnit:`](/documentation/HealthKit/HKUnit/unitMultiplied(by:)) Creates a complex unit by multiplying the receiving unit with another unit. [`- unitDividedByUnit:`](/documentation/HealthKit/HKUnit/unitDivided(by:)) Creates a complex unit by dividing the receiving unit by another unit. [`- unitRaisedToPower:`](/documentation/HealthKit/HKUnit/unitRaised(toPower:)) Creates a complex unit by raising the unit to the given power. [`- reciprocalUnit`](/documentation/HealthKit/HKUnit/reciprocal()) Returns a complex unit representing the unit’s reciprocal. ### Constants [`HKMetricPrefix`](/documentation/HealthKit/HKMetricPrefix) Prefixes that can be added to SI units to change the order of magnitude. ## Relationships ### Conforms To [`CustomStringConvertible`](/documentation/Swift/CustomStringConvertible) [`SendableMetatype`](/documentation/Swift/SendableMetatype) [`NSCoding`](/documentation/Foundation/NSCoding) [`NSObjectProtocol`](/documentation/ObjectiveC/NSObjectProtocol) [`NSSecureCoding`](/documentation/Foundation/NSSecureCoding) [`Equatable`](/documentation/Swift/Equatable) [`Hashable`](/documentation/Swift/Hashable) [`Sendable`](/documentation/Swift/Sendable) [`CVarArg`](/documentation/Swift/CVarArg) [`NSCopying`](/documentation/Foundation/NSCopying) [`CustomDebugStringConvertible`](/documentation/Swift/CustomDebugStringConvertible) ### Inherits From [`NSObject-swift.class`](/documentation/ObjectiveC/NSObject-swift.class) --- Copyright © 2026 Apple Inc. 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