HKUnit Families by Quantity Type

Every HealthKit quantity sample stores a number and the unit that number is in, and the two are inseparable — a stored double without its unit is not a weight, a distance or an energy, it is just a double. This page lists the unit family each quantity type belongs to. 116 of the 120 quantity types in the dataset resolve to one. Four do not.

Apple's HKUnit documentation, read 2026-09-04, describes the raw material: "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)." Beyond that flat list it adds: "HealthKit also supports creating complex units by mathematically combining existing units." The combining is done by explicit methods — unitDivided(by:) "Creates a complex unit by dividing the receiving unit by another unit," and there is a matching multiply, a raise-to-power, and a reciprocal. There is also a string route in, init(from:), which "Returns the unit instance described by the provided string."

That is why the families in the table below are not all simple nouns. A heart rate is not a count and not a time; it is count/time, a count divided by a minute, and the same construction shape covers respiratoryRate, restingHeartRate and walkingHeartRateAverage. vo2Max is the deepest of them, volume/mass/time — a volume of oxygen per unit of body mass per unit of time, three units combined, which no single named unit can express. peakExpiratoryFlowRate is volume/time. The sport speeds (runningSpeed, cyclingSpeed, rowingSpeed, paddleSportsSpeed, crossCountrySkiingSpeed, walkingSpeed) are distance over time, and cyclingCadence is counts per minute.

The simple families carry most of the population. Mass carries the largest share of the population, because the dietary types all live there — every logged gram of protein, sodium or fat — alongside bodyMass and leanBodyMass. Length holds the distance types, from distanceWalkingRunning through distanceSwimming to the iOS 18.0 additions distanceRowing and distancePaddleSports. Energy holds activeEnergyBurned, basalEnergyBurned and dietaryEnergyConsumed. Count holds the things that are genuinely dimensionless — stepCount, flightsClimbed, pushCount, swimmingStrokeCount, and bodyMassIndex, which is an index rather than a measurement. Smaller families are more specific than you might expect: bloodGlucose is mass/volume, electrodermalActivity is conductance, insulinDelivery is international units, the audio types are sound pressure, and blood pressure is pressure.

The four quantity types with no unit family stated are appleSleepingBreathingDisturbances, estimatedWorkoutEffortScore, physicalEffort and workoutEffortScore. Three of those four also have no abstract and no stated aggregation style. physicalEffort is the exception: Apple describes it as "A quantity sample type that measures the estimated amount of energy being used to perform a task excluding other factors such as temperature, altitude, or heart rate" and states discrete aggregation, but does not name a unit.

One note on how these values were produced. The families were derived from Apple's documentation prose, read on 2026-08-28 for the HealthKit identifier dataset, and the first version of the parser excluded the "/" character — so every compound unit resolved to nothing and heart rate rendered with no unit at all. Widening it took the resolved count to 116 of 120. Where the source does not state a unit, the field is null rather than filled in.

For the other half of reading a quantity correctly, see sum or average. Per-metric detail lives on heart rate and VO2 max, and unit handling across providers is covered in normalizing wearable data.