Every HealthKit type identifier

Four families, four sets of rules

HealthKit does not have one identifier type, it has four, and they behave differently enough that treating them alike is a common source of bugs. Only quantity types are numeric samples you can aggregate. Category types carry a value drawn from a fixed enum, and reading one without knowing which enum decodes it is meaningless — so that enum is a column here. Characteristics are read-only facts about the user that your app can never write. Workout activity types are labels for what a workout was, not data in their own right.

HKQuantityTypeIdentifier

121

HKCategoryTypeIdentifier

30

HKCharacteristicTypeIdentifier

6

HKWorkoutActivityType

84

The one distinction that changes your query

A quantity type is either cumulative — it accumulates over an interval, like stepCount or activeEnergyBurned — or discrete, a point-in-time reading like heartRate or bodyMass. Cumulative types are summed with .cumulativeSum. Discrete types are reduced with .discreteAverage, .discreteMin or .discreteMax.

Apply the wrong one and nothing throws. You get a number, it renders in your UI, and it is wrong — a summed heart rate for a week reads in the tens of thousands and looks like a bug in someone else’s code. Apple states which is which inside each type’s discussion prose rather than exposing it as a property, so the compiler cannot help you. That is why it is a column here.

Cumulative

64

sum them

Discrete

53

average them

Not stated

4

check the docs

What the unit families tell you

Apple names a unit family for 116 of the 121 quantity types — the only family that carries units at all. It matters because HKQuantity will happily convert between any two compatible units and throw only when they are incompatible — so a distance read in metres and rendered as miles is a silent bug, not a crash.

• mass 39
• length 15
• count 11
• time 6
• distance per time 6
• temperature 4

The types Apple ships undocumented

5 of these identifiers carry neither an abstract nor a discussion in Apple’s reference as of 2026-10-02: appleSleepingBreathingDisturbances, estimatedWorkoutEffortScore, heartRateVariabilityRMSSD, workoutEffortScore, hypertensionEvent. They arrived in iOS 18.0 (3), iOS 26.2 and iOS 27.0. The identifiers compile and the types exist; what they contain and how Apple derives them is simply not stated. Treat anything you infer from observed values as unverified.

Going cross-platform

An identifier existing on iOS says nothing about Android. Our HealthKit ↔ Health Connect reference maps the metrics we could verify on both platforms, including the ones that look equivalent and are not — Apple stores HRV as SDNN while Health Connect stores RMSSD, and those are different calculations that must not be normalised into one field. The heartRateVariabilityRMSSD row above, new in iOS 27.0, does not change that yet: as of 2026-10-02 Apple’s documentation does not say what it measures, how it is computed or whether Apple Watch writes it.

Questions