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

120

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

3

check the docs

What the unit families tell you

Apple names a unit family for 116 of the 120 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

4of these identifiers carry neither an abstract nor a discussion in Apple’s reference as of 2026-08-28: appleSleepingBreathingDisturbances, estimatedWorkoutEffortScore, workoutEffortScore, hypertensionEvent. All were introduced in iOS 18. 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.

Questions