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# Units and quantities

Objects used to specify a quantity for a given unit, and to convert between units.

## Topics

### Units and quantities

  <doc://com.apple.healthkit/documentation/HealthKit/defining-and-converting-units-and-quantities>

[`HKQuantity`](/documentation/HealthKit/HKQuantity)

An object that stores a value for a given unit.

[`HKUnit`](/documentation/HealthKit/HKUnit)

A class for managing the units of measure within HealthKit.

[`HKMetricPrefix`](/documentation/HealthKit/HKMetricPrefix)

Prefixes that can be added to SI units to change the order of magnitude.


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