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# HKQuantity

An object that stores a value for a given unit.

```
class HKQuantity
```

## Overview

HealthKit uses quantity objects to store numerical data. When you create a quantity, you provide both the quantity’s value and unit.

Quantities are immutable objects: Their values are set when the object is first created and cannot change.

### Converting Units

You can request the value from a quantity object in any compatible units. For example,  if you create a length quantity in feet, you can then request the length in meters. The quantity object automatically converts its value to the requested units.

### Using Quantities

As with many HealthKit classes, don’t subclass the [`HKQuantity`](/documentation/HealthKit/HKQuantity) class. To help promote sharing data between apps, [`HKQuantity`](/documentation/HealthKit/HKQuantity) objects use only the units defined by the [`HKUnit`](/documentation/HealthKit/HKUnit) class.

## Topics

### Creating Quantities

[`init(unit:doubleValue:)`](/documentation/HealthKit/HKQuantity/init(unit:doubleValue:))

Instantiates and returns a new quantity object.

### Working With Units

[`is(compatibleWith:)`](/documentation/HealthKit/HKQuantity/is(compatibleWith:))

Returns a boolean value indicating whether the quantity is compatible with the provided unit.

[`doubleValue(for:)`](/documentation/HealthKit/HKQuantity/doubleValue(for:))

Returns the quantity’s value in the provided unit.

### Comparing Quantities

[`compare(_:)`](/documentation/HealthKit/HKQuantity/compare(_:))

Compares two values after converting them to the same units.


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