# HKUserPreferencesDidChange Notifies observers whenever the user changes his or her preferred units. ``` static let HKUserPreferencesDidChange: NSNotification.Name ``` ## Discussion The preferred units are the units that the user prefers for a given measurement type. By default, the preferred units are based on the device’s current locale. For example, in the US, the preferred units for the identifier are pounds. Other regions may use kilograms or stones. However, users can change their preferred units in the Health app at any time. Each `HKHealthStore` object posts its own `HKUserPreferencesDidChangeNotification` notification. To avoid receiving duplicate notifications, always pass a health store instance to the notification center when adding observers. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)