A UDID (Unique Device Identifier) is the hexadecimal string Apple uses to identify a specific iPhone, iPad, Apple Watch, Apple TV, Mac, or Vision Pro inside your developer account. Registering a UDID adds that device to your team's device list and is the prerequisite for installing a Development or Ad Hoc build on it.
Format
- iPhone 7 and earlier, original iPads: 40 hex characters.
- iPhone 8 and later, modern iPads, Apple TV, Apple Watch, Vision Pro: 25 characters with a dash after the eighth character (e.g. `00008101-001234567890ABCD`).
- Macs: a Provisioning UDID format that does not match the hardware serial.
How to find one
- On a Mac with the device plugged in: Finder > device > the model name field flips to UDID when clicked.
- Through Xcode > Devices and Simulators > device > Identifier field.
- From the tester's iPhone alone: ask them to visit a UDID profile service that installs a configuration profile and emails the UDID back. Apple no longer exposes the UDID directly in Settings.
The 100-device cap
Apple Developer Program teams can register up to 100 devices per device class per membership year. Device classes are iPhone, iPad, Mac, Apple Watch, Apple TV, and Vision Pro, counted separately. So you can have 100 iPhones plus 100 iPads in the same year. The counter only resets at annual membership renewal; removing a device during the year frees the slot for Ad Hoc builds but the counter does not decrement.