Will your watch still work if your phone is powered off?
Can I power off my phone and still use my Apple Watch?
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Yes - subject to certain conditions, many features may remain available when your iPhone is turned off.
The full range of features - including calling and texting - is available when Apple Watch and iPhone are connected via Bluetooth. Whenever possible, the devices will connect via this method.
You don't necessarily need to have your iPhone on or immediately around you (within arm's reach, for example) for the devices to establish and maintain their Bluetooth connection. Normal Bluetooth range is around 33 feet / 10 meters (this will vary in practice due to wireless interference).
Whenever your watch can't connect to the iPhone via Bluetooth, it will attempt, as a fallback, to connect to a known, compatible Wi-Fi network.
When instead connected to Wi-Fi, the watch also still offers a number of features, including receiving and sending messages via iMessage. If your carrier offers Wi-Fi Calling and it is supported on your plan and enabled on your iPhone, you can also use Apple Watch to make calls and (subject to carrier support) send and receive texts.
If your watch is unable to connect either via Bluetooth or Wi-Fi, it again still offers some - although fewer - features.
Features that remain available in either case are detailed here:
How to use your Apple Watch without your iPhone nearby - Apple Support
For the best performance from your iPhone and Apple Watch, including the most power-efficient communication between them, Apple recommends keeping both Bluetooth and Wi-Fi permanently enabled on your iPhone.
More information: