Location of the phone

If your device is powered off, the battery has run out, or more than 24 hours have passed since its last location was sent to Apple, you can't see your device's location.

Otherwise, you can Use the Find My app to locate a missing device or item - Apple Support.

njoud5 wrote:

How do i know the location of the phone if the iphone is powered off ?

You can't if it's powered Off.