Open the Settings app
Tap General
Tap About
Find the Serial Number field
If needed, press and hold the serial number to copy it
Check the original iPhone box for the serial number label
Check the SIM tray on some iPhone models for the serial number
Connect the iPhone to a Mac or PC and view it in Finder or iTunes
Use the Apple ID website and select the device to view its serial number
If the iPhone will not turn on, check the back of the device or the purchase receipt for the serial number