If you can't open the app, look on the original packaging or box the Zip came in.
Finding the serial number (SN) on a Fitbit Zip is crucial for support, as it is often worn off or tiny on the back of the device. Fitbit Zip Serial Number
If the physical number is worn off or unreachable, you can find it in the Fitbit app by going to Account > Your Device, or via your desktop dashboard settings. If you can't open the app, look on
Having this 12-digit/letter sequence saved ensures you can easily replace it via warranty or prove it is yours if it is returned. File a warranty claim for your Fitbit device - Google Help If you can't open the app