• Go to your device setting, then click on your device. Look at the the URL of the page, the numbers after "tracker." is your Fitbits I'd.• Serial numbers are typically found on the bottom of the packaged unit under the top-most barcode.
On the back of the watch.
You can find the serial number on the thin bottom of the unit, near the input for the AC Adapter. (Ex. XAW70000000000)