Postal S10 number — validate a UPU tracking number
International postal items carry a 13-character S10 identifier: two service letters (RR, EE, CP…), an 8-digit serial whose last digit is a weighted mod-11 check digit, and the ISO 3166 code of the origin post. Type a number and see which part fails, if any.
Try it — live validator
Runs the same algorithm TrackJet’s detector uses, locally in your browser. Zero network calls.
The rule
Weights 8,6,4,2,3,5,9,7 over the first 7 digits; 11 − (sum mod 11); 10 → 0, 11 → 5. Service letters and country code come from the UPU and ISO lists TrackJet ships.