How the GTIN check digit works (and why yours might be wrong)

Updated July 2026

Every GTIN ends in a check digit computed from the digits before it. It exists so a mis-keyed or mis-scanned identifier fails loudly instead of quietly pointing at the wrong product.

The algorithm

Working right to left across the digits before the check digit, multiply alternately by 3 and 1, sum the results, then take whatever is needed to reach the next multiple of 10.

For 03600029145: the weighted sum is 58, the next multiple of 10 is 60, so the check digit is 2 — giving 036000291452.

The spreadsheet problem

By far the most common cause of broken GTINs in real catalogs is Excel. Two failure modes:

• Leading zeros stripped. 036000291452 becomes 36000291452 — now an 11-digit number that is not a valid GTIN length.
• Scientific notation. A 13-digit GTIN becomes 8.85911E+11 and the original digits are gone for good.

Both happen silently on open, before you have typed anything. Format the column as text before importing, and export as text — not "General".

Padding to 14 digits

GS1 Digital Link URIs require the GTIN as 14 digits, zero-padded on the left. A UPC-A of 036000291452 becomes 00036000291452. The padding does not change the check digit — it is computed on the original digits and stays where it is.