How the GTIN check digit works, and why yours may 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. The completed GTIN is 036000291452.

The spreadsheet problem

Spreadsheet conversion is a frequent cause of broken GTINs in catalog files. Two conversions deserve a check:

• Leading zeros stripped. 036000291452 becomes 36000291452, which is 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 the column with the Text format instead of "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 leading zeros leave the check digit unchanged in its original position.