Seller guide
How do I fix an invalid UPC or GTIN check digit before upload?
Separate wrong length, non-digit input, a bad modulo-10 check digit, and identifier ownership before a marketplace upload turns one barcode typo into a blocked launch.
Preserve the identifier as text, including leading zeroes
Spreadsheet number formatting can remove a leading zero or display a long code in scientific notation. Import UPC, EAN, and GTIN columns as text and compare them with the package or seller-owned GS1 record. Supported structures have fixed total lengths; padding a short value until it looks right creates a different identifier rather than repairing one.
Distinguish structure from the final check digit
A structural check asks whether the value contains only ASCII digits and has a supported GTIN length. A check-digit test then applies alternating weights to the body and derives the final modulo-10 digit. If letters or the total length are wrong, there is not enough information to offer a mechanical one-digit fix.
Replace only the final digit when the body is verified
When the body has the correct supported length and only the supplied check digit fails, calculate the expected final digit and compare the corrected full code with packaging and the seller-owned identifier ledger. Never change an interior digit merely to make the checksum pass: many different invented bodies can have a mathematically valid check digit.
A valid checksum is not proof of issuance or ownership
Modulo-10 math cannot tell you whether GS1 issued the code, which company owns its prefix, whether it belongs to this product or variation, or whether a marketplace exemption applies. Deduplicate the batch, reconcile the code with seller records, then validate the current category template before uploading one test listing and reviewing the marketplace response.
Tools that help with this
- Free workbook that keeps identifier fields as reviewable text (free)
- GTIN / UPC / EAN Batch Validator ($0.25) — Check GTIN-8, GTIN-12, GTIN-13, and GTIN-14 structure, duplicates, and check digits
- Four-Marketplace Catalog Readiness Check ($0.05) — Find identifier and required-field gaps before channel-file preparation
- Suppressed Listing Diagnosis Checklist ($0.25) — Build a review checklist when an identifier problem appears in a listing notice
Every paid output is a draft for your review before you publish, appeal, or activate anything. Friday does not access your seller account.