A matching check digit is useful evidence that a digit string has the expected arithmetic ending. It is not a database query, cryptographic signature, ownership certificate, or inspection of a printed barcode. Treat “valid” on this page as “passes the selected length and Mod-10 check.”
It detects a failed arithmetic check
The validator recomputes the expected final digit from the body and compares it with the supplied digit. A mismatch proves the complete string does not pass this check. A match is narrower: some different or transposed digit patterns can still produce the same remainder, so the check digit is an error-detection aid, not a guarantee against every typing error.
It does not verify identity or ownership
Passing Mod-10 does not show that GS1 issued the number, that its company prefix is licensed to a particular organisation, that a product or location exists, or that an item is genuine. Those questions require authoritative assignment or registry data outside this local calculator.
It does not inspect a barcode symbol
A correct digit string can still be printed with poor contrast, damaged bars, wrong quiet zones, an unsuitable size, or a different symbology. CheckDigit neither scans nor generates images and makes no claim about print quality or scanner readability. Preserve that boundary when sharing a result.