MOD-11 checksum
A weighted modulo-11 algorithm used by several European tax authorities to protect VAT numbers against transcription errors.
What it is
MOD-11 is a family of checksum algorithms where each digit of an identifier is multiplied by a position-dependent weight, the weighted sum is taken modulo 11, and the result determines a check digit that travels with the number. Norway's organisation number (used as the VAT identifier when suffixed with "MVA") and Switzerland's UID both rely on a MOD-11 variant to reject single-digit typos and most digit swaps.
Where you meet it
You meet MOD-11 at the format-validation stage, before any registry call. In vatverify's /v1/validate, a MOD-11 mismatch produces an invalid_format error for Norwegian (NO) and Swiss (CHE) VAT numbers, so you never waste a registry lookup on a number that cannot be valid. In code, the Norwegian check uses weights 3, 2, 7, 6, 5, 4, 3, 2 over the first eight digits; the Swiss UID uses weights 5, 4, 3, 2, 7, 6, 5, 4 over its nine significant digits.
Common confusions
- A MOD-11 remainder of 10 means the number is invalid by construction. Norwegian organisation numbers with that remainder are skipped during issuance.
- MOD-11 only proves the number is well-formed. A well-formed number can still be unregistered or deregistered; only a registry lookup confirms current status.