Checksum
An arithmetic check digit built into a VAT number so that format-level validation can reject typos before any registry call.
What it is
A checksum is a digit (or short sequence) derived from the other characters in an identifier by a fixed algorithm. For VAT numbers, the checksum lets a validator confirm the number is internally consistent without contacting a registry. Every EU member state publishes a checksum rule for its VAT number format; the algorithms vary from MOD-11 (Norway, Switzerland) to MOD-97 (Belgium) to Luhn-style weightings (Germany, Italy, Spain, and others).
Where you meet it
You meet the checksum in the first branch of a VAT number validation function. In vatverify's tax-rules engine, the format check runs before any call to VIES, HMRC, the Swiss BFS, or Norwegian Skatteetaten. A checksum mismatch short-circuits the call and returns invalid_format against /v1/validate, saving a round-trip to the registry. In code, each country typically has its own isChecksumValid(digits) helper keyed on the VAT number's country prefix.
Common confusions
- A passing checksum does not mean the number is registered. It only means the string is well-formed; registration status is a separate registry call.
- Not every country uses the same scheme. A number that validates for Germany with a MOD-11 variant will not validate for France with the SIREN-based rule.