Checksum methodology for VAT identifiers

This page documents common checksum families used by VAT identifiers. National implementations can vary; always verify registration status via official services.

MOD 97

Used for remainder-based schemes where a numeric representation is validated by modulo 97 constraints.

Concept

Rule

Valid when remainder matches a defined constant or derived check value

Inputs

Digits only, or alphanumeric mapped to digits (country-specific)

Output

Two-digit check (typical)

Diagram

Luhn family

Digit transform checksum: alternating weights with a “double then subtract 9” normalization for digits ≥ 10.

Algorithm sketch

Step

Traverse digits; double every second digit (direction depends on national rule)

Normalize

If doubled digit ≥ 10, subtract 9

Check

Sum mod 10 equals 0 (or equals control digit rule)

Weighted sums and control digits

Many identifiers use fixed weights multiplied by digits, then reduced by modulo arithmetic to produce a control digit or letter.

Example: MOD 23 control letter

Used by Spanish NIF/NIE: remainder of the numeric component determines a control letter from a fixed alphabet.

Index

i = n mod 23

Letter

alphabet[i]

Implementation boundary

Checksum logic can detect many format errors
Checksum success does not imply registration
Authorities may maintain legacy formats

Dataset: VAT formats. Validator: VAT validator.

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Failure modes and what a checksum does not prove

Checksum pass ≠ registration: a syntactically valid identifier can be inactive or reassigned.
Transcription errors: most checksum methods catch single-digit errors and many transpositions, but not all.
Normalization errors: spaces, punctuation, missing prefix, and wrong identifier types (e.g., SIRET vs SIREN) are common causes of false failures.

Implementation notes (validator)

VAT-Scan implements checksum logic client-side where the algorithm is public and deterministic. For cases where national rules require external mapping tables or authority-held datasets, the validator reports structural validity only and directs users to VIES for official confirmation.