This French SIRET validator checks a 14-digit SIRET (or a 9-digit SIREN
alone) against the Luhn checksum INSEE uses, including the documented La Poste exception.
Paste a number and get an instant pass/fail — nothing leaves your browser.
How to use it
Paste either a 14-digit SIRET or a 9-digit SIREN, with or without spaces, and click
"Validate". The tool detects which one you entered by length and applies the matching Luhn
check automatically.
How the Luhn checksum works
Starting from the rightmost digit, every second digit is doubled; if doubling produces a
two-digit result, 9 is subtracted (equivalent to summing its digits). All digits — doubled
and untouched — are then summed, and the number is valid only if that total is an exact
multiple of 10. It's the same algorithm used to validate credit card numbers.
The La Poste exception
Every SIRET beginning with SIREN 356000000 (La Poste) is a documented
exception to the standard rule: instead of the Luhn check, the sum of all 14 digits must be
a multiple of 5. This tool detects that SIREN prefix and switches checks automatically.
Frequently asked questions
What is a SIRET vs a SIREN?
A SIREN is the 9-digit number INSEE assigns to a business entity as a whole. A SIRET is 14 digits: the 9-digit SIREN plus a 5-digit NIC (Numéro Interne de Classement) identifying one specific physical establishment of that business. A company with multiple locations has one SIREN and several SIRETs.
What checksum does SIRET use?
The Luhn algorithm (the same mod-10 checksum used for credit card numbers): every second digit from the right is doubled, doubled values over 9 have 9 subtracted, and all digits are summed. A valid SIRET (or SIREN alone) produces a sum that is a multiple of 10.
Why does this tool mention La Poste specifically?
La Poste (SIREN 356000000) is a documented, official exception: its SIRET numbers do not follow the standard Luhn check. Instead, the sum of all 14 digits must be a multiple of 5. A generic Luhn-only validator would incorrectly flag every real La Poste SIRET as invalid.
Can I validate just a SIREN (9 digits) here?
Yes — paste a 9-digit number and the tool applies the same Luhn check used for standalone SIREN validation, without requiring the 5-digit establishment suffix.
Is my SIRET or SIREN sent anywhere?
No. The Luhn check runs entirely in your browser via JavaScript — nothing is transmitted, logged, or stored.