French SIREN

What it covers

Returns a valid French SIREN (company identifier) with a correct Luhn checksum. Use `format` for the 14-digit SIRET or the FR VAT number. Set `invalid=true` for a SIREN with a deliberately wrong check digit.

A SIREN is the nine-digit identifier INSEE assigns to every French business. The first eight digits are the payload; the ninth is a Luhn (mod-10) check digit — the same algorithm that guards credit-card numbers. A SIRET extends a SIREN with a five-digit establishment number plus its own Luhn digit over the full fourteen.

How it’s calculated

The ninth digit is a Luhn (mod-10) check digit over the first eight — the classic credit-card checksum.

7 (kept) = 7
3 × 2 = 6
2 (kept) = 2
8 × 2 = 16, 16 − 9 = 7
2 (kept) = 2
9 × 2 = 18, 18 − 9 = 9
3 (kept) = 3
2 × 2 = 4
sum = 7+6+2+7+2+9+3+4 = 40
40 mod 10 = 0
(10 − 0) mod 10 = 0

Edge cases

• The first digit is drawn 1–9 for the common shape, though SIRENs beginning with 0 do exist and are still valid.
• With invalid=true the Luhn check digit is offset by 1–9 mod 10, which can never land back on the correct value — so the number always fails validation while keeping the right length and shape.

Gotchas

• Luhn doubles every second digit counting from the RIGHT (from the check-digit side), not from the left — starting the doubling on the wrong end produces a different, wrong check digit.
• When a doubled digit exceeds 9, you subtract 9 (equivalently, add its two digits) — e.g. 8×2 = 16 → 7. Forgetting this step is the most common Luhn bug.