Paste a SIRET: the tool validates its 14 digits with the Luhn formula and applies the special rule for La Poste establishments. The number stays in your browser.
The calculation runs in your browser: the number you type is not sent to any server.
A SIRET identifies an establishment: the 9 digits of the SIREN (the company) followed by the 5 digits of the NIC (the site). One company has as many SIRETs as it has sites.
That the number is well-formed: the last digit is a Luhn check digit computed from the other thirteen. A typo or a made-up number fails.
That the company exists, is active, or bears the name printed on the document. For that you need the registry: the official, free Annuaire des entreprises returns the legal name, address and status of the establishment. TrustyFile queries it automatically while analysing a document.
La Poste (SIREN 356 000 000) has so many establishments that INSEE gave it a rule of its own: only the head office (356 000 000 00048) follows Luhn; for any other La Poste SIRET the sum of the 14 digits must be a multiple of 5. A checker that ignores this calls every La Poste invoice a forgery.
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