Tukuk-OS🧾 Check Digit Validator
ℹ️ What this tool does & how it works
Four everyday identifiers carry a final digit computed from all the others. This page runs each published algorithm — Luhn (cards), ISO 7064 mod-97 (IBAN), mod-11 (ISBN-10) and mod-10×3 (ISBN-13/EAN/UPC) — and prints the working. One thing it will never claim: a valid check digit means the number is real and yours. It means only that it was built correctly.
What a check digit can and cannot tell you
These digits catch typos, not fraud: Luhn catches any single-digit error and most transpositions; mod-97 catches essentially every single-character error and most two-character ones. They were designed in an age when the hostile act was a mistyped slip, not an adversarial forgery — and a check digit cannot stop anyone who computes it, which is why no payment system accepts a number on Luhn alone. For IBANs, SEPA additionally demands the country-specific length and BBAN shape; a real transfer validates against the issuer, and the fee message you get back is the only thing that ever proves an account exists.