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How the Mod-97 Checksum (ISO 7064) Actually Works

Every IBAN encodes its own checksum according to the international standard ISO 7064 (MOD 97-10), referenced by the IBAN standard ISO 13616. Validating one is a precise, mechanical process: first, the four leading characters — the two-letter country code and the two check digits — are moved to the end of the string. Next, every letter in the rearranged string is replaced by a two-digit number (A=10, B=11, ... Z=35), turning the whole IBAN into one very long numeral. Finally, that number is divided by 97; if the remainder is exactly 1, the checksum is valid.

As a worked example, take a fictional Turkish IBAN such as TR33 0006 1005 1978 6457 8413 26. Moving "TR33" to the end and converting the two letters to digits (T=29, R=27) produces one long number; dividing that number by 97 leaves a remainder of 1, confirming the checksum. Change a single digit anywhere in the account number and this same calculation will almost never land back on remainder 1 — which is exactly why the check catches the overwhelming majority of typos, transposed digits and copy-paste mistakes before a transfer is even attempted.

Beyond the checksum, the tool also checks that the total length matches what is expected for the country: Turkish IBANs are always 26 characters, German ones 22, British ones 22, French ones 27, and so on — a length mismatch fails validation even before the mod-97 arithmetic runs. Everything happens with JavaScript inside your browser; the IBAN you type is never transmitted to any server.

What This Check Can and Cannot Tell You

It is important to be precise about what a "valid" result actually means — this is a format check only, not proof that the account exists:

• A passing result means the number is structurally well-formed. It does not confirm that the account is open, active, or belongs to the person or company you expect.
• Verifying the account holder's name is a separate step. Many banks and payment systems run a name-matching check at the moment of a transfer; this tool has no access to bank records and cannot replicate that.
• A syntactically valid IBAN can still belong to a closed or frozen account. Only the account-holding bank can confirm the account's real-world status.
• Country-specific internal rules beyond length are not checked here. Some countries apply additional structure inside the BBAN portion; this tool validates the internationally standardized mod-97 part shared by every IBAN country.

In short: treat a valid result as "no typo detected," never as "payment guaranteed to arrive."