IBAN validator and checker
Verify the mod-97 checksum, country and length of an international account number before a transfer.
Nothing leaves your browser. This tool does its work on your own machine — there is no upload, no server request, and nothing is stored.
What the mod-97 check actually catches
An IBAN is not a random string. The two digits after the country code are a remainder, computed from every other character in the number. Move the first four characters to the end, replace each letter with two digits, read the whole thing as one very long integer, and divide by 97: a genuine IBAN leaves exactly 1.
That single constraint catches essentially every realistic mistake. One wrong digit fails. Two digits swapped fails. A missing character fails on length before the arithmetic even runs. It is the reason bank transfer forms can reject a typo instantly without contacting the receiving bank at all.
What it does not catch
A criminal's IBAN passes this check exactly as cleanly as your landlord's. The checksum proves the number is well formed, never that it belongs to the person on the invoice. This distinction is the whole of invoice fraud: the attacker does not forge a number, they substitute a real one they control.
If bank details on an invoice have changed, or an email asks you to pay a new account, treat the change itself as the warning. Telephone the supplier on a number you already had — from an old contract, a previous invoice, or their website typed in by hand — and confirm before any money moves. Bank transfers are effectively irreversible once settled.
Before you pay someone new
- Check the name on the account matches the name on the invoice. Many banks now show a mismatch warning at the point of transfer; do not click past it.
- Send a small test amount first when the sum is large, and confirm receipt on the phone before sending the rest.
- Run the company's website and the email address through the full check as well — a valid IBAN attached to a domain registered three weeks ago tells you far more together than either does alone.
Common questions
What does a valid IBAN actually prove?
That the number is well formed: the right length for its country and consistent with its own check digits. It does not prove the account exists, and it certainly does not prove the person named owns it. Invoice fraud uses real, valid IBANs belonging to the criminal.
How does the IBAN check digit work?
Move the first four characters to the end, convert every letter to two digits (A is 10 through Z is 35), read the result as one enormous integer and take it modulo 97. A valid IBAN leaves a remainder of exactly 1. That catches virtually every single-character typo and digit transposition.
Someone changed the bank details on an invoice. What do I do?
Stop, and telephone the supplier on a number you already had, never one printed on the new invoice. Changed payment details on an otherwise normal invoice is the defining move of business email compromise, and the money is usually unrecoverable once sent.