IBAN Validator (and CCC-to-IBAN Calculator)
Check whether an IBAN has a valid structure and checksum, for any country that uses IBAN, or calculate a Spanish IBAN from a CCC (código cuenta cliente). This only validates or calculates format: it never confirms the account actually exists.
What this actually checks
Three things: that the country code is a recognized IBAN issuer, that the total length matches what's expected for that country, and that the MOD-97 checksum (the international ISO 7064 standard) is correct.
How the MOD-97 checksum works
The first 4 characters are moved to the end, each letter is replaced with a number (A=10, B=11... Z=35), and the result, read as one very large number, must divide evenly by 97 leaving remainder 1. It's the same calculation any bank runs to catch a mistyped IBAN before attempting a transfer.
Length by country
Each country defines a fixed IBAN length (Spain: 24 characters, Germany: 22, the UK: 22...). This tool centralizes that table for over 70 countries that currently use IBAN.
How a Spanish IBAN is calculated from a CCC
A Spanish CCC has 20 digits: bank code (4), branch code (4), two check digits, and account number (10). Those check digits are calculated with their own modulo-11 algorithm (different from the IBAN's MOD-97): one validates the bank and branch codes together, the other validates the account. Once you have the full CCC, the IBAN is obtained by adding "ES" and computing its own two check digits with the standard MOD-97 — the same calculation the validator uses.
Privacy
Whatever IBAN you type never leaves your browser: it's not sent to Numelto, not logged, and not stored anywhere.