IBAN validator and check digit calculator

A clear answer. Here’s how.

Checks an IBAN in the order that makes a failure easiest to explain: the characters (spaces and hyphens are ignored, letters are upper-cased); the country code and the two check digits; whether the country is in the SWIFT IBAN Registry (release 103, text file modified 17 September 2026, 89 countries); the length that country requires; the character type of each part of its BBAN (n is digits, a is upper-case letters, c is letters and digits); and the ISO 7064 mod 97-10 check, in which the first four characters move to the end, each letter becomes 10 to 35, and the resulting number must leave remainder 1 when divided by 97. The remainder is worked out digit by digit with whole numbers, so nothing is rounded. The first failed check is reported, with every check that could run. The registry text file names only the bank identifier and the branch identifier of each country, so those two are shown as such and the rest of the BBAN (the account number and any national check digits) is shown as runs of the allowed characters. Create an IBAN gives the check digits as 98 minus the remainder of BBAN, country code and 00 divided by 97, and prints the IBAN in groups of four; a calculated IBAN is only well formed. The mod 97 check catches every change of one digit to another digit or of one letter to another letter, and every swap of two neighbouring digits or of two neighbouring letters. It can miss a swap of a letter and a digit (always for B and 1, M and 2, or X and 3, and now and then where the check digits meet the account part) and a letter replaced by a digit or the reverse, because a letter counts as two digits. This tool does not verify national check digits (Spanish, French, Italian, Belgian, Portuguese and others), does not look up banks, BICs or account holders, and cannot say that an account exists. Some countries use IBAN-style numbers that the registry does not list.