IBAN Validator

In short An IBAN is checked by moving its first four characters to the end, replacing every letter with its position in the alphabet plus nine — A becomes 10, Z becomes 35 — and dividing the resulting number by 97. A valid IBAN leaves remainder 1. That single rule is why a mistyped account number can be caught before the money moves rather than after, and it catches every single-character error and almost every swap of two neighbouring characters. Three things are worth knowing that most checkers do not say. First, the length is not universal: it is fixed per country and ranges from Norway's fifteen characters to Saint Lucia's thirty-two, so a number of the right shape and the wrong length is still wrong. Second, the check digits are always between 02 and 98 — the algorithm cannot produce 00, 01 or 99 — and a checker that only looks at the remainder will accept an impossible pair, which one of the registry's own example IBANs happens to demonstrate. Third, "mod-97 catches all transpositions" is only true for two digits: a letter expands to two digits when converted, so swapping a letter with a neighbouring digit can slip through, and this tool found exactly one such case in 1,183 tested across all 87 countries. Above all, a pass proves the number is internally consistent and nothing else. It cannot tell you the account exists, is open, or belongs to the person you are paying — a random string of the right shape gets through about once in 97 attempts.