Spain IBAN Validator

In short A Spanish IBAN is exactly 24 characters: ES, two check digits, then 4 digits, then 4 digits, then 1 digits, then 1 digits, then 10 digits — so the 4 characters after the check digits are the código de entidad — 2100 in the example — the next 4 are the oficina, 0418, and the remaining 12 are the account, 450200051332. A Spanish IBAN carries the old twenty-digit CCC intact: four digits of entidad, four of oficina, two dígitos de control, then ten of account. Those two control digits are a national checksum computed before the IBAN existed, so a Spanish number is checked twice by two different rules. The checksum is ISO 7064 MOD-97-10: move the first four characters to the end, turn each letter into its position in the alphabet plus nine, and read the result as one long number, which must leave remainder 1 when divided by 97. That catches every single-character mistake and almost every swap of two neighbouring characters, which is what it was designed for and why an IBAN is worth checking before you pay rather than after. What it cannot do is tell you the account exists, is open, or belongs to the person you think — a random string of the right shape passes about once in 97 tries, and no amount of arithmetic reaches into a bank. This page runs entirely in your browser, so the number you type is not sent anywhere.