IBAN Validator

An IBAN (International Bank Account Number) is a standardised way of writing a bank account number so it can be recognised and validated automatically no matter which country it comes from. It starts with a 2-letter country code, followed by 2 check digits, followed by the country's own domestic account identifier (a Basic Bank Account Number, or BBAN, whose internal structure varies from country to country). The format is defined by ISO 13616, and adoption is close to universal across Europe, wide across the Middle East and North Africa, and growing in parts of Latin America and the Caribbean — but it is not global, so an American or Australian bank account, for example, has no IBAN equivalent at all. The two check digits are what make an IBAN self-validating. They are calculated using ISO 7064's MOD-97-10 algorithm: the country code and check digits are moved to the end of the string, every letter is expanded to a two-digit number (A=10 through Z=35), and the resulting long digit string is reduced modulo 97. A correctly constructed IBAN always produces a remainder of exactly 1. This single property lets banks, payment processors and this tool catch the overwhelming majority of typos, transposed digits, and copy-paste errors before a payment is ever attempted — a mistyped IBAN will almost always fail the checksum rather than silently routing money to the wrong account. Because the numbers involved can run past 30 digits once every letter is expanded, a naive implementation that converts the whole string to one big JavaScript number would silently lose precision partway through and produce wrong answers on longer IBANs. This tool avoids that entirely by never forming one large number at all: it walks the expanded digit string one digit at a time, keeping a running remainder and folding in each new digit with the standard "multiply the running remainder by 10, add the digit, take mod 97" step. This is the same digit-by-digit technique real banking systems use to compute an arbitrary-precision modulus without needing big-integer arithmetic, and it is mathematically exact — no rounding, no precision loss, regardless of how long the IBAN is. The tool also checks the expected total length for each country before running the checksum, using the official ISO 13616 IBAN length registry (IBANs range from 15 characters for Norway up to 33 for Russia). If the country code is recognised but the length doesn't match, that's flagged immediately as a length error rather than being run through the checksum and reported as a vaguer failure. If the country code isn't one this tool recognises, the length check is skipped and the checksum is computed anyway, since new IBAN-issuing countries and formats do appear over time and a MOD-97 failure is still meaningful even without a length reference.