Checks length, country code, and the MOD-97 checksum. Nothing you type here is sent anywhere.
How it works
The checker moves the first four characters of the IBAN (country code and check digits) to the end, converts every letter to a number (A=10 through Z=35, per the ISO 7064 standard), and then computes that huge numeric string mod 97 — a valid IBAN always leaves a remainder of exactly 1. Since the converted number can run past 30 digits, far beyond what JavaScript can hold precisely, the mod is computed in 9-digit chunks, folding each intermediate remainder back into what's left of the string.
FAQ
Why does it process the number in chunks instead of one big calculation?
A full IBAN converted to digits can exceed 30 digits — well past JavaScript's safe integer range — so the checksum is computed by repeatedly taking a 9-digit chunk mod 97 and prepending the remainder to the rest of the string, which produces the exact same result without any precision loss.
Why does the length check sometimes get skipped?
The tool only enforces the expected length for the roughly 70 countries in its built-in lookup table; for any other country code it still validates the format and runs the MOD-97 checksum, just without a length sanity check.
Free, unlimited, no account. If this saved you a paid tool, a small thanks is appreciated.