Free Bank Account Validator

Validation here is arithmetic on the number you typed. An IBAN is checked by moving the first four characters to the end, converting letters to digits, and taking the result modulo 97, which has to come out at 1. A US routing number is checked with the weights three, seven and one across its nine digits, where the weighted total has to divide by ten. Both catch a mistyped digit and most transpositions. Neither contacts a bank, so a number can pass every check on this page and belong to an account that closed last year.

Take the routing number 021000021. The first, fourth and seventh digits are 0, 0 and 0 and contribute nothing. The second, fifth and eighth are 2, 0 and 2, giving 28 at a weight of seven. The third, sixth and ninth are 1, 0 and 1, contributing 2. The total is 30, which divides by ten, so the number is well formed. That is the whole claim. Whether the institution behind it accepts the account number sent beside it is a separate question, and in the US format that account number carries no check digit of its own.

The gap between those two facts is where a payment goes missing. Transpose two digits inside the account number and the routing number still validates, the payment leaves, and it either bounces days later or settles into a stranger account that happens to exist. The bounce is the good outcome. The other one is recovered by asking the receiving bank to approach its own customer for permission to return money that has already been spent, a request with no deadline attached to it. Validate the format here, then confirm the account against something that saw it, such as a bank letter or a statement header.