United States · Validation
ABA routing number validator.
Verify a US bank's 9-digit ABA / routing transit number against the real weighted mod-10 checksum every bank, payroll processor and ACH network uses. Everything runs in your browser: nothing you type is transmitted anywhere.
Inputs
Result
Enter a routing number and press Validate.
How the routing number checksum works
Every US ABA / routing transit number is 9 digits: a 4-digit Federal Reserve routing symbol, a 4-digit ABA-assigned institution identifier, and a final check digit. The check digit exists so a single mistyped or transposed digit is caught before a wire, ACH transfer or cheque is misrouted.
The published formula weights the nine digits 3, 7, 1, 3, 7, 1, 3, 7, 1 in order and requires the weighted sum to be evenly divisible by 10:
3(d₁+d₄+d₇) + 7(d₂+d₅+d₈) + 1(d₃+d₆+d₉) ≡ 0 (mod 10)
The weights 1, 3 and 7 are each coprime to 10, which is what lets this scheme catch every single-digit error and most digit-transposition errors — the same design goal behind check-digit schemes like ISBN's and the Luhn algorithm.
Verification
This is not a reverse-engineered guess: the formula above was cross-checked in two independent ways before shipping — against the published worked example 111000025 (a real Bank of America, Texas routing number), and against the algebraically-equivalent coefficients used by the independent open-source python-stdnum library, confirmed identical across 2,000 randomized inputs.
What a passing checksum does and doesn’t tell you
A valid check digit only proves the 9 digits are internally consistent — it does not confirm the number was ever actually assigned to a real financial institution. This tool has no institution-name lookup table; for that, use your bank's own documentation or the Federal Reserve's routing directory. Runs entirely in your browser: nothing you type is transmitted, stored, or logged.