How is an ABA routing number checksum calculated?

Multiply the 9 digits by the repeating weights 3, 7, 1, 3, 7, 1, 3, 7, 1, sum the products, and the routing number is valid if the total is divisible by 10. This is the ABA modulus-10 check used across the US banking system.

How many digits is a routing number?

A US ABA routing transit number is exactly 9 digits. The first two digits identify the Federal Reserve routing symbol and the ninth digit is the check digit that makes the weighted sum divisible by 10.

Does a valid checksum mean the bank account is real?

No. A valid checksum only confirms the routing number is structurally well-formed. It does not confirm the routing number is currently assigned to an active bank, nor does it validate the linked account number. Use the Federal Reserve E-Payments directory to confirm a live institution.

What do the first two digits mean?

The first two digits indicate the Federal Reserve district that processes the institution. Values 00–12 are Federal Reserve banks, 21–32 are thrift institutions, 61–72 are electronic transaction routings, and 80 is used for traveler's checks.

Is my routing number sent anywhere?

No. The checksum is computed entirely in your browser. Nothing you enter is transmitted or stored.

US ABA Routing Number Validator

A US bank routing number (ABA routing transit number) is the 9-digit code that identifies a financial institution in ACH and wire transactions. Before you submit a routing number in a payment form or store it in a database, you can confirm it is structurally valid with the ABA modulus-10 checksum. This free validator runs that exact check in your browser.

How it works

The ABA check digit algorithm uses repeating weights 3, 7, 1:

Take the 9 digits d1 d2 d3 d4 d5 d6 d7 d8 d9.
Multiply by weights: 3·d1 + 7·d2 + 1·d3 + 3·d4 + 7·d5 + 1·d6 + 3·d7 + 7·d8 + 1·d9.
Sum the products.
The routing number is valid if the sum is divisible by 10 (remainder 0).

The first two digits also encode the Federal Reserve routing symbol, which this tool decodes into a district description.

Example

Validate 021000021: the weighted sum is (3·0)+(7·2)+(1·1)+(3·0)+(7·0)+(1·0)+(3·0)+(7·2)+(1·1) = 14+1+14+1 = 30. 30 mod 10 = 0, so the routing number passes the checksum.

Notes

A passing checksum means the number is well-formed, not that it belongs to a live, active institution — for that, check the Federal Reserve’s E-Payments Routing Directory. The check digit catches most single-digit typos and transpositions, which is exactly what you want before triggering an ACH transfer. Everything runs locally; the routing number never leaves your device.