What Is a Routing Number? ABA Numbers, ACH & Wires Explained
Nine little digits on the bottom of every US check, and most people never think about them — until a transfer fails. The routing number identifies your bank to the rest of the financial system. Here is what it is, how it differs by transfer type, and how to check one with our Routing Number Lookup.
What a routing number is
An ABA routing number (also called a routing transit number) is a 9-digit code that identifies the bank or credit union holding an account. Combined with your account number, it tells the system exactly where money should move. It appears at the bottom-left of a check.
How it is validated
The last digit is a checksum. Using the nine digits d1…d9, the formula 3(d1+d4+d7) + 7(d2+d5+d8) + (d3+d6+d9) must be a multiple of 10. That is how a form can instantly catch a typo — and what our tool runs for you. A valid checksum means the number is well-formed, not that the account exists.
Why you may have more than one
Many banks use different routing numbers for different purposes and regions:
- ACH (direct deposit, bill pay) often uses one number.
- Wire transfers frequently use a different number.
- Large banks may assign a routing number per state where you opened the account.
Always use the exact number your bank lists for the transaction type you need.
ACH vs wire
ACH transfers are batched, low-cost (often free) and take 1–3 business days. Wires are sent individually, usually settle same day, and cost a fee. The routing number and the transfer path differ between the two, which is why the numbers can differ.
The first two digits
The prefix maps to a Federal Reserve district (01 Boston through 12 San Francisco), which is the region that processes the bank’s transactions. Our lookup shows the district and, for major banks, the bank name.
The bottom line
A routing number is simply your bank’s address in the payments network. Verify one before you send money — a single transposed digit can misroute or bounce a transfer — using our free Routing Number Lookup.