How ABA Routing Number Validation Works

Every US bank routing number contains a built-in error detection mechanism called the ABA checksum. The American Bankers Association (ABA) designed this formula so that any single-digit typo in a routing number will cause the checksum to fail, preventing payments from going to the wrong institution. The formula multiplies each of the nine digits by a repeating weight pattern of 3, 7, 1, and the sum of all products must be divisible by 10.

The ninth digit (position 9) is the check digit, calculated to make the total divisible by 10. If you change any single digit in a routing number, the sum will no longer be divisible by 10, and any payment processing system that validates the checksum will reject the number before attempting to route the payment. This is why the validator on this page can detect typos even without looking up the routing number in a database.

What a Routing Number Tells You

A 9-digit ABA routing number encodes three pieces of information. The first two digits identify the Federal Reserve district where the bank or its head office is located. Prefix 01 is the Boston Fed district, 02 is New York, 03 is Philadelphia, 04 is Cleveland, 05 is Richmond, 06 is Atlanta, 07 is Chicago, 08 is St. Louis, 09 is Minneapolis, 10 is Kansas City, 11 is Dallas, and 12 is San Francisco. Prefixes in the 21-32 range represent thrift institutions (savings banks and credit unions), and 61-72 represent additional thrift identifiers.

Digits three through eight identify the specific financial institution assigned to that Federal Reserve district. The final digit (nine) is the check digit calculated by the ABA checksum formula. So when you see a routing number starting with 12, you know it is a California or western US bank before even looking it up. Starting with 02 means New York. This is why Chase has different routing numbers for different states. The first two digits reflect where each regional bank was originally chartered before Chase acquired it.

Routing Number vs Account Number: What Each Does

A routing number identifies the bank. An account number identifies your account within that bank. To make an electronic payment to you, a sender needs both. Think of the routing number as the street address of your bank, and the account number as your apartment number within that building. The routing number gets the money to the right bank; the account number gets it to the right account.

On a paper check, the routing number is the first group of nine digits printed in magnetic ink at the very bottom left. The account number follows it. Some banks print a check number at the far right. The MICR line (Magnetic Ink Character Recognition) format uses special characters around the routing number: a start delimiter, the nine digits, and an end delimiter. When you deposit a check by mobile photo, your bank's app reads these digits automatically.

For electronic payments including direct deposit (payroll), bill pay, and ACH transfers, you provide both your routing number and your account number. The routing number tells the ACH network which bank to send the payment to; the account number tells that bank which account to credit. For wire transfers (Fedwire), you may need to specify a different routing number than the one used for ACH, as some banks use separate routing numbers for wire transfers.