How ABA Routing Numbers Work
What Is an ABA Routing Number?
An ABA routing number is a nine-digit code that identifies a specific financial institution in the United States. ABA stands for the American Bankers Association, which introduced the routing number system in 1910 to streamline the processing of paper checks. Today, routing numbers are used for ACH transfers, wire transfers, and direct deposits in addition to check processing.
Every bank, credit union, and savings institution that is federally chartered or maintains an account with a Federal Reserve Bank has at least one routing number. Large national banks like JPMorgan Chase and Bank of America have dozens of routing numbers — often one per state or region.
The Structure of a Routing Number
A routing number is always exactly nine digits. The digits are not random — they encode information about the financial institution and serve a verification purpose.
- Digits 1–4 (Federal Reserve routing symbol): Identify the Federal Reserve district and processing center. The first two digits indicate the Federal Reserve district (01 through 12), and the second two digits indicate the Federal Reserve bank office.
- Digits 5–8 (Institution identifier): Uniquely identify the specific financial institution within the Federal Reserve system.
- Digit 9 (Check digit): A mathematically derived verification digit. Banks use this digit to detect transcription errors. If you enter the wrong routing number, the check digit will not compute correctly and the transaction will be rejected before funds move.
The ABA Checksum Algorithm
The ninth digit is computed using a weighted checksum formula. Each of the first eight digits is multiplied by a weight (3, 7, 1, 3, 7, 1, 3, 7), the products are summed, and the result must be divisible by 10. If it is not, the routing number is invalid.
For example, for routing number 021000021:
- 0×3 + 2×7 + 1×1 + 0×3 + 0×7 + 0×1 + 0×3 + 2×7 + 1×1 = 0 + 14 + 1 + 0 + 0 + 0 + 0 + 14 + 1 = 30
- 30 is divisible by 10, so this is a valid routing number.
Who Assigns Routing Numbers?
The ABA assigns routing numbers through its Routing Number Administrative Board. Banks apply for routing numbers through a formal process. The Federal Reserve maintains and publishes the authoritative database of all active routing numbers through its FedACH Directory and Fedwire Directory.
Why Do Banks Have Multiple Routing Numbers?
Large banks often have different routing numbers for different states or geographic regions. This is a legacy of the pre-digital era, when checks were physically routed through the local Federal Reserve office closest to the paying bank. Even though processing is now electronic, many large banks still maintain separate routing numbers per state for operational and regulatory reasons.
Additionally, a bank may have different routing numbers for different transaction types — one for ACH transfers and direct deposit, and a different one for wire transfers.