Whenever you set up direct deposit, pay a bill from your checking account or send a bank transfer, you're asked for two numbers: your account number and a nine-digit routing number. The account number identifies you. The routing number identifies your bank. Here's what those nine digits mean, how the last one catches typos, and why a valid-looking number can still send money to the wrong place.
A routing number is an address for a bank
The American Bankers Association (ABA) devised the routing number in 1910, and the industry adopted it in 1911. ABA policy says its primary purpose is to identify the bank responsible for paying, crediting or receiving a check or electronic transaction. That's why you'll also see it called an ABA number or a routing transit number (RTN).
It started with paper checks and has spread well beyond them. Nacha notes that routing numbers "now also designate participants in ACH, wire, instant and other payment systems." On a paper check, the ABA says the routing number is the left-most number along the bottom, followed by your account number and then the check number.
Routing numbers aren't handed out by individual banks. The ABA has designated an official registrar, currently LexisNexis Risk Solutions, which processes applications and assigns, reassigns and retires numbers under a policy overseen by the Routing Number Administrative Board. The ABA puts the number of active routing numbers at approximately 22,000.
The three parts of the nine digits
ABA policy splits the routing number into three components:
- Digits 1–4: the Federal Reserve routing symbol. Regulation CC explains that the first two digits identify the Federal Reserve District where the bank is located. A bank in the First District (Boston) starts with 01, and one in the Twelfth District (San Francisco) starts with 12. The Federal Reserve's key maps each four-digit symbol to a Reserve Bank office.
- Digits 5–8: the ABA institution identifier. This identifies the specific institution within that routing symbol.
- Digit 9: the check digit. This is calculated from the first eight digits so that mistakes can be caught.
The first two digits also say what kind of number it is. Under ABA policy, 01–12 covers banks and most electronic transactions, and 00 is for U.S. government checks and drafts. The 21–32 series was used for thrift institutions, with 2 added to the first digit of the district number. ABA policy says those assignments stopped in 1985, and thrifts have since received numbers in the 01–12 series. Numbers starting 61–72 are Electronic Transaction Identifiers, used by processors that handle payments for banks. The 80 series is for traveler's checks, and many other ranges are reserved.
The check digit tells you a number is well formed. It can't tell you the number belongs to the bank you meant.
How the check digit works
The ninth digit comes from a weighted sum. Each of the nine positions gets a weight, repeating 3, 7, 1 from left to right: 3, 7, 1, 3, 7, 1, 3, 7, 1. Multiply each digit by its weight and add up the results. For a valid routing number, the total is a multiple of 10. Software libraries that validate routing numbers, such as Apache Commons Validator, describe the same modulus-10 rule.
Here's a worked example using a made-up number. It's chosen for the arithmetic and isn't meant to identify any real institution. Take the first eight digits 9 9 1 2 3 4 5 6:
- Multiply by the weights 3, 7, 1, 3, 7, 1, 3, 7: that gives 27, 63, 1, 6, 21, 4, 15 and 42.
- Add them up: 27 + 63 + 1 + 6 + 21 + 4 + 15 + 42 = 179.
- The ninth digit has a weight of 1, so it has to bring 179 up to the next multiple of 10, which is 180. The check digit is 1, and the full made-up number is 991234561.
Now suppose someone swaps two digits while typing and enters 991243561. The weighted sum becomes 27 + 63 + 1 + 6 + 28 + 3 + 15 + 42 + 1 = 186. That isn't a multiple of 10, so the number is rejected before any money moves. Because the weights differ from one position to the next, this check catches any single mistyped digit and many swaps of neighboring digits.
In ACH files, the check digit even gets its own field. Nacha's developer guide shows the receiving bank's routing number split into the first eight digits and a separate check digit (the ninth).
The directories: FedACH and Fedwire participants
The Federal Reserve Banks publish the E-Payments Routing Directory on frbservices.org. It gives basic routing information for the Fedwire Funds Service, the Fedwire Securities Service and FedACH. The Federal Reserve says the directory is synchronized with its FedACH and Fedwire databases daily. It has separate searches for Fedwire participants and for FedACH participant RDFIs (receiving depository financial institutions), by name, location or routing number.
There are two limits on how you can use it. The directory is provided as a service to financial institutions and other authorized users, and its terms prohibit selling, relicensing or otherwise using the information for commercial gain. For individuals, the ABA runs its own routing number lookup, but it's capped at two lookups a day and ten a month.
ACH and wire numbers can differ
Because FedACH and Fedwire are separate systems with separate participant lists, being listed for one doesn't guarantee being listed for the other. ABA policy also lets a bank hold its principal routing number plus up to nine additional ones. The ABA notes that the registrar's routing data includes clearing-system attributes, such as ACH transfer or wire transfer, for each number.
In practice, the routing number printed on your checks or used for direct deposit may not be the one your bank wants for incoming wires. When you're receiving money, check which number your bank publishes for each payment type. When you're sending money, use the number the recipient's bank gives for that specific payment method.
Routing data also changes. Nacha's 2024 operations bulletin says routing number changes "may happen multiple times per week," because of events such as mergers and new charters, and that out-of-date routing data can cause payments to fail.
Verifying a payee before you send
A correct routing number is necessary but not sufficient. Nacha's fraud-reduction checklist, written for payroll teams, applies just as well to anyone sending money:
- Authenticate the request whenever someone asks you to add a new payee or change existing details.
- Confirm any change through a separate channel, using contact information you already have, not details from the message asking for the change.
- Verify the routing number and the account number before the first payment.
- Have a second person confirm a payment before it's released, where that's possible.
Nacha warns that "a great deal of fraud is predicated on a change of account information to redirect a payment," so an unexpected "we've changed banks" message deserves extra scrutiny.
Spectre checks the routing number's checksum when you add a recipient for a bank transfer, so a mistyped number is caught before any money moves. Confirming that the account belongs to the person you mean to pay is still up to you.