What is a routing number, and what do the nine digits mean?
A plain explanation of the ABA routing transit number: what each digit does, how the check digit works, why banks have more than one, and how to tell a real number from a fake.
A routing transit number is a nine-digit code that identifies which financial institution holds an account. It was created by the American Bankers Association in 1910, when the problem was physically sorting paper checks into mail sacks, and the format has survived every change in banking technology since. The Federal Reserve maintains the authoritative list.
What each digit is doing
Read a routing number in three parts.
- Digits 1–4, the Federal Reserve routing symbol. The first two identify one of the twelve Federal Reserve districts — 01 Boston, 02 New York, 03 Philadelphia, 04 Cleveland, 05 Richmond, 06 Atlanta, 07 Chicago, 08 St. Louis, 09 Minneapolis, 10 Kansas City, 11 Dallas, 12 San Francisco. Numbers beginning 21 through 32 are the thrift equivalents: subtract 20 to get the district. The next two digits identify the specific Fed office or check-processing centre.
- Digits 5–8, the ABA institution identifier. This is the bank itself within that district.
- Digit 9, the check digit. A mathematical guard against typos.
The check digit, concretely
Multiply the nine digits by the weights 3, 7, 1, 3, 7, 1, 3, 7, 1 and add the products. A valid routing number always produces a total divisible by 10. Take 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, which divides by 10. The number is well-formed. This is why a payment form can reject a mistyped routing number the instant you leave the field, without querying anything.
Note carefully what this does not prove: a number can pass the check-digit test and still not belong to any bank, or belong to a completely different bank than you intended. Passing the formula means the number is plausible, not correct. Every number Buzdy publishes is additionally confirmed against the Federal Reserve's own directory.
Why one bank has many numbers
American banking consolidated hard. When a large bank buys a smaller one, the acquired bank's routing number usually keeps working for years so that existing direct deposits do not break. The result is that a bank like PNC or Chase can hold well over a hundred registered routing numbers, most of them ghosts of institutions that no longer exist as brands. This is also why "the" routing number for a big bank is a slightly meaningless idea — what matters is the number attached to your account.
Routing numbers are not private
Every routing number in the United States is public and searchable. What you should protect is the combination of routing number and account number, because together they are sufficient to originate an ACH debit against your account. That pair is printed on the bottom of every check you have ever written, which tells you something about how much protection it was ever meant to offer.
Common questions
Are routing numbers secret?
No. Routing numbers identify banks, not people, and every one of them is published by the Federal Reserve in a directory anyone can search. Your account number is the sensitive half of the pair. That said, the two together are enough to originate an ACH debit, which is why you should not post both publicly.
Why do routing numbers have exactly nine digits?
The format was fixed by the American Bankers Association in 1910 and has not changed. Digits one to four are the Federal Reserve routing symbol, five to eight identify the institution, and the ninth is a check digit computed from the other eight.
What does the check digit do?
It catches typing errors. Multiply the digits by the weights 3, 7, 1, 3, 7, 1, 3, 7, 1, add the results, and a valid routing number always divides exactly by 10. A single mistyped digit will almost always break that, which is why banking forms can reject a wrong number instantly without looking anything up.
Can two banks share a routing number?
No. Each routing number belongs to one institution. The reverse is common though — one bank can hold dozens of routing numbers, usually inherited from banks it acquired.
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General information, not financial advice. Payment network hours and bank cut-offs change — confirm current details with your own bank before relying on them for a time-critical transfer.