What Is a Routing Number? The 9-Digit ABA Number, Digit by Digit
Last verified against primary sources: September 20, 2026.
A routing number is a nine-digit code that identifies a US financial institution in a payment. It answers "which bank?" — never "which customer?" The account number, the string that sits next to it, answers the second question.
The American Bankers Association introduced the system in 1910, and the numbers are still assigned by an agent acting for the ABA. The Federal Reserve describes the arrangement plainly in Appendix A to Regulation CC: "Each bank is assigned a routing number by an agent of the American Bankers Association." That is also why the code carries three names — routing number, ABA number, and routing transit number (RTN) — for one thing.
The nine digits, in four parts
Take Navy Federal Credit Union's number, 256074974, and the structure falls out:
- First two digits — the Federal Reserve district. 01 is Boston through 12, San Francisco. A leading 2 marks a thrift: per the Federal Reserve, "Adding 2 to the first digit denotes a thrift institution. Thus, 21 identifies a thrift in the First District, and 32 denotes a thrift in the Twelfth District." So 25 is a thrift in the Fifth District — Richmond, which fits a credit union headquartered in Vienna, Virginia.
- First four digits together — the "Federal Reserve routing symbol." Digits 3 and 4 narrow it to a check-processing region, which does not always follow district lines: the Fed notes that some processing regions cross district boundaries.
- Digits 5 to 8 — the institution itself. This is the part that names the bank or credit union.
- Digit 9 — the check digit. Arithmetic, not geography. It exists solely so a typo fails fast.
The check digit: the 3-7-1 formula
The last digit is computed from the first eight with weights that repeat 3, 7, 1:
3(d1 + d4 + d7) + 7(d2 + d5 + d8) + 1(d3 + d6 + d9) ÷ 10 must leave remainder 0
Run it on 256074974:
3(2+0+9) + 7(5+7+7) + 1(6+4+4) = 33 + 133 + 14 = 180
180 divides by 10 exactly, so the number passes. Change one digit — 256074975 — and the sum becomes 181, which fails. Swap two digits — 265074974 — and it becomes 186, which also fails. That is the point: the formula catches any single-digit slip and most transpositions before a payment ever leaves.
The weights are 3, 7 and 1 because each is coprime to 10. A weight that shared a factor with 10, such as 5 or 2, would throw away information and let some errors through.
What a passing check digit does not prove
This is where people over-trust the arithmetic. The formula proves the nine digits are internally consistent. It does not prove the number belongs to a real institution, that the institution still uses it, or that it is the right number for the payment rail you are on. Plenty of well-formed numbers are unassigned. Verify against your bank's own published number — on its site, in its app, or printed on your check — before you move money.
One bank, one number?
Not necessarily, and the direction of the error matters. Large commercial banks commonly issue different routing numbers by state or by region, so a number copied from a friend at the same bank may not be yours. Credit unions more often run a single national number — Navy Federal publishes one routing number, 256074974, for every member in every state. The safe assumption is the reverse of the intuition: unless your institution says otherwise, treat a routing number as yours specifically, not as your bank's generally.
Sources: Federal Reserve Regulation CC, Appendix A (routing-number structure, thrift prefix); general ABA check-digit formula as documented in the ABA routing transit number specification and implemented in public validation libraries.