What Is a Routing Number? The 9-Digit ABA Number, Digit by Digit
Last verified against primary sources: October 17, 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 created the system — a Federal Reserve Bank of Atlanta article dates the ABA's numerical transit plan to 1911 — 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.
What the number actually does
Nacha, which operates the ACH network, calls the routing number "a unique nine digit identifier designating U.S. FIs and used for routing ACH transactions," and its December 2024 operations bulletin traces the job description back to paper:
“The American Bankers Association (‘ABA’) Routing Number was created to identify the financial institution responsible for the payment of a negotiable instrument. Although the first use of routing numbers began with checks, routing numbers now also designate participants in ACH, wire, instant and other payment systems.” — Nacha, ACH Operations Bulletin #4-2024
Two directions of travel follow from that, and both matter when something goes wrong. Outbound, the receiving institution's number is what lets a payment reach the right bank — Nacha calls it "a crucial piece of information in an ACH payment, allowing it to be routed to the appropriate financial institution to be posted to the authorized account." Inbound, a return travels the same road backwards, because "the originating institution's routing number is also included in an ACH transaction," so "the return of a payment is routed back to the sender, when appropriate."
The machinery in between is the ACH operators, and their upkeep is the part most people never see: "This routing is performed by the ACH Operators, which take in ACH payment files from and distribute files to their respective financial institution customers. ACH Operators exchange participant lists of routing numbers daily to ensure interoperability." A directory refreshed daily is what makes nine digits sufficient addressing for the whole country.
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.
If you would rather not run the arithmetic by hand, the wrong-routing-number guide embeds a checker that runs it on any nine digits you type — and stress-tests how much it actually catches.
Who assigns them — and who retires them
Assignment sits with the ABA's registrar. Nacha's bulletin names it: "LexisNexis Risk Solutions ('LNRS' - formerly Accuity) is the official ABA Registrar," and describes the process — "Financial institutions submit applications to LNRS to obtain new routing numbers upon charter approval, and to reinstate or retire existing routing numbers." Nacha's consumer-facing article on the same subject states it more simply: "Accuity, a Nacha partner, is the official registrar for ABA routing numbers."
The register is not static, which is the fact most lookup sites obscure. Nacha's bulletin counts "several hundred additions and deletions of routing numbers per year," and the article explains the causes: "Routing numbers can change with mergers and acquisitions, or even be retired when no longer needed (for example, when a bank closes)." That is the mechanism behind a whole category of failed payments — a number that was correct for decades can stop being current while its holder does nothing at all.
Nacha's recommendation for anyone building on routing numbers is to refresh "at least monthly as a best practice." For an individual the equivalent habit is cheaper: take the number from your bank's current page, app, or a check in hand — not from a saved note, a screenshot, or a message.
The payable-through wrinkle
Appendix A carries one sentence that surprises most readers. A check can be payable by one institution but payable through another, and when it is, "the routing number appearing on the check is that of the payable-through bank, not the payor bank." The nine digits name the institution that will actually handle the payment — not necessarily the brand printed on the check. Transcribing from an unusual check without knowing this is how a payment ends up at the right name and the wrong railroad.
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. It also says nothing about whether a number is still in service — that changes on the registrar's schedule, not the formula's. 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.
What a routing number is not
Four other identifiers get confused with it, and each names something different:
| Identifier | What it names | Shape | Where you find it |
|---|---|---|---|
| Routing number (ABA / RTN) | The US financial institution | 9 digits, fixed | Bottom line of a check; the bank's own site and app |
| Account number | Your specific account at that institution | Length varies by bank | Check, statements, app — often masked |
| SWIFT / BIC code | An institution, for cross-border messages | 8 or 11 characters | International wire forms |
| Debit or credit card number | The card, not the account | 16 digits | The card itself |
Nacha's consumer FAQ draws the last line for you: the routing number is "different from the number on your debit card." US institutions issue no IBANs — Navy Federal publishes none for its accounts, and the US equivalent pair is routing number plus account number. For when each code applies, see SWIFT code vs. routing number; for the pair itself, see routing number vs. account number.
Where you will be asked for one
- Direct deposit and payroll enrollment. The federal SF 1199A and Navy Federal's own form both ask for the routing number and the account number as a pair — field by field here.
- Incoming wires. Anyone wiring you money needs the routing number plus the receiving details — the full field list.
- ACH transfers between institutions. Both directions use the routing number, and the ACH vs. ABA terminology is where people stumble.
- Tax refunds and government benefits. The IRS reads it straight off your direct deposit information — Form 8888 details.
- Bill payment and peer-to-peer apps. Some rails want the pair, others deliberately never ask — which rail is which.
Sources: 12 CFR Part 229, Appendix A (Regulation CC), paragraphs A and B; Nacha, ACH Operations Bulletin #4-2024 — Importance of Maintaining Up-to-Date Routing Transit Numbers, December 16, 2024; Nacha, New Routing Numbers Impacting ACH Processing; Nacha consumer FAQ (routing number vs. debit card number); Federal Reserve Bank of Atlanta, Highway Signs for Checks (1911 transit plan); Federal Reserve Bank of Dallas circular of October 19, 1977 (check-digit formula).