ABA vs routing number — what's the difference?
Short answer: there is no difference. The two terms refer to the same nine-digit code. The reason both names persist is a century of overlapping standards bodies.
Where the names come from
The American Bankers Association (ABA) introduced the routing number system in 1910 to standardize check clearing among member banks. Each participating bank received a unique nine-digit identifier — the "ABA number" — which was printed on every check the bank issued. The Federal Reserve later adopted the same numbers for its own clearing operations, and over time the term routing transit number (RTN) — or simply "routing number" — became the official name used in regulations and software.
Today, "ABA number," "ABA routing number," "routing transit number," "RTN," and "routing number" all mean exactly the same thing: the nine-digit code that identifies a US bank within the domestic payments system.
How the nine digits are constructed
Each routing number is composed of three parts:
- Digits 1–4: Federal Reserve routing symbol. Identifies which Federal Reserve district and processing center settles transactions for this bank.
- Digits 5–8: ABA institution identifier. Uniquely identifies the bank within that district.
- Digit 9: Check digit. A mathematical checksum derived from the first eight digits using a weighted formula. This digit allows software to detect single-digit transposition errors before sending a payment.
The check-digit formula is straightforward: multiply each of the first eight digits by 3, 7, 1, 3, 7, 1, 3, 7 respectively, sum the results, and take the value modulo 10 — then subtract from 10 (or use 0 if the result is 10). Any number that fails this check is automatically rejected by every payments processor in the country, which is why a typo on a check or a deposit slip almost never lets a payment slip through to the wrong bank.
What about EFT, ACH, and SWIFT?
The terms EFT (electronic funds transfer), ACH, and wire describe types of transactions; they don't have their own numbering systems. All of them use the same ABA routing number to identify the bank involved.
SWIFT codes (also called BIC codes) are an entirely separate international identifier — typically 8 or 11 characters mixing letters and numbers — used for cross-border payments outside the US ACH network. A US bank that accepts international wires has both a SWIFT code and an ABA routing number; the two coexist and don't replace each other.
Practical takeaway
If a form asks for your "ABA number," "routing number," "RTN," or "transit number," all four are asking for the same nine digits. There's no need to ask your bank which one they mean — you can answer all four questions with the same value.