The American Bankers Association developed ABA routing numbers in 1910, originally to speed up check processing. The system has since expanded to cover ACH transactions, electronic fund transfers, and online banking — and today roughly 28,000 active routing numbers are in use across U.S. financial institutions.
U.S. routing number structure
Every U.S. routing number is nine digits, in the format XXXXYYYYC — a Federal Reserve routing symbol, an ABA institution identifier, and a check digit. The first two digits identify the category of institution:
- 00 — reserved for the United States Government
- 01–12 — standard routing numbers
- 21–32 — thrift institutions
- 61–72 — electronic transactions
- 80 — traveler's cheques
How a routing number is validated
Before a routing number is ever matched against a real institution, it has to pass a checksum. Labeling the nine digits d₁ through d₉, the formula is:
A number that fails this formula isn't a valid routing number at all — regardless of whether it matches a real bank's opening digits. That makes it one of the cheapest first-pass checks available before a number ever touches a verification service.
Outside the U.S.
Routing systems vary by country, and none of them use the American nine-digit format:
- Canada — branch and institution numbers, formatted XXXXX-YYY
- Australia — Bank State Branch (BSB) codes
- United Kingdom — six-digit sort codes