DEFINITION of ABA Transit Number

ABA Transit Numbers, also known as ABA Routing or Routing Transfer Numbers, are used to identify specific U.S. financial institutions and appear on standard checks.


ABA Transit Numbers were developed by the American Bankers Association in 1910 to indicate check processing endpoints. Since then, the numbers' use has expanded to include participants in check clearing between banking institutions, automated clearing houses and online banking activities. The numbers are also used in Fedwire transfers. 

The ABA routing number appears in two forms on most checks: as a machine-readable nine-digit number on the bottom left (followed by the checking account number), and as a fraction at the top. The first form, known as the magnetic ink character recognition (MICR) form, is the one used to actually process checks. The second is a holdover from when checks were manually processed and contains most of the same information in a different format. The second form is still used as a backup in case the machine-readable format is illegible. 

In order to qualify for an ABA transit number, a financial institution must be eligible to hold an account at a Federal Reserve bank. The routing number system is currently managed by Accuity, a subsidiary of London-based RELX Group, which is listed in the U.S. as RELX PLC (RELX).