How Aba Transit Numbers are used

American Bankers Association (ABA) transit numbers, also known as ABA routing numbers, are 9-digit numbers which can be found at the bottom of most negotiable financial instruments, such as checks, in the United States. If there are more than 9 numbers on the bottom of the check, the ABA transit number is usually the first 9 digits.

ABA transit numbers are used to identify the financial institution on which the check was drawn in order to facilitate sorting and routing of the check. In order to qualify for an ABA transit number, a financial institution must meet the conditions to hold an account at the Federal Reserve Bank.

To minimize mistakes in automated reading, the ABA transit number now appears in 2 forms on the check. The magnetic ink character recognition (MICR) form is what appears at the bottom. It always has exactly 9 digits. The fraction form can be found at the top, near the date, and serves as a backup in case the MICR is damaged. It can have 10 digits. Until electronic scanning was invented, the fraction form was the only version used for manual processing.

In the MICR form, the first 4 numbers of the code are the Federal Reserve Routing Symbol and the second 4 numbers of the code are the ABA Institution Identifier. The last digit is the Check Digit, which is a position-weighted sum of the other digits and serves as a checksum test to ensure the number’s validity.

In the fraction form, a 2-digit prefix precedes the Federal Reserve Routing Symbol and the ABA Institution Identifier. This prefix represents the city or state in which the bank is located. The ABA Institution Identifier comes next, followed by a slash and the Federal Reserve Routing Symbol. This is the reverse of their order in the MICR form.

There is no Check Digit in the fraction form. This means that unlike the MICR form, the fraction form can be up to 10 digits long: the 2-digit city prefix, the 4-digit ABA Institution Identifier, and the 4-digit Federal Reserve Routing Symbol.
Each part of the Federal Reserve Routing Symbol and ABA Institution Identifier represents specific city, state, and Federal Reserve Bank and processing center information.

Some Federal Reserve Routing Symbols and ABA Institution Identifiers are less than 4 digits long. In this case, they are left-padded with 0s to meet the 4-digit requirement.

Currently, there are nearly 27,000 active routing and transit numbers in use. Some institutions may temporarily have multiple routing numbers after a merger, which will be phased out in the 18 months following the merger.