Have you ever wondered what the numbers on the bottom of bank checks in the U.S. mean? I noticed these numbers before but had never paid much attention to them until recently. They actually mean something...

As a part of my professional life, I work on computer applications that deal with banks and personal checks. Since I come from a science background and I have spent most of my professional life working with scientists and engineers, banking and personal loans are new territory for me. So I dug around and learned some about bank routing numbers.

The numbers on the bottom of bank checks consist of the bank's routing number, the banking account number, and the check number.

For now, we are just interested in the routing number, or ABA number. This number consists of nine digits that follow the American Bankers Association (ABA) standards and it identifies the payees financial institution. It must be in the following form:

XXXXYYYYC

where XXXX is the Federal Reserve routing symbol, YYYY is the ABA institution identifier, and C is the Check Digit.

There is a formula that the routing number must adhere to in order to be valid.

(3(d1 + d4 + d7) + 7(d2 + d5 + d8) + 1(d3 + d6 + d9))mod 10 = 0

where d_x is the xth element of the routing number and "mod 10 = 0 " means that 0 is the remainder when this number is divided by 10 - in other words the number is evenly divisible by 10.


If you want to know more about routing numbers, check out the Wikipedia entry for Routing transit number.