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If you’ve ever linked online accounts together, chances are you’re familiar with the nine-digit ABA routing transit number. On a personal check, it’s usually the first in a series of three numbers at the bottom of a check. On a business check, it’s usually the second in a series of three numbers. The three numbers are the same on both, they’re just in a different order.

Personal Check

Business Check

The ABA routing transit number identifies the bank and tells the recipient’s bank where to obtain the funds. The system itself is quite complicated but the numbers are quite easy to decipher. The nine digits are separated into three sections. The first four digits designate the Federal Reserve Routing Symbol, the next four is the ABA Institution Identifier, and the last digit is a checksum digit.

Federal Reserve Routing Symbol

The first four digits are separated into two sections, the first two digits and the second two. US Government checks always begin with 00 whereas digits 01 through 12 designate one of the twelve Federal Reserve Banks. If the transaction is electronic, it’s given a 61 through 72 that follows the same order as below (61 is Boston, 72 is San Francisco).

• 01 - Boston
• 02 - New York
• 03 - Philadelphia
• 04 - Cleveland
• 05 - Richmond
• 06 - Atlanta
• 07 - Chicago
• 08 - St. Louis
• 09 - Minneapolis
• 10 - Kansas City
• 11 - Dallas
• 12 - San Francisco

ABA Institution Identifier

This is a unique four digit identifier for the bank.

Checksum Digit

This digit is used to ensure the numbers are properly formatted. It follows an equation:
(3(d1 + d4 + d7) + 7(d2 + d5 + d8) + 1(d3 + d6 + d9))mod 10 = 0

There you go, that’s how that 9 digit numbers works!