# How to Calculate Check Digits in Routing Numbers

Routing numbers on checks have "check digits" to confirm validity.

When you write a check, the bank that cashes it uses the routing number to determine which bank the check is coming from. The American Banking Association assigns routing numbers to banks, credit unions and other financial institutions. Routing numbers have been used since 1910. The routing number on a check is found in the lower left-hand corner of the check and is a nine-digit number. The final digit of the routing number is a check digit, meaning it can be calculated from the first eight digits as a way to prevent bad checks.

## Step 1

Multiply the first, fourth and seventh numbers in the routing number by 3. For example, if your routing number was 123456780, you would multiply 1, 4 and 7 by 3 to get 3, 12 and 21.

Video of the Day

## Step 2

Multiply the second, fifth and eighth digit in the routing number by 7. Continuing the example, with a routing number of 123456780, you would multiply 2, 5 and 8 by 7 to get 14, 35 and 56.

## Step 3

Multiply the third and sixth digit in the routing number by 1. In this example with, you would multiply 3 and 6 by 1 to get 3 and 6.

## Step 4

Add the products from the first three steps. In this example, you would add 3, 12, 21, 14, 35, 56, 3 and 6 to get a total of 150.