Bank Routing Number Validity

Have you ever seen a check and wondered how banks can be sure it is valid? Well, one way that banks check for validity on a routing number (ABA Routing Transit Number) is by using math! A routing number is a 9-digit number that tells what bank that money is to be drawn from. Math helps keep your hard earned cash in the bank! Here is one way that banks use math to check for valid routing numbers.

Let’s take a look at an example of a valid routing number.

Bank Routing Number: 122105278

The routing number is located at the far left at the bottom of the check.

In this case the routing number is:


Similar to a UPC (Universal Product Code), you have to multiply the routing number by the weight.

In routing numbers, the weight is 7,3,9,7,3,9,7,3.

Notice how there are only 8 weights and 9 numbers in the routing number. When multiplying with the weights, we do not use the last digit of the routing number. The last number of the routing number is known as the “check digit”. We will use this as a “check” later on in the process.

Now you multiply the routing number by the weights. (I put it in a grid so it is easier for you to visualize)

1 *7 7
2 *3 6
2 *9 18
1 *7 7
0 *3 0
5 *9 45
2 *7 14
7 *3 21

Just like in the UPC, let’s add all of the products (what is made when you multiply two numbers) together.

7+6+18+7+0+45+14+21 =


Now, take a look at the check number (the last digit in the routing number). It is 8.

(Look At Highlights)

If the last digit in the routing number is the same as the last number of the products above, then the routing number is valid!

In this case, the routing number 122105278 is valid!

(This isn’t the only way that bank’s validate routing numbers, it is just a quick way to see if it is valid at a glance)

Now that you can tell the validity of a routing number on a check at a glance, you can now know that your check will go smoothly into your bank account!