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Q: ABA routing algorithm ( Answered 5 out of 5 stars,   1 Comment )
Subject: ABA routing algorithm
Category: Computers > Algorithms
Asked by: willgreenhalgh-ga
List Price: $2.00
Posted: 21 Jul 2002 23:01 PDT
Expires: 20 Aug 2002 23:01 PDT
Question ID: 43619
What is the algorithm used to create ABA routing numbers.? How do they
check/create the nine numbers between the |:'s on checks?
Subject: Re: ABA routing algorithm
Answered By: wengland-ga on 22 Jul 2002 07:47 PDT
Rated:5 out of 5 stars

What an interesting question - I did not even know they had a checksum
until I began researching this question.

The algorithm to check the ABA Routing Number is as follows:

We'll start with a routing number like 789456124. Here's how the
algorithm works. First, strip out any non-numeric characters (like
dashes or spaces) and makes sure the resulting string's length is nine

7 8 9 4 5 6 1 2 4

Then we multiply the first digit by 3, the second by 7, the third by
1, the fourth by 3, the fifth by 7, the sixth by 1, etc., and add them
all up.

(7 x 3) + (8 x 7) + (9 x 1) +
(4 x 3) + (5 x 7) + (6 x 1) +
(1 x 3) + (2 x 7) + (4 x 1) = 160 

If the resulting number is an integer multiple of 10, then the number
is valid.  To calculate what the checksum digit should be, follow the
above algorithm for the first 8 digits.  In the case above, you would
come up with 156.  Thus, to make the total number an integer multiple
of 10, the final check digit must be 4.

From Validation Algorithms

The above page also gives a JavaScript code sample for validating the

The numbers are acutally created by the ABA, and in general, the first
four digits are a Federal Reserve routing symbol, identifying which of
the twelve Federal Reserve  districts the check was printed in (and a
city within that district). The next four digits are an ABA
institution identifier which designates the bank on which the check is
drawn, and the last number is a check digit.

From Urban Legends Reference Pages: Business (Check and Double Check)

You can read further about the ABA routing numbers and their
background at the ABA website:

The complete Routing Number Policy is found in this PDF file:

If you would like the complete key to all ABA Routing Numbers, you can
order it for $169.00 from Thomson Financial Publishing at this page:

Additionally, you can find banks via their routing number at the
Federal Reseve website:

I also found a Cold Fusion code snippet that validates ABA Routing
Numbers at the Common Function Library Project

Search Strategies:

ABA routing number algorithm
willgreenhalgh-ga rated this answer:5 out of 5 stars
Very thorough, well explained. I spent an hour and couldnt find a damb
thing. I figured it was top secret or something.

Subject: Re: ABA routing algorithm
From: chriswaterson-ga on 03 Jun 2004 11:23 PDT
Additionally, the first two digits of the routing number must fall
within the range "01" through "12", or "21" through "32". These digits
indicate the Federal Reserve District (there are twelve), with the
latter set indicating that the bank is a "thrift institution".

See FRB Regulation CC, Appendix A, which available online here:

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