Greetings!
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
digits,
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 BrainJar.com: Validation Algorithms
http://www.brainjar.com/js/validation/
The above page also gives a JavaScript code sample for validating the
numbers.
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)
http://www.snopes2.com/business/bank/number.htm
You can read further about the ABA routing numbers and their
background at the ABA website:
http://www.aba.com/Products/PS98_Routing.htm
The complete Routing Number Policy is found in this PDF file:
http://www.tfp.com/text/ABApolcy.pdf
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:
http://www.tfp.com/order_US.shtml
Additionally, you can find banks via their routing number at the
Federal Reseve website:
http://www.fedwiredirectory.frb.org/search.cfm
I also found a Cold Fusion code snippet that validates ABA Routing
Numbers at the Common Function Library Project
http://www.cflib.org/udf.cfm?ID=552
Search Strategies:
ABA routing number algorithm
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