Re: UPC Check Digit Algorithm

Carl,

>1.  Starting from the left, add all the ODD positions into SUM1.
>2.  Multiply SUM1 * 3
>3.  Starting from the left, add all the EVEN positions into SUM2
>4.  Add the product from step 2 above + SUM2  (ie (SUM1*3) + SUM2)
>5.  The modulo 10 check digit is the number when added to the result in step
>4 gives the next higest multiple of 10.  If the sum from step 4 was 83, the
>check digit is 7.

FWIW, the above instructions work but would not be technically correct
if UPC/EAN/JAN numbers had different lengths.  The counting actually
starts from the right, not the left, and excludes the check digit if
you multiply the odd positions by 3 or includes the check digit if you
multiply the even positions by 3.

Since UPC-A and UPC-E have 11 digits excluding the check digit, and
EAN-8 has 7 excluding the check digit, it works out to the same thing.
But EAN-13/JAN-13 would yield different results depending on which
side you counted from since there are 12 digits excluding the check
digit.

Doug
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