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UPC Modulo checksum formula
For those of you who wish to calculate your own mod check character,
here is the formula.
To follow, use this UPC as an example: 01234567890? 

Step 1  Starting at the left, sum all the characters in the ODD positiions
(e.g. first from the left, third from the left, etc.) STARTING WITH THE
NUMBER SYSTEM CHARACTER
.0 + 2 + 4 + 6 + 8 + 0 = 20 
Step 2  Multiply the sum we got in Step 1 by 3.
20 x 3 = 60 
Step 3  Starting at the left, sum all the characters in the EVEN positions.
1 + 3 + 5 + 7 + 9 = 25 
Step 4  Add the product of Step 2 to the sum of Step 3.
60 + 25 = 85 
Step 5  The modulo10 check digit value is the smallest number which produces
a multiple of 10, when added to the sum in Step 4.
Hence, our check digit is 5. 
Our completed UPC would then be 012345678905. 
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