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document ISO 2108:1992
The UPC/EAN checksum character
The last character in a 
Checksum calculation for UPC-A,  EAN-13  and EAN-8 
Use Worth Data’s phone number (it's not a real 
18314589938
Assign even and odd positions, starting at the right and moving left:
8  | 3  | 9  | 9  | 8  | 5  | 4  | 1  | 3  | 8  | 1  | 
odd  | even  | odd  | even  | odd  | even  | odd  | even  | odd  | even  | odd  | 
1.Starting with the leading digit, 8, take the sum of all the characters in the odd positions.
8 + 9 +8 + 4 + 3 + 1 = 33
2.Multiply the result of step 1 by 3.
33 x 3 = 99
3.Now take the sum of all the 
3 + 9 + 5 + 1 + 8 = 26
4.Add the result in Step 2 to the result in Step 3.
99 + 26 = 125
5.Subtract the result from the next higher multiple of 10.
Next higher multiple of 10 over 125 = 130
130 - 125 = 5
5 is the 
becomes:
183145899385.
This same formula is used for 
43