I found this great formula (below) for calculating the 12th (check
digit) for a 12 digit upc code and then yielding the entire code
including 12th digit. Does anybody know how to modify or alter this
formula to calculate the 13th digit (check digit) for a 13 digit UPC A
code and yield the entire code including the 13th digit?

=A1&MOD(10-MOD(SUMPRODUCT(MID(A1,ROW($1:$11),1)*
(1+2*MOD(ROW($1:$11),2))),10),10)


The long way is:
Stating as example the following code: 739007812345

Start by adding every other figure from the right and then multiply by
3.
5 + 3 + 1 + 7 + 0 + 3 = 19 * 3 = 57
After this add the rest of the figures:
4 + 2 + 8 + 0 + 9 + 7 = 30
Add the above results 57 + 30 = 87
Next reduce this amout 87 from the closest higher ten (90) and you
receive 90 - 87 = 3
3 is the check digit.

Thanks,

M. Jack