Create formula or macro to add upc check digit with leading zero

Hello and welcome to the forum,

Does the end result still need to be read by excel as number?

Depending on your answers, you can use something like:

=TEXT(A1,"0000")

And the result for 1, 10, 100 and 1000 are as follows:

1	0001
10	0010
100	0100
1000	1000

Hope this helps.

abousetta