Formula to autofill letters at end of cell

I have the same number in multiple rows and I'm looking to add _A, _B, _C, etc. to the end of each cell to make them unique. Does anyone know what formula I could use for something like this? The formula I've tried is substitute(address(1,rows(a$1:A1),4),1,"") but all i end up with is "A" rather than "12345_A"

Try this

=A1&"_"&CHAR(64+ROW(A1))

That did it. Thanks Bob - saved me a ton of time.

Bob Phillips said:

Try this

=A1&"_"&CHAR(64+ROW(A1))

Click to expand...

Follow up question. After i get to 12345_Z is it possible to then have it fill as 12345_AA, 12345_AB etc?