If cells A! to A50 contain the units and B1 to B50 indicate whether or not
they are occupied (blank meaning not occupied) then = COUNTBLANK(b1:b50) will
give number free.

Or similar logic using COUNTIF.

Would this work?

"Keith" wrote:

on the spreadsheet I have a rental complex with 50 units and I rent unit 1A I
would like that to reflect that the complex has only 49 available units left.
I understand how to subtract one cell from another but by using 1A that
formula doesn't work.

"Toppers" wrote:

First, you have posted to two groups ... please don't do this.

And second, I(!) have no idea what exactly you require; perhaps an example?

"Keith" wrote:

subtract from a cell if another cell is occupied