How do I figure what size shower to replace tub.

I need to replace a tub with a 60 inch shower. I can get a shower pan and a wall surround that will do the job. I am not sure how to tell if I have a 30 or 32 inch tub. The tub is just a few inches away from the toilet and the wall covering is up against the counter top. I have no room to put in a larger tub. I would like to be able to get the replacements before I rip out the old tub. Measuring from the back wall (against the drywall above the surround) to where the front edge of the surround meets the drywall looks to be about 30 inches. Would the correct measurement include the part that is behind the drywall, or is this a 30 inch tub? Thanks

There's usually not going to be another 2 inches hidden behind there, so you should have a 30" tub.

Remember that you will have some drain issues to deal with here. The tub is 1½" , and the shower requires 2". The tub drain is probably not in exactly the same place as the new one will be. With the tub and walls removed, all this can be done without too much difficulty. If you have any issues, post some pictures and ask us more questions after you get the old tub out.

Swan makes shower pans with a drain on the end.
http://www.theswancorp.com/index.php?prod=383