Jeff Linder wrote:

We must replace a 70-year-old toilet. In order to choose a toilet we are
told that we need to know the rough-in. What is this? On plumbing supply
web sites, such as kohler.com and americanstandard.com, there is mention of
12-inch rough-in. How do we determine our rough-in? And can we buy a
toilet without knowing that? Thank you. -Jeff Linder )
the rough in is the measurement from the back wall of the toilet to the
center of the hole in the floor( probably a 4 inch hole, pipe for drain)
it gonna be where the bolts go down to attache the toilet bowl to the
floor...i dont have it but if you do a search for american standard
toilets and find the american standard web site something like
usamerican standard or so.. look up toilets and then install
instructions.. they are pretty good at illustrating this...