Bathtubs are measured stud to stud. Bathtubs are mounted directly on the studs, so they are measured without any drywall consideration. A standard tub is 60 inches long and can vary in width as you desire. However, unless you move the plumbing, a replacement bathtub must be the same width as the old one. The waste pipe, overflow valve, and faucets are lined up centerline of the bathtub.