A standard modern bathtub drain is 1 1/2 inches in diameter — the width straight across the middle. Rough-in plumbing should be calculated for a 1 1/2-inch pipe
Need a drain for your new tub or bathroom remodel? Learn how (and what) to measure to make sure you choose the right size.
The standard size bathtub is 5 feet long by 30 inches wide but there are. On the side of framing where the tub's drain will go, a hole 12" long by 9" wide needs .