Question: are all bathtub drains the same size?
Answer: 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. ... Antique bathtubs, such as historic clawfoot tubs, sometimes have slightly smaller drain openings, often 1 3/8 inches in diameter.