What is the minimum size for a bathtub drain trap?

The minimum size for a bathtub drain trap is 1-1/2 inches. This size is specified in plumbing codes because it ensures adequate flow and prevents clogging, given the volume of water that typically drains from a bathtub. Bathtubs can discharge water rapidly, especially when filled to capacity, so a larger drain trap size helps accommodate this flow rate. Additionally, a 1-1/2 inch trap size aligns with standard practices in residential plumbing to provide a balance between efficiency and functionality, ensuring effective waste removal while minimizing the risk of any backups.