What is the minimum diameter required for a bathtub's waste overflow?

For a bathtub's waste overflow, the minimum diameter required is 1.5 inches. This requirement is based on plumbing codes and standards that aim to ensure adequate drainage and prevent clogs. The larger diameter allows for a more efficient flow of water, which is crucial in avoiding potential backups or overflow situations.

A 1 inch diameter could lead to inadequate drainage if the volume of water is significant, especially during a filling bathtub. Similarly, while 2 inches and 2.5 inches are larger than necessary, they exceed the minimum requirement and could be impractical by requiring larger fittings and potentially increasing installation complexity and cost without providing additional benefits for most residential bathtubs. Thus, 1.5 inches optimally balances the need for effective drainage with practicality within standard plumbing practices.