What is the required vent size for a 4-inch toilet drain?

The required vent size for a 4-inch toilet drain is determined by the requirements outlined in plumbing codes, which ensure that the venting system allows for proper air circulation, prevents the occurrence of siphoning, and maintains the functionality of the plumbing system as a whole. A 3-inch vent is typically adequate for a 4-inch toilet drain, as it meets the necessary standards for venting a fixture of that size.

A 3-inch vent can effectively accommodate the adequate air flow needed for a 4-inch drain, which assists in optimizing drainage and preventing potential issues such as clogs or slow drainage. This sizing is in line with industry standards and provides a balance between functionality and material usage.

The other sizes—2 inches, 4 inches, and 5 inches—would not be appropriate as they would either be insufficient for adequate venting (in the case of the 2-inch vent) or unnecessary and possibly problematic due to being larger than needed for a toilet drain (in the case of a 4-inch or 5-inch vent). Therefore, the selection of a 3-inch vent aligns with best practices in plumbing systems for a 4-inch toilet drain.