What trap size is used for a water closet with a flow rate of 1.6 GPF or less?

The appropriate trap size for a water closet (commonly known as a toilet) that has a flow rate of 1.6 gallons per flush (GPF) or less is 3 inches. This size is typically used to accommodate the larger volume of waste and water that needs to be effectively drained while also considering proper venting and minimizing clogs. The 3-inch trap not only supports the flush volume but also provides ample space for the waste to exit with minimal resistance, which is vital for maintaining efficient drainage.

In comparison, smaller trap sizes such as 1 1/2 inches or 2 inches may not provide sufficient capacity for effective waste movement, especially when considering the potential for blockages and the volume of water that is discharged during each flush. A 4-inch trap could also work, but it is generally used in commercial applications rather than standard residential conditions. Therefore, the optimal choice for a water closet with a flush rate of 1.6 GPF would be the 3-inch trap size.