What is the minimum fixture drain size for interior drainage systems typically based on?

The minimum fixture drain size for interior drainage systems is typically based on the fixture outlet diameter. This is because the drain size must accommodate the maximum discharge rate from the fixture, which correlates directly with the diameter of the fixture's outlet. When a fixture is designed, its outlet size is determined by both the expected flow rate and the type of fixture, ensuring that the drainage system can effectively handle the wastewater without risk of backup or overflow.

Other factors, such as building code requirements, may provide guidelines and standards regarding drain sizes, but the most fundamental consideration remains the fixture outlet diameter itself. It serves as the starting point for determining the appropriate drainage size, ensuring that the fixture can efficiently drain its contents into the connected plumbing system. By prioritizing the outlet diameter, designers can ensure that the drainage system is properly sized for optimal performance.