Drainage Fixture Unit Calculator

At typical residential fixture counts, toilets do -- each one contributes 4 DFU under Table 709.1 of the International Plumbing Code (IPC), more than any other single fixture type (clothes washers are next at 3; showers, bathtubs, and dishwashers are 2 each; sinks are 1). A building with an unusually large number of low-weight fixtures like sinks can still out-total a design with few toilets, since Total DFU is a sum across every fixture type, not just the highest-weighted one.

Required Drain Size is looked up from Total DFU against a stepped IPC Table 710.1 table with fixed breakpoints (1.5" up to 1 DFU, 2" up to 6, 3" up to 20, 4" up to 160, 5" up to 360). Adding a fixture only changes the recommended size when it pushes Total DFU across one of those breakpoints -- most single-fixture additions land safely inside the current pipe size's range and change nothing on the drain-size output.

No -- it only sizes the aggregate drain, vent, and stack based on Total DFU. IPC also sets minimum trap and drain sizes for individual fixtures (for example, a 2" minimum trap and drain for showers), which can be larger than what the aggregate system sizing alone would suggest, so always cross-check individual fixture connections against code separately.