TradesHow this works
Drain sizing runs on fixture units. Each fixture gets a number for how much it loads the drain, you add them up, and the pipe has to be big enough to carry the total. Pick whether you are sizing a horizontal branch or the building drain, add your fixtures, and the tool gives you the size, the capacity, and how much room is left.
Two rules do a lot of the work. Any drain with a toilet on it is at least 3 inches. And for a building drain, the slope matters: a steeper pipe carries more. The tool builds both in, and it uses IPC values, so confirm against your local code, especially if your area follows the UPC.
This sizes the drain. Vent sizing is a separate calculation and is not included here.
Common questions
What is a drainage fixture unit?
A drainage fixture unit, or DFU, is a number the code assigns each fixture to represent its drain load. A toilet is 3, a sink or tub is 2, a bathroom sink is 1. You add them up and size the pipe to carry the total.
Why is a drain with a toilet always at least 3 inches?
The IPC requires any drain serving a water closet to be at least 3 inches, no matter how small the calculated load is. The tool applies that automatically as soon as you add a water closet.
Does slope really change the pipe size?
For a building drain, yes. A steeper slope moves waste faster, so the same pipe carries more. A 4 inch drain carries 180 fixture units at a gentle 1/8 inch per foot, but 216 at 1/4 inch per foot. Steeper needs more fall, which you may not have under a slab.
IPC or UPC?
This uses IPC fixture-unit values. Some areas follow the UPC, which assigns fixture units and sizes differently. Always confirm against the code where you work.
Sources: IPC Table 709.1 (drainage fixture units) (2021) · IPC Table 710.1(1) and 710.1(2) (drain and branch sizing) (2021). Engine version 1.0.0. Risk tier 2.