How to size drainage pipe by fixture units
Drainage piping is never meant to run full. Each fixture is given a drainage fixture unit (DFU) value that represents its discharge load, the units are totaled, and the pipe is picked from a table of maximum DFU per size. This calculator uses the IPC Table 709.1 fixture values and the horizontal branch and stack limits from Table 710.1(1).
The fixture values applied here are 4 DFU for a water closet, 4 for a urinal, 1 for a lavatory, 2 for a bathtub or shower, 2 for a sink, 3 for a clothes washer on a 2 in standpipe, and 2 for a floor drain. On the capacity side, a horizontal branch carries 6 DFU at 2 in, 20 DFU at 3 in, and 160 DFU at 4 in, while a stack of three branch intervals or less carries 10, 48, and 240 DFU at the same sizes. A 40 DFU total therefore needs a 4 in branch but only a 3 in stack.
Building drains and building sewers use the separate slope-based table, so do not carry this result straight to the underground main. Venting, trap seal protection, and stack offsets can all change the final selection as well. Check the adopted code edition and any local amendments, and have a plumbing engineer confirm the layout before permit drawings are issued.
Frequently asked questions
Water in a vertical stack clings to the pipe wall and leaves an air core in the middle, so the same diameter moves more load. A horizontal branch flows only on its slope, so its allowable fixture unit load is set much lower.
No. Building drains and building sewers use a separate table where the allowable load changes with slope, such as 1/8, 1/4, and 1/2 inch per foot. Take the total DFU from this calculator into that slope table instead.