Drainage Pipe Sizing Calculator
Size horizontal and vertical drain pipes from total drainage fixture units per code.
Results
Visualization
How It Works
Drainage pipe sizing is based on the total fixture units draining into the pipe. Code tables specify maximum DFU for each pipe size based on whether it is a horizontal branch, vertical stack, or building drain, and the slope for horizontal pipes.
The Formula
Look up the minimum pipe size where Max DFU >= Your DFU in the appropriate code table.
Variables
- DFU — Total drainage fixture units
- Slope — Grade of horizontal pipe (1/8" or 1/4" per foot)
Worked Example
A horizontal branch at 1/4" slope with 20 DFU requires a 3" pipe (max 20 DFU). Adding one more toilet (4 DFU) would require upsizing to 4" pipe.
Practical Tips
- Building drains and sewers require larger pipes than branches for the same DFU
- Vertical stacks can handle more DFU than horizontal pipes
- Steeper slopes allow more DFU in horizontal pipes
- Never reduce pipe size in direction of flow