DRAIN PIPE SIZE
DRAIN PIPE SIZE
size = f(Σ DFU)
RESULT
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FILL IN ABOVE
Horizontal branch sizing per IPC Table 710.1. For full stacks or building drains, sum branch DFUs and consult code tables. Estimate only — verify with a licensed plumber and local plumbing code/inspector before purchase or installation. Not a substitute for engineered drawings.
About this calculator
This drain pipe sizing calculator returns the minimum diameter for a horizontal drain branch carrying a given set of fixtures. Sum of Drainage Fixture Units (DFU) is computed from the fixture counts you enter, then matched against IPC Table 710.1 limits. The output is the smallest pipe size that handles the load — anything smaller risks slow flow or backups; anything larger is wasted material and grade. For a soil stack carrying multiple branches, sum the branch DFUs and oversize accordingly. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
Common questions
What is a Drainage Fixture Unit (DFU)?
A DFU represents the discharge load a fixture puts on the drainage system. A toilet is 3 DFU, a lavatory is 1, a tub or shower is 2, a clothes washer is 3. DFU values come from IPC Table 709.1 and they're different from WSFU on the supply side — drainage is intermittent and gravity-driven, so the load calculation is separate.
Why does any branch with a toilet need a 3-inch pipe?
Solids. Toilet waste cannot be reliably carried in pipe smaller than 3 inches no matter how few DFUs the branch totals. IPC requires a minimum 3" branch for any line carrying a water closet, even if the DFU math would otherwise allow 2" — the calculator enforces this rule automatically.
Is the building drain sized the same way?
Same DFU table, different limits. The building drain (the horizontal main below the lowest fixtures) gets a separate column in IPC Table 710.1 with higher DFU caps — a 3" building drain handles 36 DFU at ¼"/ft slope, vs 20 for a horizontal branch. Sum every fixture in the building, look up the building drain row, and that's your main size.