Drain pipe sizing
Count the fixtures on the line and the type of run, and read the minimum drain size straight off the IPC dfu tables.
- Bathroom group (WC + lav + tub/shower) 5 dfu1
- Water closet, private (1.6 gpf) 3 dfu0
- Water closet, public (1.6 gpf) 4 dfu0
- Lavatory (bathroom sink) 1 dfu0
- Shower (up to 5.7 gpm) 2 dfu0
- Bathtub (with or without shower) 2 dfu0
- Kitchen sink, domestic 2 dfu1
- Dishwasher, domestic 2 dfu1
- Clothes washer, residential 2 dfu1
- Laundry tray 2 dfu0
- Floor drain 2 dfu0
- Service / mop sink 2 dfu0
- Bidet 1 dfu0
- Urinal (1 gpf or less) 2 dfu0
- Urinal (over 1 gpf) 4 dfu0
- Drinking fountain 0.5 dfu0
Result
The dfu math alone would fit a smaller pipe, but a water closet on the line sets a hard 3″ floor.
How is this calculated?
Using the tool
Drainage pipe isn't sized by adding up gallons — the code weighs each fixture in drainage fixture units (dfu) and caps what every diameter may carry, depending on whether the run is a branch, a stack, or the building drain. Count what's on the line, say what kind of run it is, and read the minimum size straight off the IPC tables.
The code
A fixture unit is a probability, not a flow rate — a lavatory counts 1, a bathroom group 5, because the code cares how likely fixtures are to discharge at once, not what they'd pour if they all did. That's why capacity explodes with diameter: a 3" horizontal branch tops out at 20 dfu while 4" carries 160 — the cross-section grows with the square of the diameter, and the odds of everything running together shrink as the count grows. The same pipe is also rated differently by job: vertical stacks swallow more than horizontal branches, and the building drain depends on its slope. One rule overrides the math — any line serving a water closet is 3" minimum, full stop — and pipe never shrinks in the direction of flow.
New to the math? Learn the concept: Plumbing fundamentals