Section 01 · Spatial & Fluid Dynamics
The Plumber
Every drain line is a geometry problem solved against gravity. Pick a lab — slope, offset, DFU, pressure, or pipe sizing.
Drainage Fixture Unit (DFU) Calculator
Total DFU = Σ (count × per-fixture DFU) • Pipe size ↑ as DFU ↑
The drain must carry worst-case simultaneous flow. IPC §709 assigns a DFU per fixture; the sum picks the pipe size.
Water closet (1.6 gpf, private)×4
Water closet (1.6 gpf, public)×6
Lavatory×1
Bathtub / shower×2
Kitchen sink (residential)×2
Dishwasher×2
Clothes washer (residential)×3
Floor drain×2
Urinal (1.0 gpf)×4
Total DFU
0
Min branch size
1-1/2"
≤ 1 DFU → 1-1/2"
2–6 DFU → 2"
7–20 DFU → 3"
21–160 DFU → 4"
161–360 DFU → 5"