Work out drain pipe sizing instantly with clear inputs, formula shown and shareable results.
Gravity drains are sized to run no more than half full so air can move above the water and protect trap seals. Manning's equation, Q = (1.486/n) A R^(2/3) S^(1/2), gives the half-full capacity of each standard size, and the smallest size that carries the fixture unit design flow is selected.
Manning capacity
Q = (1.486 / n) x A x R^(2/3) x S^(1/2), evaluated half full
Design flow
GPM ≈ 2.5 x DFU^0.6
Preliminary estimate only. Drainage sizing must follow the fixture unit tables and slope requirements of the plumbing code adopted locally.
Q = (1.486 / n) x A x R^(2/3) x S^(1/2), evaluated half full. Gravity drains are sized to run no more than half full so air can move above the water and protect trap seals.
Above about 4 percent the water outruns the solids, leaving them stranded in the pipe. The classic quarter inch per foot balances velocity against carrying capacity.
This calculator takes 3 inputs: Total drainage fixture units, Pipe slope, Manning roughness n. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.