Work out sewer pipe sizing instantly with clear inputs, formula shown and shareable results.
Gravity sewers are sized with Manning's equation, evaluated at full bore, then limited to a proportional depth of 70-75 percent so air can move above the flow. The smallest standard diameter whose limited capacity exceeds the design peak flow is selected, and full bore velocity is checked against the self-cleansing minimum.
Manning capacity
Q full = (1/n) x (D/4)^(2/3) x S^(1/2) x pi D² / 4
Selection
Choose the smallest diameter where Q full x fill ratio is at least the design flow
Preliminary estimate only. Sewer design must follow the drainage code and authority standards, and be verified by a qualified engineer.
Q full = (1/n) x (D/4)^(2/3) x S^(1/2) x pi D² / 4. Gravity sewers are sized with Manning's equation, evaluated at full bore, then limited to a proportional depth of 70-75 percent so air can move above the flow.
A sewer running full has no ventilation path, so pressure fluctuations break trap seals upstream and septic conditions and odour develop. It also leaves no capacity for peaks.
This calculator takes 4 inputs: Design peak flow, Pipe gradient, Manning roughness n, Maximum proportional depth. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.