Sewer Pipe Capacity Calculator

A sewer line is a controlled slide: slope, diameter and pipe smoothness decide how much wastewater a gravity line can carry before it backs up. Manning’s equation puts a number on that slide.

Sewer Pipe Capacity Calculator returns full-pipe flow for a given diameter and grade, in the units your drawings use.

• Manning full-flow: capacity for your pipe.
• Four flow units: GPM, L/s, L/min, m3/h.
• Velocity: self-scouring check.
• Material n: plastic, concrete or corrugated.
• Size for a target: smallest standard pipe for a flow.
• Export: Copy or download the report.

1. Enter the internal diameter in inches.
2. Enter the slope in percent (1/4 in/ft = 2.08%).
3. Pick the pipe material (Manning n).
4. Read the full-flow capacity and velocity.
5. Optional: give a design flow to see the minimum pipe.

Example - 6 in at 1%. Plastic sewer (n=0.013) at 1% grade carries roughly 200+ GPM full. The same line at 0.5% drops to about 140 GPM - a whole branch of the demand curve depends on that grade.

• Real hydraulic numbers: not just fixture-unit guesses.
• Match any spec: GPM or metric, your choice.
• Grade sensitivity: see how slope moves capacity.
• Free and private: no uploads, no logging.