Sewer Full-Flow Capacity Calculator
How much can that gravity sewer carry? Enter diameter, slope, and roughness to get the full-flow capacity and velocity by Manning’s equation.
Full-Flow Capacity
| Item | Value |
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How the calculation works
Manning’s equation, circular pipe flowing full (US units)
Q = (1.486 / n) × A × R2/3 × S1/2
A = π × D² / 4 · R = D / 4 (full flow)
V = Q / A
1 cfs = 448.831 gpm = 0.646317 MGD
Q = (1.486 / n) × A × R2/3 × S1/2
A = π × D² / 4 · R = D / 4 (full flow)
V = Q / A
1 cfs = 448.831 gpm = 0.646317 MGD
Notes & limitations
Manning’s equation is the standard gravity-sewer capacity relation (Metcalf & Eddy; Ten State Standards). Full-flow capacity is the reference condition — sewers normally run partly full, where the velocity at a given depth differs from the full-flow value.
- Design sewers to flow partly full at the design flow (commonly d/D ≤ 0.75–0.85) so capacity remains for peaks — full flow is the upper reference, not the operating target.
- Peak velocity in a circular pipe actually occurs slightly below full depth (around 0.8–0.9 d/D); full-flow velocity is a conservative check.
- The n value for aged pipe with slime and deposits runs higher than new-pipe values — use 0.013–0.015 for design unless your agency specifies otherwise.
- This tool does not check cover, structural loading, or minimum diameter requirements — see your state standards.
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