Storm Drain Pipe Sizing Calculator
Manning's Equation — Full & Part-Full Circular Pipe
Compute full-pipe capacity and velocity for a circular storm drain, check part-full flow at any depth, and find the smallest standard diameter that carries your design flow. US customary units (CFS, GPM, ft).
1Pipe Properties
Slope as a percentage (1% = 0.01 ft/ft). A 0.5–2% slope is common for storm drains.
100% = just flowing full. Circular pipes carry peak discharge near 94% depth.
Get this from the runoff calculator (Q = CIA), then size the pipe to carry it.
2Full-Pipe Capacity
Capacity (full)
22.68
CFS
Capacity (full)
10181
GPM
Velocity (full)
7.22
ft/s
Area / Hyd. Radius
3.14
ft² • R = 0.50 ft
Part-Full Flow (100% depth)
Flow at depth
22.78
CFS
Velocity at depth
7.25
ft/s
Recommended Diameter
18" pipe
Smallest standard size whose full capacity (10.53 CFS) meets the 10 CFS design flow.
Full-flow velocity at this size: 5.96 ft/s.
About This Calculator
Capacity uses Manning's equation for a circular pipe in US customary units. Part-full flow uses the exact partly-filled circular geometry, so a circular pipe's peak discharge occurs near 94% of full depth.
Q = (1.49/n) × A × R^(2/3) × S^(1/2)
- Q = Flow rate (cfs)
- n = Manning's roughness coefficient
- A = πD²/4 (full) — cross-sectional area (ft²)
- R = D/4 (full) — hydraulic radius (ft)
- S = Pipe slope (ft/ft)
Related Calculators & Products
Pair With These Tools
- Runoff Calculator — find the peak design flow (Q = CIA) this pipe must carry.
- Open Channel Flow Calculator — check Manning's flow and Froude regime for open channels.
- Detention Volume Calculator — size storage when the outlet pipe restricts release rate.
- Weir & Orifice Calculator — size the outlet structure controlling flow into this pipe.
- Flow Regimes Guide and Load Ratings for design references.
Specify the Hardware
- Catch basins collect runoff and connect to your sized pipe runs.
- Drainage grates cap inlets ahead of the pipe network.
- ACO pipe systems and JOSAM push-fit pipe for the conveyance itself.
- Detention systems when downstream capacity caps your release rate.