Storm Drain Sizing Calculator
Size storm drain pipes using the Rational Method for flow estimation and Manning's equation for pipe capacity. Selects nearest standard pipe size.
Inputs
in/hr
acres
ft/ft
Results
Design Flow (Q)
10 cfs
Selected Pipe Size
18 in
Required Diameter17.7 in
Flow Velocity5.94 ft/s
Pipe Capacity10.5 cfs
How to Use This Calculator
- Enter runoff coefficient (C) for the drainage area land use.
- Set rainfall intensity (in/hr) for the design return period and drainage area (acres).
- Review design flow (Q = CiA) and required pipe diameter for your chosen slope.
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