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

Runoff Coefficient (C)

ASCE: roofs/pavement 0.85–0.95; lawns (flat) 0.10–0.35; cultivated land 0.30–0.50

Rainfall Intensity

in/hr

Drainage Area

acres

Pipe Slope

ft/ft

Manning's n

Tab to move between fields

Results

Design Flow (Q)

10 cfs

Selected Pipe Size

18 in

≈ 5 credit cards

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.

How the result changes with Runoff Coefficient (C)

Runoff Coefficient (C)	Design Flow (Q)	Selected Pipe Size
0.19	3.8 cfs	15 in
0.42	8.3 cfs	18 in
0.69	13.7 cfs	21 in
0.91	18.2 cfs	24 in

What each input means

Runoff Coefficient (C): Rational Method runoff coefficient C per ASCE 5/7 and local drainage manuals. Fraction of rainfall that becomes runoff: impervious pavement ≈ 0.90; flat lawns ≈ 0.20; mixed residential ≈ 0.40–0.60.
Rainfall Intensity: Design rainfall intensity from IDF curves for the chosen design storm frequency and time of concentration.
Drainage Area: Total contributing drainage area. The Rational Method is valid for areas up to about 200 acres.
Pipe Slope: Longitudinal slope of the pipe. Typical minimum is 0.005 ft/ft for self-cleaning velocity.
Manning's n: Manning's roughness coefficient. Smooth concrete ≈ 0.012; corrugated metal ≈ 0.024; PVC/HDPE ≈ 0.010.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Runoff Coefficient (C) = 0.5, Rainfall Intensity = 4, Drainage Area = 5, Pipe Slope = 0.01 = 5 input(s) provided
Calculate Design Flow
Design Flow
10 = 10
Calculate Selected Pipe Size
18 = 18
Calculate Required Diameter
17.7 = 17.7
Calculate Flow Velocity
Flow Velocity
5.94 = 5.94

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