Stormwater Runoff Calculator

Calculate peak stormwater runoff discharge rate (Q) in Cubic Feet per Second (CFS) using the civil engineering Rational Equation Q = CIA.

Drainage Area

Surface Runoff Coefficient (C)

Rainfall Intensity (In/Hr) From local IDF design curves

Peak Discharge Flow Rate (Q)

5.10 CFS

2,289 Gallons per Minute (GPM)

Hourly Volume 137,340 Gal Per hour of peak storm

Min Culvert Pipe 15" Pipe @ 4 ft/sec velocity

The Rational Formula Constant: In US customary units, 1 acre-inch per hour equals 1.008 cubic feet per second (CFS). Because this is so close to 1.0, the Rational Formula is simply Q = C × I × A.

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The Rational Method Formula Q = CIA

First developed in 1889 by Emil Kuichling, the Rational Method remains the universal standard for sizing storm sewers, culverts, street gutters, and retention basins for watersheds under 200 acres:

Q = C × I × A

Q: Peak discharge rate in Cubic Feet per Second (CFS)
C: Dimensionless runoff coefficient representing the fraction of rainfall that runs off rather than infiltrating into the ground
I: Rainfall intensity in inches per hour for a duration equal to the basin's time of concentration ($t_c$)
A: Drainage basin area in Acres

Frequently Asked Questions

What runoff coefficient C should I use for mixed developments?

Calculate a composite weighted average: C_comp = Σ(C_i × Area_i) / Total Area (e.g. weighting roofs at 0.90, asphalt parking at 0.85, and landscaped grass at 0.20).

How many GPM are in 1 CFS?

1 Cubic Foot per Second (CFS) equals exactly 448.831 Gallons per Minute (GPM).

What is Time of Concentration (tc)?

The time required for stormwater to flow from the hydraulically most distant point of the watershed to the outlet. Peak discharge occurs when the entire watershed is contributing runoff simultaneously.