Stormwater Runoff Calculator
Stormwater runoff is the rainwater that flows over a surface instead of soaking in, and estimating it is the first step in sizing drains, swales, and detention. This calculator uses the Rational Method, Q equals the runoff coefficient times the rainfall intensity times the drainage area, to estimate the peak runoff rate in cubic feet per second. It also computes the total runoff volume from a rainfall depth using the same coefficient and area. The runoff coefficient, the fraction of rain that runs off rather than infiltrating, depends on land cover, so it is a user-editable input you set from a local drainage manual. The method suits small, site-scale catchments.
Rational Method formula
Peak runoff Q (cfs) = C * intensity (in/h) * area (acres)
Area (sq ft) = area acres * 43,560
Runoff volume (cu ft) = C * (depth in / 12) * area sq ft
Runoff volume (gallons) = volume cu ft * 7.48052
In US customary units the Rational Method conversion factor is about 1.008, conventionally taken as 1, so Q in cubic feet per second equals C times intensity in inches per hour times area in acres.
Stormwater context
- The runoff coefficient C is the fraction of rainfall that becomes surface runoff.
- Impervious surfaces such as pavement and roofs have coefficients close to one.
- One acre equals 43,560 square feet; one cubic foot equals about 7.48 gallons.
- The Rational Method suits small catchments with roughly uniform rainfall.
- EPA NPDES stormwater programs govern many municipal and construction discharges.
Stormwater runoff: frequently asked questions
What is the Rational Method?
The Rational Method estimates peak runoff rate as Q = C times i times A, where C is the dimensionless runoff coefficient, i is the rainfall intensity, and A is the drainage area. In US customary units with intensity in inches per hour and area in acres, Q comes out in cubic feet per second, because the unit conversion factor is very close to one.
What runoff coefficient should I use?
The runoff coefficient is the fraction of rainfall that becomes surface runoff rather than infiltrating or evaporating. It ranges from low values for forest and lawns to near one for paved or roofed surfaces. Because it depends on land cover, it is a user-editable input you set from a local drainage manual.
How is total runoff volume found?
Runoff volume equals the runoff coefficient times the rainfall depth times the area. This calculator multiplies the coefficient, the rainfall depth in inches converted to feet, and the area in square feet to give a volume in cubic feet, then converts to gallons for convenience.
Why does impervious area matter?
Paved roads, roofs, and parking lots prevent rainwater from soaking into the ground, so a much larger share runs off quickly. More impervious area raises the runoff coefficient and increases both peak flow and total volume, which is why stormwater design focuses on managing impervious surfaces.
Is the Rational Method suitable for large watersheds?
The Rational Method is intended for small drainage areas, traditionally up to a couple of hundred acres, where rainfall can be treated as uniform. Larger or more complex watersheds need hydrograph methods. Use this tool for site-scale estimates and check your local design standard.
Official sources
- U.S. Environmental Protection Agency: Stormwater Discharges from Municipal Sources.
- U.S. Geological Survey: water resources and runoff data.
Reviewed by the CalculatorHub team, edited by James Graham, 17 June 2026. See our methodology.