Rainfall Runoff Calculator (Rational Method)
The Rational Method is the most widely used formula for estimating peak stormwater runoff from small urban and suburban drainage areas. It expresses peak discharge Q in cubic feet per second (cfs) as the product of three factors: C, the runoff coefficient (ranging from 0 for completely pervious ground to 1.0 for fully impervious surfaces); i, the design rainfall intensity in inches per hour for a storm duration equal to the watershed's time of concentration; and A, the drainage area in acres. The formula Q = CiA is exact in US customary units. Enter your values to calculate peak runoff and runoff volume for a storm event.
Rational Method formula
Q = C * i * A
Q = peak runoff (cfs), C = runoff coefficient (dimensionless)
i = rainfall intensity (in/hr), A = drainage area (acres)
Volume (acre-ft) = Q * duration (hr) / 1 [cfs * hr = acre-ft / 12.1, approx]
Volume (acre-ft) = Q (cfs) * duration (s) / 43560
The volume is calculated as Q x duration in seconds divided by 43,560 (square feet per acre), converting cfs-seconds to acre-feet. This is an approximation assuming constant peak flow throughout the storm duration.
Runoff coefficient guide
- Lawns and grassland (flat, sandy): 0.05 to 0.10
- Lawns and grassland (steep, clay): 0.25 to 0.35
- Suburban residential (lot size 1/4 to 1/3 acre): 0.25 to 0.40
- Commercial and business areas: 0.70 to 0.95
- Paved parking lots and roofs: 0.85 to 0.95
- For mixed land uses, use a weighted average C by sub-area.
Frequently asked questions
What is the Rational Method?
The Rational Method is the standard engineering formula Q = CiA for estimating peak stormwater runoff from small drainage areas (typically under 200 acres). Q is peak flow in cubic feet per second (cfs), C is the dimensionless runoff coefficient, i is rainfall intensity in inches per hour, and A is drainage area in acres.
What runoff coefficient should I use?
Runoff coefficients (C) range from 0.05 for flat, open grassland to 0.95 for impervious pavement. The ASCE Manual of Engineering Practice No. 36 and local stormwater design manuals provide tables by land use. Urban commercial areas typically use 0.70 to 0.95; residential lawns use 0.25 to 0.40.
How do I find rainfall intensity?
Rainfall intensity depends on storm return period and duration. NOAA's Precipitation Frequency Data Server (hdsc.nws.noaa.gov) provides intensity values by location. For the Rational Method, use the intensity for a storm duration equal to the time of concentration of the watershed.
What are the limitations of the Rational Method?
The Rational Method is best suited for small, homogeneous drainage areas under 200 acres with a single land use type. For larger or more complex watersheds, the NRCS Curve Number (TR-55) method or hydrologic modeling software is more appropriate.
What is the result unit?
The Rational Method gives peak runoff Q in cubic feet per second (cfs) when C is dimensionless, i is in inches per hour, and A is in acres. The formula is exact in US customary units by definition of the rational method constant (approximately 1.008, rounded to 1.0).
Official sources
- NOAA: Precipitation Frequency Data Server.
- USGS: USGS Water Resources.
- NRCS: NRCS Water and Climate.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.