Stormwater Runoff Calculator

The rational method uses the formula Q = CiA, where Q is the peak runoff flow rate in cubic feet per second (cfs), C is the dimensionless runoff coefficient, i is rainfall intensity in inches per hour, and A is the drainage area in acres. It is widely used for small urban catchments up to about 200 acres.

The runoff coefficient represents the fraction of rainfall that becomes surface runoff. Values near 1.0 (like rooftops or pavement) mean almost all rain runs off, while low values like 0.15 (forest) indicate most water is absorbed. Local drainage manuals and the ASCE often provide C values by land use type.

Rainfall intensity is determined from an Intensity-Duration-Frequency (IDF) curve specific to your location and design storm return period (e.g., 10-year, 100-year storm). The EPA's National Stormwater Calculator and NOAA Atlas 14 are reliable sources for local precipitation data in the United States.

Impervious surfaces — like rooftops, roads, and parking lots — prevent water from soaking into the ground, dramatically increasing runoff volume and peak flow rates. As development increases impervious cover, downstream flooding, erosion, and water quality problems worsen. Stormwater management aims to mitigate these impacts.

Green infrastructure practices such as rain gardens, bioretention cells, permeable pavement, and green roofs help infiltrate, evapotranspire, and detain stormwater close to where it falls. These approaches reduce peak flow rates, filter pollutants, and help mimic pre-development hydrology.

Peak flow rate (Q, in cfs) is the maximum rate at which water flows at a given point during a storm event — critical for sizing pipes and channels. Runoff volume (in cubic feet or gallons) is the total amount of water generated over the storm duration — important for sizing detention ponds and storage systems.

The rational method is best suited for small, urban catchments generally under 200 acres with relatively uniform land use. For larger or more complex watersheds, methods like TR-55 (SCS Curve Number), HEC-HMS, or EPA SWMM provide more accurate results by accounting for storage, variable land use, and longer travel times.

Many stormwater regulations require that post-development peak runoff rates do not exceed pre-development rates for a given storm event. Comparing both helps engineers and planners size stormwater control measures — like detention basins or bioretention — to compensate for increased impervious cover from development.