How to use the rational method stormwater runoff calculator
The rational method is still the most common way to size storm sewers, inlets and small detention facilities in the United States. With the drainage area, the design rainfall intensity and a runoff coefficient for the ground surface, you get the peak discharge directly, which makes it well suited to site-scale drainage design. This tool also returns the total runoff volume over the storm duration so you can check detention or cistern sizing.
In customary units the formula is simply Q = 1.008 · C · i · A, where Q is in cubic feet per second, C is the dimensionless runoff coefficient, i is rainfall intensity in inches per hour and A is the drainage area in acres. The conversion constant works out to 1.008, which is why textbooks write it as Q = CiA and treat the constant as one. Metric practice uses a different constant because the units differ, so never mix the two forms.
Runoff coefficients rise as the surface becomes less permeable. Mixed sites should use an area-weighted average, and the values listed here are typical design references rather than approved numbers. Local stormwater manuals, the state DOT drainage manual and the reviewing agency all publish their own tables, so confirm C and the design storm frequency before submitting. Intensity should be read from the NOAA Atlas 14 IDF curve for your location at the time of concentration, and final pipe sizing belongs to a licensed civil engineer.
Frequently asked questions
The rational method assumes the duration equals the time of concentration, which is the inlet time plus the travel time through the pipe. Small urban sites are typically 5 to 15 minutes, and larger basins run longer.
It is normally limited to small urban watersheds of roughly 200 acres or less. Larger basins have enough storage and lag that a unit hydrograph or a full runoff model gives more reliable results.