Stormwater Runoff Calculator
The peak flow a storm sends off your site — the number you size a drain, swale or culvert to. The Rational Method, Q = C i A, with a runoff coefficient set by the surface.
How Much Water Runs Off, and How Fast
When rain hits a site, some soaks in and some runs off. The peak rate of that runoff — not the total volume — is what determines whether a drain, gutter, swale or culvert can keep up. The Rational Method is the classic, fast way to estimate that peak for a small catchment.
Q is the peak runoff, C the runoff coefficient (the fraction of rain that runs off), i the rainfall intensity, and A the drainage area. In US customary units the constant works out so neatly that Q comes out directly in cubic feet per second; in metric the formula carries a divide-by-360.
The Runoff Coefficient
C is what makes a parking lot flood and a meadow shrug off the same storm. Pavement and roofs shed almost everything (C around 0.9); lawns and forest soak up most of it (0.1 to 0.25). For a site that mixes surfaces, use an area-weighted average. C also creeps up on steeper ground and during long storms as the soil saturates.
Picking the Rainfall Intensity
Intensity i is not just any rainfall number — it is read from an intensity-duration-frequency (IDF) curve for your location and design return period (say a 10-year storm), at a duration equal to the catchment time of concentration. Short, intense bursts drive the peak on small sites.
Frequently Asked Questions
What is the time of concentration?
The time for runoff to travel from the most remote point of the catchment to the outlet. The design storm intensity is taken at this duration, because that is when the whole area is contributing at once.
How big a catchment can I use this for?
The Rational Method is intended for small, uniform areas – commonly up to about 200 acres. Larger or complex watersheds need a unit-hydrograph or full hydrologic model.
Does it give runoff volume?
No – it gives the peak flow rate, which is what pipes and channels are sized on. Detention and storage volumes need a different, volume-based method.
Related calculators
- Pipe Flow Calculator — size a pipe to carry the runoff.
- Hydraulic Diameter Calculator — for open channels and swales.
- Excavation Calculator — earthwork for ponds, swales and trenches.
- Engineering Unit Converter — flow and area units.