Drainage (Rational Method) Calculator
Estimate peak stormwater runoff with the Rational Method to size gutters, drains, and culverts.
The Rational Method is a planning estimate. Verify against local drainage codes and consult a qualified civil engineer before construction.
How it works
- 1Select the surface type — this sets the runoff coefficient C (the fraction of rain that runs off).
- 2Enter the design rainfall intensity (mm/hr or in/hr) and the catchment area.
- 3The Rational Method Q = C × i × A returns peak flow in L/s, m³/s, US gpm, and m³/hr.
Use cases
- Sizing roof gutters and downpipes for a given design storm.
- Selecting a culvert diameter for a gravel car park or access road.
- Comparing runoff from a grassed site versus a paved development.
Frequently asked questions
What is the Rational Method formula?
Q = C × i × A, where C is the runoff coefficient (0–1), i is rainfall intensity, and A is catchment area. 200 m² of pavement (C = 0.9) under 50 mm/hr gives Q ≈ 2.5 L/s.
What runoff coefficient should I use?
C is the fraction of rain that runs off rather than soaking in. Roofs and asphalt use 0.85–0.95, gravel 0.50, bare soil 0.45, grass 0.20–0.35. Mixed sites use an area-weighted average.
How accurate is the Rational Method?
It suits small catchments with uniform land use. For large or complex catchments, dynamic models are more appropriate. Always verify against local drainage codes.
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