Stormwater Runoff Calculator
Estimates the peak stormwater runoff from a small catchment using the Rational Method, Q = C·I·A / 360. Used by civil and drainage designers, developers and council engineers to size pipes, pits, channels and culverts for a chosen design storm.
Enter Values
Before you rely on this: First-pass guide only. Verify safety-critical or regulated work against the relevant standards, your project requirements and a qualified professional.
How to use this calculator
- Enter the runoff coefficient C (0–1) for the surface — around 0.9 for paved/roof, 0.1–0.3 for grass; use an area-weighted value for mixed catchments.
- Enter the rainfall intensity I in mm/hr, read from the design-ARI IFD curve at the catchment's time of concentration.
- Enter the catchment area A in hectares and read the peak flow Q in m³/s and L/s.
How it works
The Rational Method assumes rainfall of constant intensity falls uniformly over the catchment for at least the time of concentration, so runoff reaches a steady peak. Peak flow Q = C·I·A / 360, where C is the fraction of rainfall that becomes runoff, I is the intensity (mm/hr) and A is the area (hectares). The 360 constant reconciles the units to give Q in m³/s.
It is intended for small catchments (roughly under 50 ha, ideally under about 5 km²). Larger or more complex catchments need a routed hydrograph model. The design intensity comes from the IFD curve for the chosen ARI/AEP at a duration equal to the time of concentration.
Worked example
Mixed catchment, 2.5 ha. With C = 0.85, I = 50 mm/hr and A = 2.5 ha: Q = (0.85 × 50 × 2.5) / 360 = 106.25 / 360 = 0.2951 m³/s, or about 295 L/s.
Common mistakes
- Entering the area in m² or km² instead of hectares — the 360 constant only works with A in ha, I in mm/hr and Q in m³/s.
- Using a single C for a mixed catchment instead of an area-weighted average of paved and pervious surfaces.
- Applying the method to a large catchment where flow times and rainfall vary too much for the constant-intensity assumption to hold.
Frequently asked questions
What is the runoff coefficient C?
The dimensionless fraction of rainfall that runs off rather than infiltrating or being stored — near 0.9 for impervious roofs and pavement, 0.1–0.3 for grass and gardens. Use an area-weighted C for mixed surfaces.
Where do I get the rainfall intensity I?
From the intensity–frequency–duration (IFD) curve for the site and the chosen design storm (ARI/AEP), read at a duration equal to the catchment's time of concentration.
Why divide by 360?
It converts the mixed units (C dimensionless, I in mm/hr, A in hectares) into m³/s. It comes from 1 mm/hr over 1 ha ≈ 2.78 L/s.
How big a catchment can the Rational Method handle?
A small-catchment method — roughly under 50 ha and ideally below about 5 km². Beyond that, use a hydrograph/routing model.
Is this suitable for detailed design?
It gives a defensible first-pass peak flow. Final design should follow the relevant standard (e.g. Australian Rainfall and Runoff) and your local authority's method.
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