Rational Method Runoff Calculator
The peak stormwater runoff from a catchment — the rational method, Q = C i A — from the surface type, the design rainfall intensity and the area, in cubic metres and litres per second.
The rational method assumes that at the peak the whole catchment contributes: peak flow equals a runoff coefficient (the share of rain that runs off — near 0.
How the rational method runoff calculator works
The rational method assumes that at the peak the whole catchment contributes: peak flow equals a runoff coefficient (the share of rain that runs off — near 0.9 for roofs and asphalt, 0.2–0.3 for lawns) times rainfall intensity times area. With intensity in mm/h and area in hectares, dividing by 360 gives m³/s. It suits small catchments up to a few tens of hectares; larger ones need routing.
Formula: Q (m³/s) = C × i (mm/h) × A (ha) ÷ 360
Worked examples
| Inputs | Peak runoff | Note |
|---|---|---|
| Suburban mix, 80 mm/h, 2.5 ha | 0.278 m³/s | 0.278 m³/s |
| A 1 ha car park at 120 mm/h | 0.283 m³/s | 0.283 m³/s |
| 10 ha of parkland at 50 mm/h | 0.347 m³/s | 0.347 m³/s |
FAQFrequently asked questions
What intensity do I use?
The design storm for your standard — a 10- or 20-year event for minor drainage, 100-year for major — at a duration equal to the catchment’s time of concentration (often 5–15 minutes for a small site). Intensity–frequency–duration tables are published by the national weather service.
Why do the coefficients vary?
They combine how much rain soaks in with how fast the rest runs off: roofs and asphalt shed nearly everything, lawns absorb most of a moderate storm. Steep or saturated ground pushes the coefficient up; use the higher end of the range for big storms.
What are the method’s limits?
It gives a peak, not a hydrograph; it assumes uniform rain over the whole catchment for at least the time of concentration; and it becomes unreliable above a few tens of hectares or where storage and routing matter.
How do I size the pipe properly?
The 1 m/s row is only a first estimate. Pipe capacity depends on slope and roughness (Manning’s equation) and codes require minimum velocities for self-cleansing; a drainage engineer sizes it.
Where these figures come from
- UN-Habitat — Urban Indicators Guidelines — density, green space and public space benchmarks
- World Health Organization — Urban green spaces — the 9 m² per person minimum
- ITE — Parking Generation Manual — parking demand rates by land use
- Office for National Statistics — Population estimates — the UK's national statistics agency
Last checked: September 2026. Default per-capita rates are mid-range figures from national statistical agencies and utility reports; every one is an input you should replace with the local figure.