Work out rainfall runoff instantly with clear inputs, formula shown and shareable results.
The rational method gives peak runoff as Q = CiA/360 with intensity in mm/h and area in hectares. A 1.5 hectare catchment with 70% runoff under 25 mm/h produces 0.073 m3/s, or 73 L/s. The coefficient captures how much rain runs off rather than infiltrating - near 0.95 for asphalt and 0.1-0.3 for woodland.
Rational method
Q = C x i x A / 360, with i in mm/h and A in hectares
Rainfall volume
volume = intensity x area x 10 m3 per hour
About 0.95 for paved surfaces, 0.7-0.9 for roofs, 0.4-0.6 for suburban mixed, 0.2-0.35 for grassland and 0.1-0.2 for woodland.
Above about 80 hectares, and where storage or routing matters. Larger catchments need a hydrograph model.