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Rational runoff

Q = 2.78 C i A. Intensity in millimetres per hour, area in hectares, result in litres per second.

Runoff coefficient C

Intensity i (mm/h)

Area (ha)

CalculateReset

Method

Worked case: C 0.8, 50 mm/h, 0.4 ha is 44.5 L/s. The 2.78 converts mm/h over a hectare into L/s.

Not FEH, not Sewers for Adoption, not a SuDS design. See where the rational method stops.

Last checked 2 October 2026. A scenario, not a design. See methodology, standards and the disclaimer.