Rational runoff
Q = 2.78 C i A. Intensity in millimetres per hour, area in hectares, result in litres per second.
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.
How to read a rational peak
The rational peak is Q = CiA, with the units made consistent. It is a peak-flow screen for a small catchment. It is not FEH and not a sewer design.
C is a runoff coefficient you choose. A is the catchment area. i is the rainfall intensity for the duration you believe matches the time of concentration. The page does not compute that duration.
A roof and a field do not share a coefficient. Using 0.9 on a landscaped area overstates the peak. Using the daily rainfall total as if it were an intensity understates it.
Use it to see the order of a peak before a drainage design. Attenuation, climate change allowances and a network model are not in the product.
What to write beside the result
A review can only check this figure if the inputs sit next to it. Write the value, the unit and the assumption the form used. A diameter in millimetres and a diameter in metres are not the same reading. A force in kilonewtons divided as if it were newtons is a thousand-fold error.
The result is the arithmetic on this page. It is not a utilisation, not a selected product, and not a clause from a standard. If the number is going into a shared container, name the page, the date and the inputs. The file-name checker does not read the calculation.
Last checked 3 October 2026. A scenario, not a design. See methodology, standards and the disclaimer.