Method: The Rational Method estimates peak runoff as: Q = C × i × A where Q is peak discharge, C is the dimensionless runoff coefficient, i is rainfall intensity and A is drainage area. When rainfall intensity is entered in mm/h and area in m²: Q (m³/s) = C × i × A ÷ 3,600,000 The method assumes that rainfall intensity is approximately uniform over the catchment and that the entire contributing area is producing runoff at the peak condition. For design applications, rainfall intensity should normally correspond to a storm duration approximately equal to the catchment time of concentration and to the required design return period or probability. Runoff coefficients vary with surface type, slope, soil, antecedent moisture, development and local design practice. The preset values in this calculator are indicative only. The Rational Method is primarily intended for relatively small catchments. Large or complex watersheds, storage effects, detention systems and detailed flood studies generally require more comprehensive hydrologic modelling.