Rational Method Runoff Calculator

The standard formula for small drainage area design

The Rational Method is the widely used standard formula for estimating peak storm runoff from small drainage areas - the runoff coefficient captures how much rainfall becomes surface runoff versus soaking into the ground.

Worked example

For a 10-hectare drainage area with a 0.7 runoff coefficient (moderately developed land) and 50 mm/hr rainfall intensity:

Q = (0.7 x 50 x 10) / 360 = 0.9722 m3/s

Surface Type	Typical Runoff Coefficient
Natural woodland	0.1 - 0.2
Residential (moderate density)	0.4 - 0.6
Impervious pavement/rooftops	0.85 - 0.95

This method remains standard practice for sizing storm drains and culverts on small sites, typically under about 80 hectares - larger watersheds require more sophisticated hydrograph-based modeling methods instead.