One millimeter of rain on one square metre of roof equals one litre of water run-off
To calculate the approximate amount of rainwater you are likely to harvest, use the following formula:
Estimated Net Runoff from an Impervious Catchment Surface Adjusted by its Runoff Coefficient
catchment area (m2) x rainfall (mm) x runoff coefficient = net runoff (litres)
Run-off (litres) = A x (Rainfall - B) x Roof Area
A is the efficiency of collection and values of 0.8-0.85 (i.e. 80-85% efficiency) have been used (eg Martin, 1980).
B is the loss associated with adsorption and wetting of surfaces and a value of 2 mm
per month (24 mm per year) has been used (eg Martin, 1980).
Rainfall should be expressed in mm and Roof Area in square metres (m2)
In an area receiving 600 millimetres of rain a year with a rooftop catchment surface that is 11 metres long and 4.55 metres wide, and you want to know how much rainfall can realistically be collected off that roof in an average year. You want a conservative estimate of annual net runoff, so you use a runoff coefficient of 80% or 0.80.
catchment area (m2) = length (m) x width (m)
2 x (length (m) x width (m)) x rainfall (mm) x 0.80 = net runoff (litres)
2 x (11 m x 4.55 m) x 600mm x 0.80 = net runoff (litres)
100m2 x 600mm x 0.80 = 48,112 litres
48,112 litres = net runoff
A realistic estimate of the volume of water that could be collected off this 11 meter by 4.55 meter roof in a year of average rainfall is 48,112 litres.