Work out rainwater harvesting potential instantly with clear inputs, formula shown and shareable results.
One millimetre of rain on one square metre is one litre, so the harvest is area times rainfall times collection efficiency. A 120 square metre roof under 750 mm of annual rain at 85% efficiency yields 76,500 litres a year - about 210 litres a day, or 219 days of a 350 litre household demand.
Harvest volume
litres = area(m2) x rainfall(mm) x efficiency
Key identity
1 mm of rain on 1 m2 = 1 litre
First-flush diversion, evaporation, splash loss and gutter overflow during intense storms all reduce capture. 0.8-0.9 is realistic for a hard roof.
Usually sized on the longest dry spell rather than annual yield - typically 20-40 days of demand for a garden supply.