Rainwater harvesting calculator
Yield = roof area × rainfall × runoff coefficient. Works for annual or monthly rainfall figures.
One millimetre of rain on one square metre is exactly one litre — the runoff coefficient discounts evaporation, splash and first-flush diversion. To size storage, decide how many dry days you want to bridge and multiply by your daily household need from the IS 1172 calculator.
FAQ
How much rainwater can my roof harvest?
Roof area (m²) × rainfall (mm) × runoff coefficient — a 100 m² roof in 800 mm annual rainfall at 0.8 runoff yields about 64,000 L a year.
What runoff coefficient should I use?
0.8 is the common figure for sloped metal or tiled roofs; concrete flat roofs run 0.7–0.8; first-flush diverters and losses justify the discount from 1.0.
How big should the storage tank be?
Match it to the dry-season demand you want to bridge: daily household litres × dry days you're covering, capped by what your roof actually yields.