Rainwater harvest from a roof, with storage sized for the dry spell. A millimetre of rain on a square metre is exactly one litre, which makes the base calculation simple; the runoff coefficient is what varies.
Also called: rainwater collection calculator, roof water harvesting.
86,400 litres a year from 120 m² at 900 mm, after a runoff coefficient of 0.8 for a tile roof. That covers 43.84% of a 1,97,100 litre demand. A 45 day dry spell needs 24,300 litres of storage. Divert about half a litre per square metre of roof at the start of each event to discard debris.
One millimetre of rainfall on one square metre is one litre by definition, so the harvest is area times rainfall times a coefficient for what the surface actually delivers. Metal sheet runs at about 0.9 and a green roof at 0.3, with most of the loss to evaporation and wetting. Storage is sized on the dry spell rather than the annual total, because a system that harvests plenty across the year and runs dry in May has not solved the problem. First flush diversion discards the initial runoff carrying roof debris.
catchment area times rainfall depth times a runoff coefficient for the surfaceEach of these is asserted on every build. If a change to the engine ever moved one of these answers, the build would fail before the page could print it.
Annual harvest86,400
120 x 900 x 0.8; 540 litres a day x 45
Open this exampleAnnual harvest97,200
boundary: the highest coefficient
Open this exampleFormula version 1.0.0 · definition 1.0.0 · India · Report a problem with this calculator