Work out rainwater harvesting requirement instantly with clear inputs, formula shown and shareable results.
Harvestable volume is catchment area times rainfall depth times a runoff coefficient that accounts for evaporation, first flush and losses: about 0.85 for a hard roof. Storage is sized on a few days of intense monsoon rather than the annual total, because a tank large enough to hold a year's rain would be absurd.
Harvestable volume
m³ = roof area m² x rainfall mm x runoff coefficient / 1000
Storage
Tank m³ = peak daily yield x storage days, peak daily taken as annual yield over 60 rain days
Preliminary estimate only. Rainwater harvesting requirements, first flush arrangements and recharge design are set by local regulations.
m³ = roof area m² x rainfall mm x runoff coefficient / 1000. Harvestable volume is catchment area times rainfall depth times a runoff coefficient that accounts for evaporation, first flush and losses: about 0.85 for a hard roof.
For a supply-led system yes. Where the aim is recharge rather than storage, the sizing question becomes percolation capacity of the pit rather than tank volume.
This calculator takes 4 inputs: Catchment roof area, Annual rainfall, Runoff coefficient, Storage as days of peak rainfall. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.