Rainwater Harvesting Calculator
Water you can collect from a roof and the barrels to store it.
m²
mm
%
L
How it works
Water collected (litres) = roof area × rainfall (mm) × efficiency. Each millimetre of rain on one square metre of roof gives one litre. Efficiency drops with first-flush diverters, leaks, and overflow, and 80–90% is typical. Example: with the default values, 60 m² of roof in 50 mm of rain yields 2,550 litres, or 13 barrels of 200 L.
FAQ
Is this calculator free?
Yes. No signup is needed and nothing you enter is stored.
Is the result exact?
Results are rounded for display. For financial, medical, or legal decisions, confirm with a professional.
Do my numbers leave my device?
No. The calculation runs in your browser.