Rainwater Harvesting Calculator
Estimates a roof’s yearly rainwater yield and a sensible tank size.
Yield per year
— L
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
Average per day — L
Suggested tank — L
Share of your use covered — %
Reading this —
The formula
yield (L) = area (m²) × rainfall (mm) × runoff × filter efficiency
One millimetre of rain on one square metre is exactly one litre.
What to watch out for
The yearly average hides dry spells, so seasonal climates need a bigger tank.
Frequently Asked Questions
How much water can I collect from my roof?
An 80 m² roof with 800 mm of rain, at 0.8 runoff and 90% filter efficiency, yields 46,080 litres a year.