Rainwater Harvesting Calculator
Estimate how much rainwater you can collect from a roof in a storm, a month or a year — in gallons or litres — and the tank size you'd need.
How to use
- 1Enter the roof's footprint area (not the sloped area) and the rainfall.
- 2Set the collection efficiency (80% is typical for gutters and first-flush losses).
How it's calculated
Imperial: gallons = area (ft²) × rain (in) × 0.623 × efficiency. Metric: litres = area (m²) × rain (mm) × efficiency.
Frequently asked questions
Why use the footprint, not the roof's real area?
Rain falls vertically, so what matters is the horizontal area the roof covers.
How much water is 1 inch on 1,000 sq ft?
About 623 gallons before losses (~2,360 litres).
Is rainwater collection legal?
Almost everywhere, but a few US states limit volumes or uses. Check local rules and don't drink untreated rainwater.