Rainwater harvesting calculator
Yearly rainwater yield and tank size from roof area and rainfall.
How it is calculated
Yield (litres) = catchment area (m²) × rainfall (mm) × runoff coefficient × filter efficiency. Tank estimate = yield of the wettest month, or demand × dry days, whichever you choose.
Check locally: Runoff coefficients are typical values for the roof finish. Local rainfall data gives the best result.
Questions
Why does 1 mm of rain on 1 m² give 1 litre?
1 mm × 1 m² is 0.001 m³, which is one litre.
Is anything I enter saved or sent?
No. The calculation runs in your browser. Nothing you type is sent anywhere. You can copy the result, print it or download it as a CSV.