Rainwater Harvesting Calculator
Estimate annual harvestable rainwater from roof area, rainfall and runoff coefficient.
About this calculator
Sizing a rainwater harvesting system, or simply gauging whether it's worth installing one, starts with knowing how much water a roof can realistically capture over a year, and that figure depends on catchment area, local rainfall and how much of that rain actually runs off the roof surface rather than evaporating or soaking in. This calculator multiplies your roof's catchment area by average annual rainfall and a runoff coefficient, the fraction of rainfall that actually becomes usable runoff, generally high for smooth roofing materials like metal or tile, lower for rougher or more absorbent surfaces, to estimate total litres harvestable per year. Enter roof area, your location's average annual rainfall and a runoff coefficient matching your roof material, and get an annual harvest estimate immediately instead of researching and multiplying the three factors by hand. Because rainfall varies significantly by region and even year to year, use your specific local average annual rainfall figure from a meteorological source rather than a generic default for a realistic estimate.
Worked example
1,500 sqft roof, 900mm annual rainfall, 0.85 runoff
Result: Annual harvest ≈ 1,06,650 litres
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