Rainwater Harvesting Calculator
Estimate how much rainwater your roof can collect from its area, local rainfall and collection efficiency.
How it works
Litres = roof area (m²) × rainfall (mm) × efficiency · (1 mm over 1 m² = 1 litre)
The key fact is that one millimetre of rain falling on one square metre delivers exactly one litre. So the harvest is simply your roof’s plan area times the rainfall depth, scaled down by a collection efficiency that allows for splash, the first-flush diverter and evaporation. Use the flat footprint of the roof, not the sloped area — the rain that lands is set by the horizontal area it covers.
Worked example
A 100 m² roof in a place that receives 800 mm of rain, at 85% efficiency, collects 100 × 800 × 0.85 = 68,000 litres — about 68 m³, or roughly 17,960 US gallons over the period.
Frequently asked questions
Do I use the sloped roof area or the flat footprint?
The flat (plan) footprint. Rainfall is measured as depth over horizontal ground, so a steep roof and a flat roof with the same footprint catch the same volume.
What efficiency should I assume?
Most systems capture 80–90% of the theoretical maximum after first-flush diversion, splash and evaporation losses. Smooth metal roofs are toward the high end; rough or absorbent surfaces are lower.
Is harvested rainwater drinkable?
Not without treatment. Roof runoff picks up dust, debris and droppings, so it is best used for irrigation, flushing and washing unless properly filtered and disinfected.
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This calculator is for educational purposes. Double-check important results before acting on them.