Rainwater Harvesting Calculator
Estimate how much rainwater your roof can collect from a rainfall total — in litres or gallons.
Use the roof footprint (the area seen from above), not the sloped surface. Collection rules for rainwater differ by region, so check local regulations before using it for drinking water.
About this tool
Enter the roof footprint and the rainfall for a storm, a month or a year. The calculator multiplies area by rainfall and a runoff efficiency to show how much water you can capture, so you can size a water butt or rain tank.
Frequently asked questions
One millimetre of rain on one square metre is exactly one litre. A 100 m² roof therefore collects up to 100 litres per mm of rain, or about 90 litres after typical losses.
One inch of rain on one square foot is 0.623 US gallons. A 1,000 ft² roof can collect about 623 gallons per inch, or roughly 560 gallons at 90% efficiency.
Around 90% is a common figure for metal, tile, concrete and asphalt-shingle roofs, allowing for evaporation, splash and first-flush losses. Use a lower value for gravel or green roofs.
Measure the footprint seen from above (length × width of the building plus overhangs), not the sloping surface. Rain falls vertically, so the pitch does not add catchment area.
Related tools
Mulch CalculatorWork out how much mulch, bark or compost you need for a garden bed — and how many bags.Aquarium Volume CalculatorFind how many litres or gallons your fish tank holds — and how much the water weighs.Pool Volume CalculatorFind how many litres or gallons your swimming pool holds — rectangular, round or oval.