How much rainwater you can collect from your roof each year, and what tank size to use.
How this works
Litres per year = roof area (m²) x rainfall (mm) x runoff coefficient x 0.9 filter efficiency. 1 mm of rain on 1 m² is 1 litre. Runoff: metal roof 0.9, concrete 0.8, tiles 0.75, gravel 0.6. Lahore gets about 700 mm a year, Karachi 200, Delhi 750, Mumbai 2,400, London 600.
Questions people ask
How do I use the Rainwater Harvesting tool?
Fill in the fields, starting with “Roof area”, “Rainfall per year” and “Roof surface”, and change any other options that match your home. The answer, a drawing and practical tips update as you type, and the Copy link button saves your numbers in a link you can share.
Is the Rainwater Harvesting tool free?
Yes. The Rainwater Harvesting tool is free, with no sign-up, and works on phones, tablets and computers. The results are estimates to help you plan, so check with a qualified professional before structural, electrical, gas or plumbing work.