Rainwater Harvesting Calculator
Estimate yearly rainwater from roof and rainfall.
Runs in your browser. Nothing is uploaded — your input is processed on this device and discarded when you close the tab.
Rainwater Harvesting Calculator
How to use this tool
- Enter roof area in square metres and annual rainfall in millimetres.
- Enter a runoff coefficient, such as 0.85 for a hard roof.
- Press Generate to see the yearly collectable volume.
Worked example
With a 100 sq m roof, 800 mm of annual rainfall and a runoff coefficient of 0.85, collectable water is 100 x 800 x 0.85 = 68000 litres per year, or 68 cubic m.
What this tool does not do / Limits
This is an estimate from the figures you enter. It assumes even rainfall and one runoff coefficient for the whole roof, and it ignores first-flush losses, storage limits, evaporation and water quality treatment.
Frequently asked questions
Why does 1 mm over 1 sq m equal 1 litre?
A millimetre of depth over a square metre is a thousandth of a cubic metre, which is exactly one litre.
What runoff coefficient should I use?
Hard roofs sit near 0.8 to 0.9; rough or green roofs lose more water, so use a lower figure.
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