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# Rainwater harvesting calculator

Estimate monthly rainwater yield from your roof area and rainfall with a runoff coefficient, then size a tank against daily demand.

Enter a roof area, twelve rainfall values, and a daily use.

The collection sum is roof footprint times rainfall times runoff coefficient, one millimetre over one square metre giving one litre. The monthly balance is deliberately simple: it spreads demand evenly and ignores drought sequences longer than a year, pump placement, and plumbing losses beyond the coefficient. It is a sizing estimate, not an irrigation design.
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## More unit & math tools

- [Unit converter](/converters/units)
- [Aspect ratio and DPI](/converters/aspect-ratio)
[All unit & math tools](/converters)

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