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  1. Home
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  3. Rainwater harvesting calculator
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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.

This tool is in beta

Still being tested. If a result looks wrong, tell us.

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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.

Roof and use

Roof surfacePresets the runoff coefficient
12 averages, January to December
More settings▾
Rounding step

The tank field runs the entered tank through the year month by month. Rainfall is what you enter; nothing is fetched, so use climate normals from your local weather service.

Area is the roof footprint, between 1 and 1,000,000 m². First-flush diverters and leaky gutters are losses you price in by lowering the coefficient yourself.

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