Rain Barrel Calculator

One millimetre of rain on one square metre is exactly one litre. An 80 m² roof under 800 mm of annual rainfall receives 64,000 litres, less runoff losses.

The plan area — the footprint the rain falls on. Rain falls vertically, so a steep roof intercepts no more of it than a flat one covering the same ground.

The fraction that actually reaches the barrel after splash, evaporation, wetting the surface and the first flush. Eighty to ninety percent is typical for a tiled or metal roof.

Large enough to bridge dry spells rather than to catch a whole year. Storage rather than roof area is usually the binding constraint, which this makes visible by reporting overflow.

For ornamentals and established plants, generally yes. For edibles, roof material and bird droppings matter, and a first-flush diverter that discards the initial dirty runoff is a sensible addition.

For toilets and washing machines in many places, with a separate marked pipe network and often regulatory approval. Potable use needs treatment and is regulated far more tightly.

A device that discards the first few litres of a rain event, which carries the dust, pollen and droppings washed off the roof. It improves water quality substantially for very little cost.

A sealed lid and a fine mesh screen over the inlet. Standing water with any access is a reliable mosquito site within days in warm weather.

For garden use rarely; for edibles it can. Older bitumen and treated timber roofs can leach compounds, while tile and coated metal are generally considered fine.

Connecting them at the base lets them fill and draw as one, which is far more useful than several independent barrels — and cheaper than one large tank.

Water expands as it freezes and can split a full barrel. Draining below half or fitting a bypass to the downpipe over winter avoids it.

For acid-loving plants, generally yes — it is soft and free of chlorine. For most plants the difference is small, and the saving is the main reason to harvest it.