Convert rainfall in mm to litres per m² — useful for gardeners, farmers and water butt sizing.
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How This Works
1mm of rainfall = 1 litre of water per square metre. This is because 1mm depth over 1m² area = 0.001m³ = 1 litre. Collection efficiency ~80% for roof systems (accounting for first-flush discarding, evaporation, overflow). UK average annual rainfall: ~885mm (range 600mm in East Anglia to 3,000mm+ in Highland Scotland).
The first 1–2mm of rainfall from a roof contains the most contaminants — bird droppings, dust, particulates, lead from old flashing. First flush diverters automatically discard this initial runoff before directing cleaner water to storage. This significantly improves water quality for garden use. For drinking water use, rainwater requires filtration and UV sterilisation regardless of first-flush diversion.