Convert millimetres of rain to litres of water collected over a given area.
One millimetre of rain over one square metre is exactly one litre of water. This simple relationship makes it easy to calculate how much water a roof collects, how much a garden receives, or how much a cistern needs to hold.
A 100 m² roof in a city that gets 1,200 mm of rain per year collects 120,000 litres — enough for toilet flushing and garden irrigation for a small household, if properly stored. The practical yield is 80-90% of the theoretical figure, accounting for evaporation and first-flush losses.
The tool also shows the result in cubic metres, which is the unit most water utilities bill in. One cubic metre = 1,000 litres.
Volume (litres) = rainfall (mm) × area (m²). 1 mm × 1 m² = 1 L = 0.001 m³.
Because 1 mm of water over 1 m² is a volume of 0.001 m × 1 m² = 0.001 m³ = 1 litre.
Use the horizontal projection of the roof, not the sloped area. A 10 × 8 m house has a 80 m² capture area regardless of roof pitch.
Yes — subtract the rainfall from the crop water requirement to find how much irrigation you need.
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