The formula
1 mm of rain = 1 litre per m² · collected = mm × area × efficiency
Where it comes from
The equivalence is not an approximation: it is exact by definition. A litre of water spread over a square metre forms a layer one millimetre deep, because a litre is a thousand cubic centimetres and a square metre is ten thousand square centimetres. That is why forecasts speak in millimetres and in litres per square metre interchangeably: they are the same number.
How to work it out by hand
- The millimetres of rain are already the litres per square metre
- Multiply by the square metres of roof
- Multiply by the collection efficiency, because some water evaporates or is lost at the gutter
What is worth knowing
The efficiency factor covers what is lost to evaporation, to the first wash of the roof and to gutter overflow. 0.8 is reasonable for a tiled or metal roof; on gravel or green roofs it drops to 0.5. And it is worth remembering that collected rainwater is not drinkable without treatment: it is fine for irrigation, cleaning and flushing, not for drinking.