Water Storage Tanks. How do I calculate the volume of water (in litres) that runs off my roof when a certain depth of rain (in mm) falls on it?
One millimetre (mm) of rain falling on one square metre produces one litre of water. Knowing this, you just multiply the depth of rain in mm by the horizontal area in square metres covered by the roof and your answer is in litres. For those who want to know more, the well-known formula is: Volume = Depth x Area. But the units of depth and area must be ‘consistent’, such as depth in metres, area in square metres and volume in cubic metres. Rainfall is usually measured in millimetres and one millimetre is 1/1000th of a metre. So for a rainfall of one millimetre on a horizontal area of one square metre: Volume = Depth x Area Volume = 1/1000 x 1 (answer in cubic metres) Volume = 1/1000th of a cubic metre. But 1/1000th of a cubic metre is exactly one litre so, converting to litres….
