This is a simple calculation. 1mm of rain on every square metre of surface area results in 1 litre of water into your rainwater tank. Let’s say that your house roof is 12 metres long by 10 metres wide – 12m x 10m = 120m².

If you had a very brief storm that dropped 1mm of rain on the roof, you would receive:

1mm x 120m² = 120 litres of rainwater into your rainwater tank.

A larger storm comes through and drops 25mm of rain,25mm x120m²= 3,000 litres of rainwater into your rainwater tank.