To estimate the amount of rainfall you can expect to harvest off your roof, there is a simple calculation you can do.

One millimetre of rain falling on one square metre of roof will give you approximately one litre of water in your tank.

Find out your annual average rainfall, and you can easily work out how much rain water you will harvest in a year.


Remember that :

  • You need gutters to get the water into your tanks!
  • Not every roof surface will easily divert into your tank.
  • For more accurate calculations it is better to multiply the result by a factor of 90%, as some rainwater will be lost to evaporation and splashing.

For example: If your annual rainfall is 675mm, and your  roof is 220 square metres -then the total rain water you can potentially harvest over the course of a year is 148 500 litres. For better planning use monthly rainfall figures.