There is a simple rule of thumb for calculating the amount of rainwater you can collect annually with your rainwater tank or cistern. Per m2 this is approximately 650 litres. So if you have a roof of 100m2, you can collect approximately 65.000 litres or 65 m3 of rainwater for use.

Then you can calculate how big your rainwater tank should be. Often, 5% of the annual volume is used, which would result in a tank of 0.05 x 65 = 3.25 m3. However, due to the changing climate and longer drought periods, we feel this is too little and therefore calculate a minimum of 8% (or a month’s supply). In the example, this means that you need a rainwater tank or cistern of 0.08 x 65 = 5.2 m3.