How much water can i catch?

For every square meter of roof area, you harvest 1 litre of water for every mm of rain. Think of it as 1:1:1……one litre of water per 1 mm of rain falling on 1 square meter of roof area.

For example, if you had 90m2 of roof area (eg 6 meters x 15 meters) and your annual rainfall rate is 800mm then over the year you would capture 72000 litres. 

This is ideal situation and assumes your tank is always empty. Many rain events will fill your tank in the first few hours and then will be overflowing. However even if you only caught half of this it is much better to capture it than let it run to the ocean.

To work how much water, you can capture work out the meters squared of your roof area and then check out your rainfall rate using the folowing diagram provided by the bureau of metrology.