To calculate how much potential water you can harvest you need to know the size of your roof in square meters of your roof and the average rainfall in your area.
1mm of rain falling on 1m2 of catchment area provides 1 litre of water
(click on this link for Farmonline http://www.farmonlineweather.com.au/climate/stationdrill.jsp )
Catchment area (m2) x Rainfall (mm) = potential number of litres to be harvested
For example – An average suburban roof of 200m2 in an area that receives 500mm of rain per year, could potentially harvest 100,000L of rainwater (200m2 * 500mm = 100,000L).
Knowing how much water you will use also helps. Pioneer estimates that an average rural home needs about 30,000L per person per year