The amount of rainwater you can collect is dependant on three critical factors:
Average annual rainfall for your location – The Bureau of Meteorology website www.bom.gov.au/climate/averages is a great place to find the annual rainfall for your area
The roof surface area supplying rainwater to your tank – Measuring the external dimensions of your home will give you a reasonable approximation
The number of down pipes connected to your tank – and the percentage of roof area they will deliver. The formula to bear in mind when calculating how much water you can collect is this; One millimetre of rain falling on one square metre of roof (1m x 1m) will collect one litre of water. Remember, you don’t need to purchase a tank with a capacity equivalent to the total annual rainfall as you will gain the most benefit from your tank by constantly using the water and topping it up when it rains.