Tankmasta has a great little rainwater calculator to help you calculate how much water could be collected from your roof into a rainwater tank annually. I was out in an industrialized area near Melbourne recently and I was amazed at the about of roof area some factories had with no sight of a rainwater tank.

Some of these factory roofs would literally cover hectares. Tankmasta's raineater calculator suggests that a 1 hectare (1o,000 sq meters) roof in Melbourne would collect 5,877,000 litres of water annually. That's not to be sneezed at!

You can find the rain water calculator at the bottom of Tankmaster's home page.