How to calculate how much water you can collect from your roof

Melbourne's average rainfall is around 650mm. a year. The pattern is remarkably even, with the lowest month (February) having 45 mm., and the highest month (October) having 65 mm. In Melbourne, your water tank can capture rainfall all year round.

Here's how to calculate how much water you can collect from your roof. Take the number of square metres of roof, muliply by the number of millimetres of rainfall, to give the number of litres. e.g. If you have 150 square metres of roof, and the average Melbourne rainfall is 650 mm, then you could collect over 97,000 litres a year. A 10mm rainfall would yield 1500 litres! In both cases, it's safest to subtract 10% to allow for absorbtion, evaporation etc.