Calculate annual rainfall collection and tank sizing for Australian homes using BOM state averages.
Rainwater tanks are one of those investments that make more financial sense the more water costs in your area — and with water prices steadily increasing across Australia, the maths is improving. This calculator estimates how much rainwater you could collect from your roof based on your roof area and local rainfall.
It accounts for a collection efficiency factor to reflect that not all rain that falls on a roof ends up in the tank — losses from the first flush, evaporation, and overflow reduce the theoretical maximum. The result tells you approximately how many litres per year you could expect to collect and what that's worth at current water prices.
Tank sizing is about matching storage to your collection pattern — a tank that fills up and overflows every time it rains isn't doing much for you. This calculator helps you understand whether you're likely to be limited by tank capacity or by rainfall availability.
You don't need to measure it directly. Use the floor plan area of your house as a starting point — a 200m² house footprint has roughly 200m² of roof catchment area (slightly more for a pitched roof, but it largely balances out for this purpose). Measure the external dimensions of the house and multiply length × width.
The Bureau of Meteorology (bom.gov.au) has historical average annual rainfall data for thousands of locations across Australia. Search for your nearest weather station to find your local figure.
In most Australian states, collected rainwater can be used for garden irrigation, toilet flushing, laundry, and car washing without restrictions. Using it as drinking water requires the tank and plumbing to meet health standards — check your state's regulations.
In most cases no — standard residential tanks are exempt development in most councils. Above-ground tanks up to about 10,000 litres typically don't require approval, but check your local council's exempt development rules as they vary.