Rainwater tank calculator
Estimate how much rain your roof can collect each year and how long a tank will last.
Rain falls unevenly through the year, so a bigger tank stores wet-season water for dry months. Find your area's rainfall on the Bureau of Meteorology website.
How this calculator works
1 mm of rain on 1 m² of roof = 1 litre. So yearly harvest = roof area × rainfall × efficiency. Efficiency allows for first-flush diverters, evaporation and overflow. A round tank's capacity is π × r² × h × 1,000.
Worked example: a 150 m² roof with 600 mm of rain at 80% collects 72,000 L a year. A tank 2.2 m wide and 2 m high holds about 7,600 L, or 38 days at 200 L a day.
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