Calculate annual rainfall collection and tank sizing for Australian homes using BOM state averages.
Enter roof catchment area, local annual rainfall, collection efficiency and tank assumptions, then calculate potential capture and compare it with expected demand.
First-flush diversion, gutter losses, overflow and roof wetting reduce usable collection.
A useful rule of thumb is that one millimetre of rain falling on one square metre of roof represents about one litre of water before losses. The calculator combines roof area and rainfall with a collection-efficiency assumption to estimate how much may reach the tank.
First-flush devices, gutter overflow, wind, roof wetting, leaks and a full tank can all reduce usable capture. Annual rainfall also says little about how long dry periods last, so total yearly collection should not be confused with guaranteed water security. For tank sizing, compare capture with household demand and local rainfall timing. For drinking-water systems, follow appropriate tank, roof, filtration and water-quality guidance for your location.