Rainwater Catchment Area Calculator
Calculate roof catchment area needed for a target Australian rainwater yield from annual rainfall and collection efficiency.
Your results
Calculation breakdown
- Catchment area
- target litres ÷ (annual rainfall mm × collection efficiency)
Worked example
Collecting 80,000 L a year from 600 mm rainfall at 80% efficiency requires about 166.7 m² of roof plan area for a 219.2 L daily average.
Assumptions
- One millimetre on one square metre equals one litre.
- Annual rainfall and efficiency are representative.
Sources
Rates effective 23 August 2026.
How this calculator works
Calculate roof catchment area needed to collect a target annual rainwater volume from local rainfall and collection efficiency. The inverse calculation starts with the intended water supply and exposes the roof area required before tank capacity is considered. The calculation exposes its units and intermediate assumptions so Australian users can reconcile the estimate with bills, meters, labels, design records or site measurements.
Frequently asked questions
What should I measure for the rainwater catchment area result?
Use long-term local Bureau rainfall, horizontal roof plan area and an efficiency reflecting gutters, first flush and overflow. Keep the measurement period and unit basis consistent across every input.
What can make this rainwater catchment area estimate differ?
Annual totals hide seasonal dry periods, event intensity, tank overflow, water quality, roof suitability and demand timing. Recalculate with measured operating data before making a purchase, design or reporting decision.
Does this rainwater catchment area calculator confirm compliance?
No. It provides a transparent planning calculation only. For this topic, use long-term local Bureau rainfall, horizontal roof plan area and an efficiency reflecting gutters, first flush and overflow. Obtain qualified advice where electrical, plumbing, building, safety or reporting obligations apply.
Actual prices, usage, efficiency and savings vary by provider, equipment, location and behaviour.