How this rainwater tank calculator works
The calculator estimates your annual rainwater yield using the formula: Roof area (m²) × Annual rainfall (mm) × 0.85 efficiency factor = Annual yield (litres). The 0.85 factor accounts for evaporation off the roof, first-flush losses from the diverter, and overflow during heavy downpours — a standard figure used by Australian tank sizing guides.
Daily demand is then estimated from your household size and intended water uses. This gives a demand-to-supply ratio that drives the tank size recommendation.
What size rainwater tank do I need?
For most Australian homes using rainwater for garden and toilet, a 5,000–10,000 litre tank is the most common choice. A 5,000 L tank fits comfortably alongside most homes; a 10,000 L round poly tank needs roughly 2.4 m of clear ground. In water-scarce regions (Perth, Adelaide, inland areas), bigger is always better — the extra storage makes a meaningful difference through a 6-week dry spell.
For full household use, especially with laundry, look at 22,000–30,000 litres or larger. Slimline tanks can be installed along fence lines where space is tight.
Australian tank materials
- Polyethylene (poly) — the most common. UV-stabilised, food-grade, 10–25 year warranties. Available in round, slimline and underground. Brands: Bushmans, Kingspan, Tank Boss.
- Corrugated steel (Colorbond / Zincalume) — robust, long-lasting, suits rural and period properties. Requires a food-grade liner. Often more cost-effective at large sizes (30,000 L+).
- Fibreglass — strong and long-lasting, good for underground or above-ground installations. Generally more expensive.
- Concrete — cast in situ or pre-cast. Excellent for very large capacity. Passively cools the water.
Council rebates and requirements
Most Australian councils and state governments offer rainwater tank rebates. Queensland's Home WaterWise Rebate Scheme, NSW's Rainwater Tank Rebates, and various SA Water and Yarra Valley Water programs can reduce purchase cost by $150–$1,500 depending on tank size and connection type. Check your local council or water authority website before purchasing.
In most states, tanks above 10,000 L connected to household plumbing require a licensed plumber. Standalone garden tanks generally don't.