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.
Rainwater tank calculator — frequently asked questions
What size rainwater tank do I need for a house in Queensland?
For garden irrigation only, 5,000–10,000 litres is sufficient for most suburban blocks. For toilet flushing and laundry top-up, 10,000–22,500 litres is more appropriate. For whole-of-house supply in a rural area, 22,500–45,000+ litres depending on roof area and annual rainfall. The calculator works out the right size based on your actual catchment and usage.
How do I calculate how much rainwater my roof collects?
Multiply your roof catchment area (in m²) by annual rainfall (in mm) by 0.85 (a typical efficiency factor for losses from the roof surface, gutters and first-flush diverters). A 200 m² roof in Brisbane receiving 1,100 mm of annual rain collects about 187,000 litres per year — more than enough for non-potable household use.
Is rainwater tank installation mandatory in Queensland?
Since 2007, new homes in Queensland have been required to install a rainwater tank connected to at least one internal use — typically the toilet or laundry. Requirements vary slightly between councils, so check your local planning scheme for specifics. Rebates have been available in some areas — worth checking with your local council.
What is a first-flush diverter and do I need one?
A first-flush diverter automatically discards the first few litres of rainwater from each rain event — this flush carries the most contaminants from the roof surface (dust, bird droppings, debris). They're strongly recommended for any tank used for drinking or cooking and are a good idea for any tank connected to internal uses.
How long does a rainwater tank last in Australia?
Polyethylene (poly) tanks typically last 20–30 years. Slimline poly tanks may have slightly shorter lifespans depending on UV exposure. Corrugated steel (Colorbond or Zincalume) tanks last 20–30 years with appropriate liner maintenance. Concrete tanks can last 50+ years but are more expensive to install.