What size rainwater tank do I need for my house?

For a typical Australian home with 200 m² of roof area in a 650 mm rainfall zone, a 5,000 L tank is recommended for garden watering. For full household use (toilet flushing, laundry), consider 10,000–20,000 L depending on occupancy.

How much rainwater can I collect from my roof?

Multiply your roof area (m²) by rainfall (mm) by 0.9 (runoff coefficient). For a 200 m² roof in Melbourne (650 mm): 200 × 650 × 0.9 = 117,000 L (117 kL) per year — well over most household needs.

Do I need a rainwater tank in Australia?

While not mandatory in most areas, rainwater tanks are highly recommended. Many new homes in water-restricted areas and some council jurisdictions (e.g., parts of QLD and NSW) require tanks as part of development approval. They also reduce water bills and provide water security during droughts.

Rainwater Tank Size Calculator Australia — Free Estimator

💧 Rainwater Tank Size Calculator

Roof Area (m²)

Annual Rainfall (mm)

Default: 650 mm (Melbourne average)

Number of People

Recommended Tank Size

0 L

Water collected per year

0 L

Household water use / year

0 L

% of needs met by tank

Water saved per year

How to Choose the Right Rainwater Tank Size

Sizing a rainwater tank depends on three factors: your roof catchment area, local annual rainfall and your household water demand. This calculator uses the standard Australian formula:

Water collected (L/year) = Roof area (m²) × Rainfall (mm) × Runoff coefficient (0.9)

The runoff coefficient accounts for losses from evaporation, first-flush diversion and roof absorption. A coefficient of 0.9 is standard for metal and tile roofs.

The recommended tank size is calculated to capture the first 2–3 weeks of average rainfall, which provides a good balance between storage capacity and cost. Larger tanks are recommended if you rely on rainwater for indoor use.

Annual Rainfall by Australian City

City	Average Annual Rainfall
Melbourne, VIC	650 mm
Sydney, NSW	1,213 mm
Brisbane, QLD	1,149 mm
Perth, WA	733 mm
Adelaide, SA	546 mm
Hobart, TAS	614 mm
Darwin, NT	1,727 mm
Canberra, ACT	616 mm

Worked Examples

Example 1 — Melbourne Home, 200 m² Roof, 4 People

Item	Value
Roof area	200 m²
Rainfall	650 mm
Water collected annually	117,000 L
Household use (garden + toilet)	~80,000 L/year
Recommended tank size	5,000 L

Example 2 — Sydney Home, 150 m² Roof, 3 People

Item	Value
Roof area	150 m²
Rainfall	1,213 mm
Water collected annually	163,755 L
Household use (garden only)	~30,000 L/year
Recommended tank size	3,000 L

Frequently Asked Questions

What can I use rainwater for in Australia?▾

Rainwater can be used for garden irrigation, toilet flushing, laundry, car washing and (with appropriate filtration and treatment) drinking water. Most Australian states allow rainwater use for non-potable purposes without a permit. Check local regulations before connecting to internal plumbing.

How much does a rainwater tank cost in Australia?▾

A standard 5,000 L polyethylene tank costs $800–$1,500, plus $300–$600 for delivery and $500–$1,500 for installation and plumbing connections. Slimline tanks (for narrow side passages) are more expensive. Steel tanks range from $1,200–$3,000. Government rebates may be available — check with your state or local council.

Do I need a first-flush diverter?▾

Yes — a first-flush diverter is essential to prevent debris, bird droppings and roof dust from entering your tank. It diverts the first 20–40 litres of runoff from each rain event. This calculator's runoff coefficient (0.9) accounts for these losses. Budget $50–$150 for a quality diverter.

Can I connect multiple tanks together?▾

Yes — you can connect multiple rainwater tanks in series or parallel to increase total storage capacity. The calculator recommends a single tank size. For larger requirements, consider two smaller tanks (e.g., 2 × 2,500 L) which may be easier to transport and install than one 5,000 L tank.

Disclaimer: Rainwater tank sizing depends on many factors including roof pitch, gutter condition, local rainfall patterns and intended use. This calculator provides general guidance. Consult a licensed plumber or rainwater tank specialist for a site-specific assessment. Water savings assume current water rates and may vary.

Related Tools

Square Metre Calculator — Measure your roof area in m²
Paving Cost Estimator — Cost estimates for permeable paving around your tank
Decking Material Calculator — Build a platform for your tank stand

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