How to Size a Rainwater Tank for an Australian Home

By Sam Alex, civil engineer · About the author

A rainwater tank is one of the most useful additions to an Australian home, but sizing one is often done by guesswork. Too small, and it overflows every storm and runs dry by January. Too big, and you've paid for capacity your roof can never fill. The right size comes from balancing three numbers.

Number 1: how much water your roof can collect

Every millimetre of rain falling on a square metre of roof is one litre of water. After losses (first-flush diversion, evaporation, splashing and overflow), a good planning figure is about 80% of that:

Litres per year ≈ roof area (m²) × annual rainfall (mm) × 0.8

Measure the roof area that drains to the downpipes you'll connect. It's the plan area (the footprint seen from above), not the sloping surface area.

Number 2: how much water you'll use from it

Connecting the toilets and laundry gives the most reliable savings, because that water is used every day of the year.

Number 3: rainfall, and when it falls

Annual averages differ widely between cities. Roughly: Darwin about 1,700 mm, Sydney about 1,200 mm, Brisbane about 1,000–1,100 mm, Perth about 700 mm, Canberra about 600 mm, Melbourne about 600–650 mm, Hobart about 600 mm and Adelaide about 550 mm. Check the Bureau of Meteorology for the station nearest you.

When it falls matters as much as how much. Perth and Adelaide get most of their rain in winter, so a tank needs to carry water through a long dry summer. Darwin's rain comes almost entirely in the wet season. Sydney's is spread more evenly.

Worked example: a family of four

Roof area connected: 180 m²
Uses: toilets and laundry, about 200 L/day (73,000 L a year)

In Sydney (about 1,200 mm): 180 × 1,200 × 0.8 = 172,800 L a year. Supply comfortably exceeds demand, and rain is spread through the year, so a 5,000–10,000 L tank will stay topped up most of the time.

In Melbourne (about 650 mm): 180 × 650 × 0.8 = 93,600 L a year. Supply still exceeds demand, but with drier spells, a 10,000 L tank gives about 50 days of buffer (10,000 ÷ 200).

In Perth (about 700 mm, mostly in winter): about 100,000 L a year, but very little falls from December to March. To carry the house through a 90-day dry summer at 200 L/day would need about 18,000 L. Many households instead size for winter use and accept that the tank will run dry in summer, with an automatic switch to mains water.

The rainwater tank capacity calculator works out the storage from daily demand and the number of dry days you want to cover.

Don't oversize for the garden

Garden demand peaks exactly when rain is scarce. Trying to water a large garden through summer from a tank alone usually needs a very large tank and roof. Drip irrigation, mulch and water-wise plants often achieve more for less money.

Installation checklist

Council rules: many councils require approval above a certain size or near boundaries, and new homes in some states must include a tank or meet water targets (such as BASIX in NSW).
Base: a level, compacted base. Full tanks are very heavy, at 1 tonne per 1,000 L.
First-flush diverter and leaf guards: keep the water cleaner.
Overflow: must connect to an approved stormwater system.
Pump and switch: an automatic mains-switching device keeps toilets working when the tank is empty.
Drinking water: follow your state health department's advice. Treatment may be needed.

Frequently asked questions

How much water can I collect from 100 m² of roof?

About 80 litres for every millimetre of rain. At 600 mm a year, roughly 48,000 L.

What size tank do I need for toilets and laundry?

For a family of four using about 200 L/day, 5,000–10,000 L is common in cities with year-round rain. Drier or seasonal climates may justify more.

How heavy is a full water tank?

One kilogram per litre, so a full 10,000 L tank weighs about 10 tonnes plus the tank itself.