Water Tank Size Calculator
🌧️ Free Tool — Australia-Specific
Water Tank
Size Calculator
Find the right rainwater tank capacity for your Australian property — based on your roof size, local rainfall, and household usage.
Based on your roof
Australian rainfall data
Instant results
How to size a rainwater tank for your Australian property
Getting the right tank capacity means balancing how much rain your roof collects against how much water you'll actually use — and how many dry days you need to plan for. This calculator uses average annual rainfall figures for each Australian state and territory, your roof catchment area, and your household usage to give you a personalised minimum tank size recommendation with a sensible range either side.
Your State / Territory
We use actual average annual rainfall data for your region
Roof catchment area The area of roof draining to your tank (m²)
Number of people in household Used to estimate indoor water demand
Primary intended use What will you mainly use your rainwater for?
How much backup supply do you want? Days of supply if it doesn't rain
✦ Recommended Tank Capacity
5,000L
Minimum recommended capacity for your situation
Or consider: 3,000L – 7,500L range
— Annual Capture
— Daily Demand
— Dry Buffer
Your roof can capture approximately X litres per year — enough to cover your usage with surplus for most months.
💧 Installation Tips for Your Setup
- Tip goes here
Calculations use average annual rainfall data per state/territory. Actual catchment varies based on roof pitch,
guttering, first-flush diverters, and seasonal distribution. Results are a guide only.
Frequently Asked Questions
The right water tank size depends on your roof catchment area, your state's average annual rainfall, and how you plan to use the water. For garden use only, a 2,000–5,000L tank suits most Australian homes. For toilet flushing plus garden use, 5,000–10,000L is typical. For full household use including laundry and drinking water, 10,000L or more is generally recommended. Use the calculator above to get a personalised recommendation based on your specific situation.
Annual rainwater collection is estimated using: Roof Area (m²) × Annual Rainfall (mm) × 0.85 efficiency factor = Litres per year. For example, a 120m² roof in Sydney (averaging 700mm of rain per year) can capture approximately 71,400 litres annually. The 0.85 efficiency factor accounts for evaporation, roof debris, and water discarded by a first-flush diverter. Keeping gutters clean and unobstructed significantly improves your actual collection rate.
A family of 4 using rainwater for garden and toilet flushing typically needs a 5,000–10,000L tank in most Australian states, with a 60–90 day dry buffer built in. In drier states such as South Australia or Western Australia, sizing toward the upper end — or even 15,000L+ — is advisable to carry the household through extended dry periods. In higher-rainfall states like Tasmania or Queensland (wet season), a smaller tank may suffice if you also have mains backup.
In most Australian states, rainwater tanks under a certain capacity installed on residential properties do not require development approval — but rules vary by council and state. South Australia actively encourages installation and offers rebates; Queensland has also run rebate schemes. Always check with your local council before purchasing. Tanks connected to indoor plumbing (toilets, laundry, drinking water) typically require installation by a licensed plumber under local regulations.