Rainwater Tank Size Calculator Australia

Choosing a rainwater tank size can feel like guesswork. Too small and it runs dry between downpours; too big and you have paid for capacity you never fill. This free calculator gives you a practical starting point, using the three things that actually decide tank size in Australia: your roof catchment area, your local rainfall, and how much water you want to use in the garden or home. Enter your details below for an estimate, then read the guidance underneath to sanity-check the number for your block and climate.

The calculator combines two figures: how much rain your roof can collect, and how much water you plan to use. Roof collection is straightforward — for every millimetre of rain that falls on one square metre of roof, you capture about one litre before losses. So a 150 m² roof in a suburb receiving 600 mm of rain a year could, in theory, collect around 90,000 litres across the year. The calculator uses your inputs to estimate a tank size that balances what you can collect against what you will realistically draw down between rain events. Treat the result as a planning guide, not a fixed rule.

• Roof catchment area — the roof footprint in square metres that drains to the tank, not the whole house. Even one or two downpipes can feed a tank.
• Local rainfall — your average annual or seasonal rainfall. This varies enormously across Australia, so use a figure for your own area rather than a national average.
• Garden and lawn watering needs — how much you water, and whether the tank is mainly for the garden. If you need to work out watering time, our irrigation runtime planner can help.
• Household and outdoor use — toilets, laundry, car washing and topping up pools all change the sums if the tank is plumbed inside.
• Available space — the footprint and height you can fit, which often decides between a slimline and a round tank.

As a rough starting point: a garden-only tank for a small courtyard or townhouse often sits in the 1,000–2,000 litre range, while a typical suburban block watering lawns and beds is usually better served by 3,000–5,000 litres. Homes plumbing rainwater to toilets and laundry, or on acreage with no mains backup, commonly go well beyond that. Rainfall pattern matters as much as volume: areas with long dry spells need more storage to bridge the gaps, while reliable year-round rain lets a smaller tank refill often. Match the tank to your driest stretch, not your wettest month. Reducing how much water you need in the first place helps too — our water savings calculator shows where the biggest savings come from.

• Sizing to the roof, not the rainfall. A big roof in a low-rainfall area still won’t fill a huge tank reliably.
• Ignoring the dry season. A tank that copes in winter can run empty through a long, hot summer.
• Forgetting the footprint. People choose a capacity, then find it won’t fit the side of the house — check dimensions early.
• Overlooking first-flush and losses. Some rain is lost to evaporation, overflow and the first-flush diverter, so real collection is lower than the theoretical figure.
• Skipping council rules. Some areas require or incentivise certain tank sizes or plumbing connections.

This calculator gives an estimate to guide planning, not a guaranteed figure. Rainfall varies year to year and place to place, roof pitch and material affect how much water actually runs off, and plumbed-in systems involve rules the calculator doesn’t assess. Before you buy or install, check local requirements — many councils have rules or rebates for rainwater tanks — and confirm the final size and setup with a qualified plumber or tank installer. For more planning tools, see all our garden and landscaping calculators.