What size water tank do I need?

How big should my rainwater tank be?

One of the first questions many people have when considering purchasing a water tank for any purposes, is how big it should be, and how much space that means it’s going to take up.

This post has been put together to help answer some of these questions, and help you onto the next step of the considerations for your water tank purchase.

Factors to consider

If only it were as simple as deciding how much rainwater you felt like you were going to use, and buying a suitably sized water tank, and leaving it at that.

There are, however a few factors that you should consider when purchasing a water tank. Available land, roof surface area, water usage and your budget.
Available Land:
One of the seemingly more obvious, but sometimes less considered factors when buying a water tank. If you’re putting your rainwater tank in a large flat paddock, this won’t necessarily apply to you, but if you’re placing it near a building or a fence line, it is important to ensure you have enough space to place a rainwater tank.
A Southern Cross 26kL TL model liner tank has a 4.4m diameter, and the 125kl model has a 9.5m diameter. Larger water tanks will have larger diameters, and other manufacturers will have different diameters and heights for these water tanks.
Roof Surface Area:
How much water you can catch in your tank is directly related to your total roof surface area. This is including any sheds that are able to run into your tank or any additional tanks.
To work out how much water you can catch, multiply your catchment area by your annual rainfall. For example, someone in Toowoomba with a 230m² roof, who gets an average rainfall of 860mm has a total yearly rainwater harvesting capacity of 197,800L. With more than half of this rainfall coming in four months, you would want to have sufficient storage for this water to last during the dry months with minimal top-ups.
Water Usage:
The amount of water you are using the tank to subsidise will also be an important determinant in what size water tank you will need to purchase. If you’re just looking to replace your toilet and gardens with rainwater, then you may not need a tank large enough to capture your full annual rainfall. If you are looking to replace all or as much as possible of your water usage with your water storage, then the larger the water tank the better.
Finally, your budget is possibly the most important factor of what size water tank you can purchase. While most people may be able to stretch their budget slightly, if you can’t afford the largest water tank you’re looking at, you’ll likely settle for one within your price range.


If you are looking to subsidise part of your rainwater usage, it is important to work out how much, and aim for a rainwater tank in that range.

If you are looking to use your water storage for all future water use, the best rule of thumb is to get the largest tank that you can afford, fill, and fit in your available space.

For a quote on a rainwater tank today, call Southern Cross Water Tanks on (07) 3441 5600.

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