How to Choose the Right Water Tank Size

Tank Rainwater Washing CarThere are two approaches that you can take when determining the size of rainwater tank that you need. You will need to decide for yourself which approach is most suitable.

Approach #1: The Bigger The Better

We have obviously all heard this phrase, when it comes to rainwater tanks.

Many in the industry recommend buying the largest and most practical tank that will fit in your available space. One reason is that rainwater does not fall from the sky at a uniform daily rate. Rather, when it rains during a month most of the rainwater falls all at once over just a few days.

This means if you want to harvest as much water as possible when it rains, then you may as well buy the biggest and most suitable tank within your budget.

Approach #2: Calculate Rain Harvesting Capacity

First, you need to work out how much water your catchment area can produce. 1mm of rain falling on 1m2 of catchment area provides 1 litre of water, so you just need to work out your catchment area size and rainfall.

An average suburban roof of 200m2 in an area that receives 500mm of rain per year, could potentially harvest 100,000L of rainwater (200m2 * 500mm = 100,000L).

Of course, houses are often built much bigger today, and some areas collect up to 350mm+ of rain in just one month. This amount of rain can even fall in a few days. Every area is different, so you can use the Bureau of Meteorology website to find out your exact rainfall.

Once you know your catchment area and your rainfall, just multiple the two numbers together to know how many litres of rainwater can be collected. You will likely still need to make a judgement about how much overflow is acceptable when it pours.

Knowing how much water you will actually consume can also help. Each person living on your property will use approximately 250L per day, although water restrictions can obviously reduce this. To help you out we have created an online water usage calculator. Try it out.


Once you have decided upon which approach you will use to determine the best size of rainwater tank for your needs, then it is just a matter of selecting an appropriate style and colour for that capacity.

Make sure you factor in any rainwater tank rebates available to you. There are often capacity requirements that need to be met, which could obviously affect your choice.

Should you want more practical advice on rainwater tanks, feel free to contact us via the question form at the bottom of this page or call us on 1800 758 709.