09 February 2023

What size water tank do I need?

Installing a water tank at home is a great way to cut down on your water bills as well as help the environment – the more rainwater you collect and use, the less reliant you are on the main water supply.

Water tanks are a great source of water for use around the home and garden. From watering the garden to cleaning the car, rainwater harvested from your roof can come in handy, especially in times when there are water restrictions in place as there have been in Auckland over the past 18 months.

Beyond the garden and outdoors, water tanks can also be connected to the household supply and used for the laundry and flushing the toilet. If the water is properly treated, it can also be used for drinking and other household uses.

Who should install a water tank?

If you have a property that isn’t connected to the mains water supply, you will need to install a tank to get fresh and clean drinking water supplied to your property. For some properties that are located a long way from the mains water supply, they may also need to install a water tank as their most viable water supply.  

Unless you live in a very rural area, the chances are, any property you either purchase or build will have access to the mains water supply, however, this is not to say that you shouldn’t also consider a water tank.

Installing a tank at your property, even if it is already connected to the mains, is a great way to reduce the demands on the mains water supply and in some areas, to reduce water charges.

If you are still not sure if a water tank is for you, check out our recent post, Is it worth getting a water tank?  Where you will find lots of good advice about the benefits of installing a water tank as well as a look at some of the different options available.

Collecting rainwater

The process of collecting rainwater is a fairly simple one. Typically, rainwater is collected from your roof – either your house, garage or shed – and stored in a tank until you need it.

Water tanks come in a huge range of sizes and applications – from 200-litre rain barrels that are typically connected to your shed and used for watering the garden to 30,000-litre tanks that might be used for farming or watering particularly large plots.

How you plan to use the rainwater you collect will be one of the factors that dictate the size that you need along with the amount of average rainfall in your area.

Legal requirements

It also makes sense to check on the council regulations in your local area before you start to think about installing a water tank. Some councils will only let you use rainwater for potable purposes (e.g., drinking and cooking) if it is treated.

In other areas, you may need to get council consent to install a water tank depending on the size and elevation of the tank. If you are going to connect your water tank to the main water supply in a property, you will also need to seek building consent.

What size water tank do I need?

Once you have determined the legal requirement around installing a water tank, you will need to consider the size of the tank you want to install. There are several factors to consider including:

·         The annual rainfall in your area

·         What you intend to use the water for

·         Whether you have access to the mains water supply

·         The number of people in your property

·         The size of your property (including the garden)

·         How efficient your water-using appliances are

As a rule of thumb, if you are going to be installing a water tank as your main source of water, then bigger is better.

The National Institute of Water and Atmospheric Research (NIWA), estimates that the number of drought days we will experience will double in most regions in New Zealand over the next 20 years. That means restricted water use, especially during the summer months, is going to become more common. Investing in a water tank now will future proof you, especially if you plan on staying in your property for the next 10-15 years or more.

To work out what size water tank you will need, as a minimum requirement, you need to first work out how much water you can potentially catch and store.

How much water can you catch and store?

There are several different calculations that people use to work out how much water can be captured and stored but we like this model:

1.       Estimate the size of your roof by calculating the length x width of your property. You can also visit websites like homes.co.nz where you can find out the overall size of your property and use this as a guide i.e., 170m²

2.       Look up the annual rainfall for your area. Head over to the NIWA website where you will find that information.

Once you know these two bits of information, then it’s a simple equation to calculate your harvestable water potential:

Litres per square metre of catchment x catchment area (roof area) = litres of harvestable water

You can calculate the litres per square metre of catchment based on the annual rainfall for your area. The annual rainfall for your area in mm is equal to the litres per square metre of catchment.


The average rainfall in Auckland is 1,114mm per year. My property is 172m².

1,114 litres x 172m² = 191,608 of harvestable water per year

How much water do I use?

The amount of water used can vary significantly by household, however, there are some averages we can use to make some rough calculations. In Auckland, for example, the average water usage per person is 140-170 litres per day.

That means each person uses on average, somewhere between 51,100 litres and 62,050 litres per year. If you have a family of four, you will need to harvest between 204,400 and 248,200 litres a year if a water tank is your main or single source of water. In our example above, we would not be able to collect enough water in an average year to meet demand so would need to top that up, either with a water delivery or through the mains water supply.

How much space do I have for a tank?

One other factor to consider when choosing the size of your water tank is how much space you have to install one. Whilst new builds may have the luxury of installing a tank underground, the cost of doing this retrospectively can be very expensive, so above ground tanks are usually a more viable option.

Above ground tanks, however, can take up a lot of space and look unsightly.  There is a range of options when it comes to water tanks and the new slimline designs can be installed next to a property or fence and made to blend in more than the traditional and bulky round tanks. You will still need a decent amount of space to install any water tank at your property, so this is a key consideration.


Hopefully, this helps you to calculate the size of the water tank needed at your property and gives you some insight into the potential uses of rainwater that is harvested and stored.

If you already have a tank installed or once you decide to take the plunge and install one at your property, make sure you keep on top of your water usage as well as accurately measure the amount of water in your tank with a Smart Water sensor.

Our award-winning products provide state-of-the-art monitoring systems that not only provide you with an accurate reading of the water level in your tank but also work intuitively for the everyday user, providing you with all the information you need about your water consumption on an easy-to-read display or through our app. Read more about measuring the water level in your tank in our recent post.