What Size Water Tank Do I Need?

by Mar 4, 2021Help & Advice, Select News, Statistics & Data

Wondering “what sized water tank do I need”? It’s a good question, because the answers vary depending on how much water you use! Obviously, the bigger your water tank is, the more water you can store in it. Some of the things you should keep in mind when decided how big your rainwater tank should be include:

How Much Water Do You Need?

On average a four-person family consumes around 600 Litres per day, which comes to 120 to 200 Litres per person. The way we use our water in Australian households is broken down as follows: Source: GWM Water

  • 50% – Outdoor use (car washing, hosing driveways, garden use)
  • 17% – Showers
  • 13% – Toilets
  • 11.5% – Laundry
  • 8.5% – Other (includes drinking)

However, what sized water tank you need will depend on how you’re planning to use your tank water. It might surprise you to find that the vast majority of the water your household uses is probably outside, or in your shower, toilet or laundry. While all of the water that comes from the main water supply and into your home is treated and can be safely consumed by your family, most of it doesn’t actually have to be; basically, you don’t have to water your garden or wash your car with drinking water.

Many Aussie households with water tanks will exclusively use untreated rainwater for their outside water consumption – and since that’s half the water that’s contributing to your water bill, it can represent serious savings.

Can you use rainwater tanks for all your water needs?

80% of households with rainwater tanks consider the volume of water they collect in their tanks to be sufficient for all their needs, including:

  • Drinking water
  • Garden watering
  • Toilet flushing
  • Laundry usage
  • Replenishing domestic pools or spas
  • Car washing
  • Supplying the hot water system
  • Thermal buffers to insulate houses
  • Ventilation for buildings
  • Protecting homes from bushfires
What Size Water Tanks Do I need - Watering a Garden

Rainwater that you collect from a well-designed and operated system can be suitable for any and all domestic water uses. For example, you can use you tank water for your garden and some indoor uses (toilet flushing & showering), while still sourcing your drinking water from your water supply. And if you want to reduce your reliance on mains water even further, check out our whole-house water filtration system which can make your tank water 100% safe for drinking as well.

Water Tank Dimensions and Sizes

Depending on the space available, and your personal preferences, you can choose whether to order a round or slimline water tank(s). Round water tanks are a common sight across Australia, particularly in regional areas, while slimline water tanks are popping up more and more in urban Aussie backyards and are loved for their space-saving, sleek design.

Slimline Water Tanks

Slimline Water Tanks are ideal for where you have limited space (under eaves, for example) making this water tank design particularly well-suited to urban areas with smaller backyards. They’re also widely considered a more aesthetic choice.

However, there is a caveat: the maximum water storage for a single slimline water tank is 10,000 litres. Fortunately, there’s a way around this – just link up multiple tanks side by side to increase water storage capacity.

Slimline Water Tank Dimensions
Slimline Water Tank Dimensions

Round Water Tanks

Round water tanks can store up to 37,000 litres of water and are ideal where you have plenty of space and where you want a large volume of water – they’re a regular sight in country towns, where they’re helping farmers to combat drought, and significantly reducing the load on mains water supply in regional areas. But if you’ve got a large-enough backyard, they’re a great choice for urban and suburban homes as well!

Lisa Hagean

Select Water Tanks Google Review Lisa Hagean 

"I recently ordered a 7500 litre stainless steel round tank from Select Water Tanks and couldn't be happier with it. The process from order to delivery was perfect and I was kept in the loop with manufacture times and a set delivery date. Communication was great through the process and I had to change the delivery date to suit me and there was no issue with this.."

Gary Turner

Select Water Tanks Google Review Gary Turner 

"A big thank you to Jason, Linda, Loyd, Damon and the team at Select Watertanks.The whole process ran on time, in fact you were able to deliver all our tanks prior to Christmas.The boys did a great job negotiating our tricky block and setting up the tanks.We highly recommend Select Watertanks"

Paul Inglis


"The team from Select Water Tanks were superb. The way they handled my new tank was especially careful and thorough. It was great watch them take it down a hill and locate on the site. Highly recommended for good product, service and interest in the customers needs."

Barbara Geitz

Select Water Tanks Google Review Barbra Geitz 

"We have just bought 3 tanks and have now installed them. We found Select Tanks very easy to work with right through to delivery. We had to change the delivery date on a number of occasions and it was never a problem. We are very happy with the quality of the tanks which look great."

How do I calculate the size of my water tank?

Round Water Tank Dimensions & Examples of Sizes | Capacities
The right size water tank for you will depend on the rainwater storage capacity you need, as well as the space you have available. For a full list of all our Round Water Tank Sizes & Dimensions visit our Steel Water Tank Pricing Page.

Capacity (litres) Diameter x Height (mm)  
2,071 1300 x 1560
5,332 1900 x 1880
15,015 3500 x 1560
22,467 3900 x 1880
37,047 3900 x 3100

Slimline Water Tank Dimensions & Examples of Sizes | Capacities
The right size water tank for you will depend on the rainwater storage capacity you need, as well as the space you have available. For a full list of all our Slimline Water Tank Sizes & Dimensions visit our Steel Water Tank Pricing Page.

Slimline Water Tank Dimensions
Capacity (litres) Diameter x Length x Height (mm)  
2,145 850 x 1800 x 1560
5,067 700 x 4000 x 1880
7,712 1150 x 3100 x 2350
10,144 1150 x 4000 x 2350

So, what size water tank do I need for a 3-bedroom house?

Now that we’ve covered the basics, let’s look at what size water tank the average Aussie 3-bedroom house will need. Assuming you have three people living in your home, you’ll need about 15,000 litres of water storage (one month’s storage capacity for 3 people, at 165 litres per person per day) to cover your basic water usage needs.

This is a bit more than one slimline tank can manage; although if you’re only using your water tank for outside purposes, your rainwater tank storage needs will drop to about 7,500 litres, in which case a slimline water tank will do the job nicely. Otherwise, you could consider a large single tank at 3200 x 1880 (mm), which can hold 15,126L litres of water.

One big tank or several smaller ones linked together?

It’s easier to transport and place smaller tanks, and we can place them in different locations to capture rainwater from different aspects or sides of a roof (or multiple roofs). One large tank will take up less space, but it can also mean that you can only capture the water from one roof location, limiting your harvesting capabilities.

From our many years of experience in supplying and installing water tanks, we have found that having three round tanks holding 30,000 litres each (total 90,000 litres) will easily keep even a large family in water all year round.

How Much Water Can You Harvest from Your Roof?

Another important factor to consider when answering the age-old “what size water tank do I need” question is how much rainwater you can actually harvest from your roof. Fortunately, there’s a rule of thumb for that too. Each square metre of roof space collects around 1 litre of water for everyone millimetre of rainfall.

So, if hypothetically you have 10 millimetres of rain in a storm, and 150 square metres of roof space actually available to collect rainwater, you can then collect 1500 litres of rainwater from that storm (150sq metres x10 millimetres x 1 litre).

To average things out over a longer period of time, you can actually look up the monthly rainfall pattern for your geographical area – for example, let’s say 100mm per month on average over a 12-month period.

The average area for rainwater catchment for Australian households:

  • 100-150 ㎡ – Small House
  • 150-200 ㎡ – Medium House
  • 200 ㎡ + Large House

Don’t want to get out a pen and paper, or open up Calc? Can’t blame you. Fortunately, we’ve done the hard yards and automated that process, so just check out our handy rainwater harvesting calculator.

Long term savings

Harvesting rainwater can reduce your water bills, while taking the pressure off main water supplies. With Australia facing a freshwater crisis, water rates are more than likely to go up in future years, but even at current rates getting a good-quality water tank that lasts for decades means serious long-term savings for you and your family. Below is a guide to the money that you can potentially save by reducing your water bill with a water tank.

Household Size Average Per year
1 $884
2 $1,072
3 $1,248
4 $1,292
5 $1,488
Saving Money with Water Tanks

So, what about the upfront cost in a water tank?

Is a water tank a good investment? How long does it take to pay your water tank back? All good questions. With around half of Brisbane homes having a water tank nowadays, water tanks are a trend that’s really caught on. And it’s not just about the environmental impact; they’re actually a great buy.

Let’s say that you spend $4,296 on a 3500 x 1560 round water tank with a 15,015L capacity (sounds oddly specific? That’s because it’s an actual water tank we sell – head to our pricing page). If you use 15,000L per month for 3 people, and you save $1,248 per year, your water tank will pay for itself in less than four years – and with a 30-year warranty, you can expect another 21 years of savings. #betterthanbitcoin? We think so!

What is the best water tank to buy?

You’re already asking yourself the question “What size water tank do I need?”, but size isn’t the only factor to think about. Cost is another important factor, but remember, there’s a trade-off – a water tank made of cheaper materials like poly (i.e. plastic) will degrade faster and so has a shorter lifespan, whereas for a bit of a higher upfront cost, you can choose a steel water tank with a 30-warranty and enjoy substantial savings on your water bills for years to come.

Other than the materials, the shape of a water tank will affect the pricing – slimline water tanks will generally be more expensive than a round water tank of equivalent sized. That’s because unlike round water tanks, slimline water tanks require internal reinforcing rods to keep their sides stable when the tank is full of water. This makes these water tanks slightly more expensive, but on the other hand, they do have the advantage that they can be fitted into narrow spaces.

So, what is the best material for a water tank?

Stainless Steel Tanks
At Select Water Tanks we firmly believe that Stainless Steel Water tanks are the best choice for a number of reasons. They’re longer-lasting, offer you fantastic value for money over their lifespan, and at the end of their life are fully recyclable – better for you, better for the planet!

Choose steel water tanks for longer lifespan
A stainless steel water tank from Select Water Tanks has a minimum life of 30 years, and the corrosion-resistance properties of Stainless Steel mean your tank will not only last for a long time, but it will always look brand new.

What Size Water Tank Do I Need - Picture of Stainless Steel Water Tank
Benefits of Steel Water Tanks - Recyclable

Our stainless-steel tanks have absolutely no plastic, and they also create a low-carbon emission footprint in production. They’re fully recyclable, and the main elements can be easily recovered and re-used.

Looks Great
Stainless steel is a favorite material of architects and designers of beautiful homes. Your stainless steel tank will maintain its appearance for its natural life, making it an asset to the look of your backyard.

Round Water Tanks
Steel Water Tanks Cost Effective

Cost Effective
Over its impressive life cycle, a Stainless Steel water tank with a minimum life expectancy of 30 years will end up very cost effective. For a family of three, assuming a 15,000L tank and savings of $1,248 off your water bill in a year, that’s about $37,440 in savings – and 15 thousand litres per month of harvested water means about 5.4 million less litres used by your family in mains water. Amazing for your budget and the planet alike!

Colorbond Water Tanks
Colorbond water tanks also offer you great value for money. They’re built in the classic corrugated finish we have grown up with (so many childhood memories!) and come in a range of colours that can be matched or contrasted to your existing colour scheme.

What size tank do i need - colorbond water tank

At Select Water Tanks, our Colorbond Water Tanks have a 20 year non-corrosion warranty and a 10 year manufacturers warranty, making them an economical choice for your water tank. Being made from steel, they are built to withstand Australia’s harsh conditions and they will maintain structural integrity in the event of a bushfire, which makes them a fantastic choice for firefighting tanks. We can even custom-build to your requirements to meet your council’s regulations for bushfire overlays.

Want to know more?

We’d love to chat with you about how a water tank from Select Water Tanks can help save you money, ease the burden of water restrictions and help you contribute towards Australia’s water future. Get in touch!

Looking for Help or Advice?

We are passionate about Steel Water Tanks!
We are always happy to help. Contact us today or Email us below

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