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how big should my water tank be


Just before we start, we want to point out that we’ve written this blog specifically for situations where a water tank is the sole source of water supply for your new home, but of course the principles will apply if you’re connected to the town water but want a water tank to supplement your water usage.


So if you’re in the position of needing water tanks to supply all your water, then you’re likely out of town on a rural or country property. Which is great – we love the country and our homes are designed to fit in with the great outdoors!


But you’re asking yourself how big should my water tank be? Or perhaps you’re thinking more than one tank. So you start looking at tank sizes and you realise that there’s a massive range of sizes, styles and types, and it’s a bit overwhelming.


In this blog, we’re going to direct you to another website called Tankulator to actually do the calculations. It’s a brilliant online calculator that takes most things into consideration and gives you meaningful data that makes sense.


But before you hit the link below, just for your interest, the basic formula behind the online calculator is rainfall x catchment x usage.


Once you enter the postcode, it works out the rainfall for your area. It’s important to be accurate with the location, as you’d be surprised how much the rainfall differs between town to town, even a few kilometres can make a significant difference.


Then you’ll need to enter the surface area of the house and other buildings such as a garage, shed or carport that will catch the rain that falls. Simply work out the square metres of roof area of the buildings – these numbers are often on the plans from the builder. Make sure you include the eaves overhang. You want 1 number for this part of the calculator, which will be in square metres.


And it then shows common estimated usage figures that you can use, but if you want it to be accurate you’ll need to do some homework and adjust these figures to your own application. In fact, it’s the single variable out of the 3 that can produce wild fluctuations in telling you what tank size you need. Think about everything that uses water. You might have to spend some time googling average water uses for various things to try and make sure it’s accurate.


And the other thing that Tankulator will do is use a standard tank size of 10,000 litres to start with. This is way too small to run a home off, so you need to enter some bigger tank sizes – but you can play around with this as you go.


Here’s the link Tankulator Website


Ok, you’ve spent some time on it and it could be telling you that all is well with a 50,000 litre tank or it could be telling you about some problems.


So here’s a few other considerations for you –

1. Tank size is not everything. Unfortunately there’s a myth out there that the bigger the tanks the better. You can have the biggest tank in the world, but if you don’t have the rainfall and / or enough catchment area then the huge tank is not going to be of any benefit at all. A tank that has way more capacity than it needs to means very slow turnover (stagnant water), and also spending more money than you need to.

2. Most plumbers / tank installers will put some water in when the tank is installed – that can be quite a boost to start things off, and the calculator can take that into consideration.

3. You may have to have a steel tank due to a bushfire regulation. If that’s the case, make sure you work on these special tank sizes. But on another note, some of these tanks have a curved roof which actually acts as extra catchment area – good idea!

4. In spite of what we said a few clauses back, it’s obviously a good idea to make sure the tank has some buffer zone in size. Through trial and error you might have worked out that you need a 41,000 litre tank. We would suggest going for 45,000 or even 50,000.

5. Another consideration is whether you go for multiple tanks, that are linked together. There would be a host of reasons for this, including size restriction – you only have a 3.5mtr wide strip to put the tank, and 3 smaller tanks will work instead of one with a much bigger diameter. Or what happens if one of the tanks gets damaged – at least you can shut this off and you still have the other tank. Or what if a possum dies in 1 of the tanks? once again you can close it off and still have water. Or it might be head height restrictions which means you have to go for smaller tanks to make sure the inlet height of the downpipes on the home are higher than the inlet height on the tanks. You get the idea.


Well, it’s a long story, but unfortunately it’s not a straightforward question of how big your water tank should be.


Of course, you can always ask someone else to work it out for you!

Keen to find out more?

Reach out to us. We’re here to help.

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