What Size Water Tank Should I Buy?

This is one of the more frequently asked questions on our FAQ Page.

There is quite a few factors that determine my answer to this.

Is the tank to feed water to the home for toilets, washing machines?
How many garden taps are to be connected?
Will the garden taps be feeding an irrigation system?
Will the rainwater harvested be used to keep a swimming pool topped up?

Obviously these questions need to be answered to gauge an idea of the amount of water that will be required to achieve the goals of the customer.

As an example lets assume the following:

A new home is been renovated with a new bathroom and laundry, so easy access to install new pipework will be possible.

Pipework from the tank location will be installed to feed one toilet and also a top loader washing machine. A garden tap will also be installed to assist in watering the back garden.

A Look At Water Usage

Lets look at the toilet first, and estimate the amount of water that will be required for a week. A full flush of a standard toilet uses 4.5 litres of water, while a half flush uses 4 litres of water. An average person flushes the toilet 20 times per week.

Lets assume we have a family of 4 living in the home. A simple calculation could be  as follows
20 flushes x 4.5 litres = 90 litres x 4 people = 360 litres per week

So from this simple calculation we can estimate that we will need at least 360 litres of water per week to supply water to the main toilet for a family of four.

Now lets look at the washing machine. The laundry is to be fitted with a top loading machine, which does tend to use substantially more water than a front loading machine.
This top loader will require approximately 140 litres per full cycle. The family on average is doing 5 loads per week.

Our calculation is as follows:

5 loads x 140 litres = 700 litres per week

We can now see that will the rainwater tank been fitted to the main toilet, and the new top loading washing machine, we are going to require a minimum of 1,060 litres per week.

Now it can be difficult to estimate the amount of water that will be required for the back garden tap. Obviously if a quality irrigation system is been installed, less water will be required than if a hose is used to water the garden. For our example lets allow another 200 litres per week for watering the back garden

We now have a total of 1,260 litres per week that will be required to supply water to the home and the garden.

From the above information we are going to attempt to install a minimum capacity tank of 3000 litres.

What Space is Available For The Water Tank

The available space for the tank is now going to be the determining factor on what type of tank we purchase. As a general rule, slimline rainwater tanks, are the most suitable types of tanks for inner city properties. We have measured the maximum space we have for the tank along the side passage and it is 5m in length, 2m in height and we have 980mm available between the building and the boundary fence.

With only 980mm in width available to us we are going to be looking for a very slim tank, as we don’t really want to block off total access to the side passage. The more common 3000 litre poly water tanks are usually between 720-750mm in width/depth. While these wider tanks are quite inexpensive, we are only going to be left with around 200mm to get past the tank. Most people will not be able to pass, so installing one of these tanks will effectively block off the side passage.

Once solution would be to utilise the entire length of 5m which we have available, and install two thinner slimline tanks. The new 2000 litre ThinTank would be the tank of choice in this instance. At 2400mm in length we could fit two end to end, which would take up 4.8m. The ThinTank is 1850mm in height, and this is less than the 2m in height we have available. The major benefit of the ver slimline ThinTank, apart from its aesthetically pleasing design, is how thin or slim it is at 480mm. This would give us half a metre in space left after the ThinTank is installed, allowing easy access past the tanks.

With the two 2000 litre ThinTanks been installed we would have a total capacity of 4000 litres. Our final calculation of 4000 litres tank capacity divided by 1,260 litres water required would give us approximatly 3.17 weeks of water from a full tank. For an inner city property this is quite acceptable.

One very important thing that I always tell my customers during Site Assessments, is that it is not all about the size of the tank, but more importantly it should be about how much roof area we are going to harvest to feed the tank. The larger the roof area harvested the quicker the tank will fill and replenish.

Every square metre of roof collects 1 litre of water for every 1 millimetre of rainfall received. Therefore, a tank connected to 50% of the roof area of a 200 m2 roof would collect 1000 litres after 10 millimetres of rain.

Calculations, formulas and charts are usefull but as a golden rule, the more of the roof that we be harvested and fed to the tank the better. This can be done by connecting downpipes directly to the tank inlet or by using charged lines.

Of course the example given above has been used to illustrate the water usage of particular fixtures and show how the space available can be used wisely to achieve the required outcome. Every home is different and this is why using our Site Assessment service is a great idea. There are a lot of different slimline and modular tanks available on the market today and our experience staff, if on site, will be able to give you all options.

Please feel free to add your own comments or questions below and I will respond in a timely manner.

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