If you have made the choice to invest in rainwater tank your next step is to work out what size you need.

The first issue to consider is what space do you have available?

If the space available is limited, then you might need to decide whether to order a round or slimline tank. The round tanks range in diameter from 900mm to 3900mm and in height from 300mm to 3100mm while the slimline tanks range from 1000mm to 4800mm in length, 550mm to 1180 mm in width and 300mm to 2350mm in height. See diagrams:

The maximum water storage for slimline tanks is 10,000 litres and for round tanks is 37,000 litres. If you work out that you need more litres, then you could consider linking up maybe three tanks side by side to triple the water storage capacity.

The second issue to consider is how much water do you need?

While each circumstance is different a general rule is to plan for at least four weeks supply of water. For example, if you use 5000 litres per week then you should get a tank at least 20,000 litres in size. There are some approximate calculations you can do to get an idea of your water usage. For example, for a four-person family each person uses 120-200 litres each per day = 600 litres per day.

Another way to approach this is to consider – for a 22,800 litre tank:

It will provide for approximately 4600 toilet flushes or 350 showers or 380 wash cycles or 23 hours of tap water

The third issue to consider is do we order one large tank or a number of smaller tanks

It is easier to transport and place smaller tanks. We can place them in different locations so we can capture rainwater from different aspects or sides of a roof of roofs. One large tank takes less space but it could mean that you can only capture the water from one roof location.

From experience we have found that having three round tanks holding 30,000 litres each (total 90,000 litres) will easily keep that family in water all year round.

The fourth issue to consider is how much water can I harvest from my roof?

As a general rule, each square metre of roof space collects around 1 litre of water for every one millimetre of rainfall.

So if we had

• 10 millimeltres of rain in a storm, and
• 150 square metres of roof space actually available to collect rain water


We could collect 1500 litres of rain water in that storm (150sq metres x10 millimetresx1 litre)

To add to this calculation you can look up the monthly rainfall pattern for your geographical area, maybe say 100mm per month on average over a 12 month period.

The fifth issue to consider is what are the comparative costs?

In general, round tanks are less expensive than similar size slimline tanks because slimline tanks need internal reinforcing rods to keep the sides stable when they are full of water. This make them slightly more extensive but then they have the advantage that they can be fitted into narrow spaces.

Stainless steel tanks are slightly more expensive than tanks made from Aquaplate/Colorbond but then stainless steel tanks have a longer 30-years warranty compared to the 20-year warranty for tanks made from Aquaplate/Colorbond. There are other differences. For example, Aquaplate/Colorbond tanks come in different colours while Stainless steel tanks are better in marine environments. Both tanks maintain structural integrity during bush fires and can be made bushfire compliant.