What size tank will I need?

Rule of thumb
A good rule of thumb is to purchase a tank that holds a minimum of four weeks supply. For instance, if you use 1,000L of tank water each week in the garden, toilet and laundry combined, you should consider purchasing a 4,000L tank.

Other factors to consider...

• How big is your roof?
• How much rainfall do you get in your area?
• How much space do you have for a tank?
• What type and size is your garden?
• How many people live with you?

If your tank is going to be large, consult a builder or engineer as it may require structural support. You may also need a building permit from your local council.

The pattern of rainfall is important. That is, if you have very regular rainfall all year round, you can use a smaller tank. Australia's long-term average rainfall is 465mm, however the average rainfall for any particular area within Australia may be very different to this. For information on rainfall in your local area, go to the Bureau of Meteorology website using the below link and check out the rainfall maps.

• Bureau of Meteorology
  Check out the Bureau of Meteorology website

Each square metre of roof area collects 1 litre of water for every 1 millimetre of rainfall received. This is determined by the area of roof connected to the tank via the stormwater down pipe.

As an example:

• An average house has a roof area of approximately 160 square metres.
• Assume that 50% of this roof area can be easily connected to a rainwater tank i.e. 80 square metres.
• Each square metre of roof area collects 1L of water for every 1mm of rainfall received.
• If you have 10mm of rain, the water collected by the tank is 800L (80 square metres x 10mm = 800L).

Rule of thumb guide: roof collection versus tank capacity for an area receiving 600mm of rain per year courtesy of
EME group.