How to Calculate the Rainwater Tank Size You Need

22 June 2015
 Categories: Environmental, Blog


One of the most important considerations when buying a rainwater tank is selecting the right size. Too big, and you might pay more than is necessary, but even worse is a tank that's too small, which could see your water supply literally drying up. There are a number of factors to consider in helping to determine the right size rainwater tank. These are explained below.

Roof Size

For every millimetre of rain, each square metre of roof will collect 1 litre of water. The average roof size for a 3-bedroom medium house in Australia is 150-200 square metres. Usually the catchment area of the roof that collects rainwater is around 50% of the roof, therefore around 80-100 square metres. Therefore, for every 10mm of rain in a short shower, 800-1000L (around 1 day's supply) of water would be collected.

Rainfall in Your Area

It's important to look at average rainfall readings in your area since this will dictate how important it is to have a larger tank. Areas of high rainfall will be able to choose a smaller tank (less than 10,000L) however in areas with prolonged dry spells it is recommended a tank greater than 20,000L is used. You can check the average rainfall figures in your area to get a better idea.

Number of People in the Household

The average home of four people uses around 200 litres of water per person per day with only 15 litres each needed each for basic survival including drinking and food preparation. This means around 800 litres per household each day! For a family household that relies entirely on rainwater a large tank will be necessary, especially in drier areas. This will avoid water shortages and having to pay for water to be brought into the property. 

What the Water Is Used For

If you are depending entirely on rainwater for your water supply, a 100,000-litre tank will provide about 125 days of water for a four-person household with no rainfall. Other households that only want to use rainwater for drinking or watering the garden will need significantly less than this and may be able to depend on a smaller 2000L tank.

The Space Available

Not all homes will have lots of space for a rainwater tanks, especially those in suburban areas. This means a smaller tank will need to be used and likely one that only has enough capacity for garden watering and drinking.

It's important to investigate your own water usage habits and seek advice based on your geographical location, roof size and family size. A rainwater tank specialists will be able to provide more advice on gaining the right size tank based on your needs. As a general rule for those living on rural properties that depend entirely on rainwater, the largest tank you can afford and have room for may be recommended as it will shield you from the future costs of having to shipping water in should your supply run out.