If you are installing your water tank to collect rainwater then you will need to work out your catchment area (roof area) and find out your average annual rainfall.
Then use the ‘Water Catchment Calculator’ to work out approximately how many litres of water you can catch per year.
Obviously, the household water consumption varies considerably for everyone but as a guide, an average family of 4 with a small garden use approximately 280,000 – 300,000 Litres per year for general household use.