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What Size Water Tank do I Need?

Posted by Rod Judge on

There’s not a one size fits all answer to the size rainwater tank you’ll need, as many factors can influence your decision. Below, we’ll run you through some simple calculations and average usage estimates to help you decide what size tank to buy.

Generally speaking, you should purchase the largest water tank that will realistically fit at your home or property, and could fill with your local area rainfall and the size of your water collection area on your roof.

How To Calculate the Right Size Water Tank for your Home

First off, let’s review how much water you actually already use.

How much water do I need?

Using your existing water bills, review how much water your household uses annually. You can do this by viewing the back of your water bill. Remember that your water usage can vary seasonally, so best to take an annual calculation.

For example, a small household of 2 people without a pool uses 170,000 Litres per year. That’s approximately 14,100 litres per month.

How Much Rainwater Can I Harvest from my Roof?

You can quickly calculate the size tank you will need based on the formula that 1m² of roof catchment area multiplied by 1mm of annual rainfall is one litre of harvestable water.

The calculation formula

[Roof Catchment Area (m²)] x [Annual Rainfall (mm)] = [Total Capacity (L)]

A working example of how much water you could collect in Sydney in one year with a roof catchment size of 150m² would be more than 57,000L of water. An example of a water tank that would be suitable for this amount of water would be our 5000L Slimline Water Tank. This size tank is suitable for a small family. Utilising the average yearly rainfall of approximately 383ml to calculate.

Discover what your average annual rainfall is on a weather site.

What if my catchment area and average annual rainfall doesn’t supply enough water for my home?

Even if you can’t physically harvest the amount of water you use around your home, installing a water tank is still cost effective and environmentally beneficial. In the State of NSW, new homes are mandated to install a water tank under the BASIX scheme.

Based on the average calculations listed above, you could save 60,000 Litres of water per year on your water bill. That’s also 60,000L of stormwater runoff saved from causing flooding or running out to sea.

Some average water usage rates are outlined below and offer ideas on how you can save water around your home, simply by reducing shower lengths and fixing leaking plumbing.

How you can save water

Inside the home

Outside the home

  • Regular showerhead 10L a minute
  • Water efficient showerhead 6-7L a minute


  • Filling a backyard pool Up to 54,000L
  • Sprinkler 999L an hour
  • Tap 15L a minute


Average bath 110L

Car washing

  • Hose 180L a wash
  • Bucket 99L a wash


  • Single flush 11L a flush
  • Older dual flush 11L a full flush/5.5L a half flush
  • Modern dual flush 4.5L a full flush/3L a half flush

Leaks (taps and pipes)

  • Dripping tap27-198L a day
  • Leaking pipe 99L a day

Hand basin

  • Running tap 4 L a minute

Leaks (toilet cisterns)

  • Slow, barely visible 9L a day
  • Leak visible in toilet pan 36L a day
  • Visible just audible 144L a day
  • Quite visible constant hiss 261L a day


  • Hand wash 15L
  • Older machine 13L a load
  • Average new dishwashing machine 12L a load

Clothes washing

  • Front loader washing machine 65L a load
  • Top loader washing machine 110L a load

Information sourced from Sydney Water.

Remember, if you would like any advice regarding the purchase of a water tank for your home, speak to our knowledgeable sales team.