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How to Calculate the Amount of Rainwater You Can Capture

How to Calculate the Amount of Rainwater You Can Capture

Calculating the amount of rainwater you can collect is an interesting task to perform when considering rainwater storage tanks. Knowing how much rainwater you can capture provides a good upper maximum limit of water you will need to be able to store, which is often much higher than people realise.

Calculating Your Rain Harvest

Given 1mm of rain falling on 1m2 yields 1 litre of water, you need to know how many millimetres of rain falls in your area. To discover the rain fall in your area:

  1. visit the Australian Bureau of Meteorology website
  2. select monthly rainfall
  3. enter your suburb then select your closest bureau station
  4. click on button to get rainfall data for your area.

Unless you are in a drought stricken area, you will normally see 50% of the months provide up to 200mm and beyond. Much of the rain (100mm+) will often fall in just one day. Using 100mm of rain fall, here are some average calculations:

  • Small house (150m2 * 100mm rain): 15,000L
  • Medium house (200m2 * 100mm rain): 20,000L
  • Large house (250m2 * 100mm rain): 25,000L

These are just estimates, and each locality is different, but we quickly see that to capture the most water on just one day could quickly fill up a fairly large tank in even the smallest of houses.

How Much Water People Use

Another factor to consider is your property or household water usage. Typically in Australia, the average suburban house uses about 250,000 litres of water per year (20,833 litres per month). Given a person can consume about 300 litres of water per day, if you have a larger number of people in your house, gardens or a swimming pool then your water usage can run much higher.

So if you have an average household, then you will use the majority of all the rainwater you collect in your rainwater tank, especially as you hit the dryer months in the year.

What Tank Size Is Best?

In Australia rain doesn’t fall at a nice and even rate, but normally pours down in bucket loads or not at all. It is therefore often recommended to buy the biggest size tank that you can reasonable fit to meet your water usage demands. This will allow you to capture tens of thousands of litres of rain when it does fall, reducing your disheartenment when it overflows into your storm water drainage.

Keep in mind that you will likely end up using all of it, especially as the dryer months hit. Guilt over the environment today is also common. So if you are one to feel guilty over using too much water, then collecting your own water supply is a great way to alleviate your guilt when watering your garden, if having an extra long shower or accidentally leaving your tap running.