Calculating Your Average Rainfall & Rainwater Tank Size

Calculating Your Average Rainfall & Rainwater Tank – many persons when looking at rainwater harvesting ask as to calculating your average rainfall & ideal tank size. Using rainwater can bring many economic benefits as it can help you save money on your water bills for many uses around the home and in commercial premises. In many areas, there are grants/rebates and tax allowances for those who install a rainwater tank and rainwater systems.

There are many factors that can affect this, including the following:

  • How much water do you typically use around your property?
  • The size of your roof which you are looking to collect rainwater from
  • Whether you will be relying solely on rainwater for your water needs or will there be a means top-up

All these will depend on how much rainfall you get each year.

Historical rainfall data 

The Met Office is an excellent source of historical rainfall data for your area as this will show several things.

  • How much rainfall each year does my area get? This will dictate how much rainfall I can collect.
  • Does where I live get rain consistently through the year or is it seasonal? Will I have enough water at the times of the year when I need it or do I need to harvest a lot more water and store it?
  • Using an average of rainfall is best as although we get a similar amount of rainfall each year, it rarely comes at the same time each year so you may want to consider using a larger tank or tanks that can handle the larger volume.  

Calculating the ideal tank size

As a guide, 1mm of rain will give approximately 1 Litre (L) of water from each square metre or roof area collected.  This will show whether you can collect the volume of water you require or whether you may need to consider a mains top up so you don’t run out of water when it isn’t raining.

For businesses and industry, the uses of round rainwater tanks for rainwater harvesting remain the same as domestic, including laundry, watering gardens, toilet flushing and cleaning. Water for businesses is becoming an increasingly expensive proposition with sewerage and trade effluent charges rising and an efficient rainwater harvesting system can help reduce costs by as much as 50% when it is combined with other water-saving measures.

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