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Click on the TV above to watch a Graf video on rainwater harvesting

New video showing assembly of a Graf Carat tank

Download our fitting instructions for the Carat system

The tank size calculator is provided by RainWaterHarvesting.co.uk for the information of their customers and can be used to establish the size of tank you might decide to install. You will need to know the approximate dimensions of your house.

1.   To start the calculation, click on the Tank Size Calculator link here and download the Excel spreadsheet.
2.   Insert your home's measurements, in metres, in the yellow boxes. Try to estimate the dimensions of that part of the house from which you will be collecting rain water.
3.   Refer to the table on Tab "Rainfall" and find out the average annual rainfall in centimetres in your part of the country, then type it in the 3rd yellow box; e.g., if you live in East Anglia, the driest part of the country, use 63.
4.   Insert in the red box the number of days drought protection you would like from your rainwater harvesting system. You can change the number of weeks at will to see what impact it has on the tank size you need.
5.   The results of the calculations are shown in the three light blue boxes:
      5.1. The first blue box shows the amount of water you need to protect your home use and garden from drought.
      5.2. The 2nd blue box shows YES if your roof is big enough to provide the water you need to fill the tank!
      5.3. The 3rd blue box shows the tank size we would recommend.

You're done! Congratulations. Go to the product page or search our site for underground tanks, the size you require, and the mains backup option you prefer. Place an order online (or by phone if you have queries) and have the system installed. If you cannot work the spreadsheet calculator please email us the 6 figures to calculator@rainwaterharvesting.co.uk and we will email you the required tank size.


More details on rainfall round Britain on the Met Office web site at: http://www.metoffice.gov.uk/climate/uk/2006/annual/averages1.html
More details about the amount of water required for plants at the Royal Horticultural Society web site at:
http://www.rhs.org.uk/advice/profiles0406/water.asp

If the calculator said that you need tank 6500 litres or bigger, don't be surprised. Most suburban homes in Britain will need this amount of water to protect against drought for just a week or two. If you want longer protection, then buy multiple tanks and connect them together (a few inches from the bottom) to increase your capacity.