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Tanks Size Calculator

The tank size calculator service is provided by RainWater Harvesting so you can establish the size of tank you might decide to install. You will need to know the approximate dimensions of your house in metres. If in doubt, just pace out the house length and width.

Collectable Roof Area (m²)

Main Building Width: Length: Rain Collection Area: 0
Extension one Width: Length: Rain Collection Area: 0
Extension Two Width: Length: Rain Collection Area: 0
Extension Three Width: Length: Rain Collection Area: 0
Or the total roof area, if you already know it: Total area of collectable roof space: 0
Select Your Region Average rainfall per year in your region: 0 L
Collectable rainwater per annum in litres - discounted by 20% to account for water loss 0 L

Use of rainwater in the building

Number of people or bedrooms in the building - people: bedrooms:
0 Cycles 0 L
0 Flushes 0 L
Outdoor use in litres, per person per day (recommended 5 litres per person per day) 0 L
Amount of water you require every day 0 L
Amount of water you require every year DEMAND 0 L

Final Figures

How many days drought protection do you need? Typically 21 (18 minimum)
Capacity of water storage in litres required for drought protection 0 L
The lesser of YIELD (blue) or DEMAND (green) per annum 0 L
Therefore, volume of rainwater storage required 0 L


Is there sufficient roof water available:

Please Complete the Form

Recommended tank size from our shallow dig range: F-Line Range:

[F-Line Tank Range]

Select the carat range if you require a deep dig tank: Carat Range:

[Carat Tank Range]

We can do the calculation for you if you supply the information requested below.

For us to do the calculation please give us a call on 01733 405111 and provide the following information :

  1. Collectable Roof Area
  2. Number of bedrooms in the property
  3. Your location within the country

If you wish to work out the tank size yourself please click on the link below to download the Tank Size Calculator Spreadsheet.

Click Here

  1. Insert your home's measurements, in metres, in the green boxes.
    Try to estimate the dimensions of that part of the house from which you will be collecting rain water.
  2. 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 grey box; e.g. if you live in East Anglia, the driest part of the country, use 58.
  3. Insert into the red box the number of people in the home,
  4. Insert in the yellow 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 section 12.

You're done! Congratulations.
To find the tanks suitable for your application, type the size of tank into the search box - say 2700 - and the search should bring up the tanks and systems of that tank size.

Further Advisory Notes - (Spreadsheet Version)

Many other companies, especially those publishing information on their websites, do us the honour of cribbing our tank size calculator. The only difference we can claim is that we keep ours up to date with the development of regulations, rainfall statistics, new tank products on the market and other aspects which might alter the calculations:

  1. Lines 8 to 52 replicate the calculations of the UK Communities and Local Government paper "The Water Efficiency Calculator for new Dwellings" May 2009 available on the internet at
    This is the official version of the same calculator required by the Code for Sustainable Homes as issued by BREEAM and by the Building Regulations Part G.
  2. Our tank size calculator is simplified in only one respect, that is the amount of water used by typical toilets. The official calculator provides spaces for you to define single flush or dual flush toilets and their capacity or flow rate, whereas ours simplifies this to an average flush of 5 litres (at best, cistern of 6 litres and low flush of 2 litres, but often larger flush volumes). We retain the Code's 4.42 flushes per day ("Use Factor") times this.

    5 litres average volume per flush. If in doubt or if required by the authorities you should run the complete calculation which you can find at Our washing machine usage is based on a wash for everyone in the home every 4 days.
  3. The official calculator requires a minimum outdoor water of 5 litres per person per day. If you insert a larger figure in cell F42, as we suggest, and if the rainwater Yield of your roof is greater than the Demand, then the calculator will recommend a suitably larger tank. A small garden needs 200 litres a day and half an acre more like 400 litres a day (in spring and summer).
  4. Paragraph 12 provides you the volume of rainwater storage required according to the "Code" calculation as referred to in 1 and 2 above. Our tank size recommendations are based on this.
  5. We add in the light green box at the bottom the recommendations of the British Standard BS 8515-2009. This will typically give a smaller tank size because the drought protection is about 18 days not 21 days. Use it as a check, or select this volume, as you choose. Our calculator uses the BS 8515 "intermediate approach".
  6. Paragraph 8 uses a roof and filter coefficient of 80% (cell F35) which is 10% loss of water off the roof (mostly evaporation) and 10% filter loss (water going to drain with debris and leaves). Roof loss is much greater if you have a sedum or other organic roof.