Rainwater Harvesting Ltd.
0800 074 7234

Tank 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.

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

Want to do the calculation yourself? It will only take a minute or two.

Insert your building's data in the yellow boxes.
From the roof area of your property, you assess the amount of water available, and equate it to the amount you will use.

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: 0L
Collectable rainwater per annum in litres - discounted by 20% to account for water loss 0L

Use of rainwater in the building

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

Final Figures

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


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]

Please save the form. This allows you to return later without losing your entered data.
We do not keep any information you enter here and all data saved is only temporary.

Spreadsheet Version Of The Calculator

An Excel spreadsheet version of the calculator is available to download.

  1. Download the Excel spreadsheet Tank Size Calculator.
  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 58.
  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:
    1. The first blue box shows the amount of water you need to protect your home use and garden from drought.
    2. The 2nd blue box shows YES if your roof is big enough to provide the water you need to fill the tank!
    3. The 3rd blue box shows the tank size we would recommend.

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. You can place an order online (or by phone if you have queries) and even have the system installed.

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 http://www.planningportal.gov.uk/uploads/br/water_efficiency_calculator.pdf
    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 http://www.wrcplc.co.uk/PartGCalculator/Calculator.aspx 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.