1. Your Name*
2. Telephone No.*
3. Email Address*
4. Roof Area in Sq. Metres* (Add up all the rain collecting area that can be piped to your tank)
5. Your Region from Chart* (Fill in your Regional Rainfall from the blue chart - eg. Midlands =80)
6. Number of People* (How many people in your household?)
7. Drought Protection* (How many days of drought cover to be stored in the tank? E.g. 10)
8. Any Additional Information?
9. Please type 3 missing letters from the word rainwater in the box below.
10. r***water here>* (Type the three * letters in the box to stop bad bots)

Further Advisory Notes:

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 for download here. 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 3 litres (cistern of 6 litres and low flush of 2 litres, low flush used twice as often as full flush). We retain the Code's 4.42 flushes per day ("Use Factor") times this 3 litres average volume per flush. If in doubt or if required by the authorities you should run the complete calculation which you can find here. 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 500 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".

5) Paragraph 8 use 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 is you have a cedum or other organic roof.