How much rain can you harvest?

water meter at the Greenwood Forest ParkIt all depends on your roof and the amount of rain that you receive. Roughly 80% of the rain falling on the plan area of your roof ends up in your gutters. In North Wales  with an annual average 1000mm of rainfall, a typical two bedroomed house with a roof area of 30m2 could collect 30 cubic meters of rainwater a year. That's enough to irrigate a similar area of garden or flush the toilet over 5000 times or wash 500 washing machine loads. A potential saving of  £60 a year, however the volume of rainwater that you can collect does depend your local rainfall, your pattern of water use and the size of  your tank. The more evenly distributed rainfall and your rainwater use is, the more water you can save.

In general your tank should be large enough to supply your daily rainwater use through the driest periods and in most situations the ideal tank size is 10-15% of the volume of rainwater. For the typical 2 bed house mentioned above, a water harvesting system with a 2 cubic meter tank would collect enough water to flush the toilet 10 times a day, which is enough for one person.

If your water use is irregular throughout the year (e.g. gardening) then to use all the rainfall a much larger tank is required, as water must be stored through the winter to be used during the summer.

This spreadsheet will help you estimate the amount of water you can collect and the tank size required. You will need the average monthly rainfall for your area, the roof area of your house, and an idea of how many times you flush the toilet a day.

Help us find out how many time on average people flush the toilet by taking part in our poll.


In the UK if you are on a metered supply then you will be charged by the volume of water your household uses. At present the rate charged by Dwr Cymru is £1.23 per m3 for your water. If you are on mains sewerage you will also be charged £1.31+ per m3 to treat the water you have been supplied with. In the example above 2 bedroomed house would save around £50 per year. Water and sewerage prices are set to rise by 10% till 2010.


Water company charges for metered water and sewerage (pence)
Water Sewerage Total Charge
Anglian standard 115.14 112.04 227.18
  SoLow 147.17 201.38 348.55
  Aquacare Plus 61.80 53.38 115.18
Dwr Cymru 122.60 132.30 254.90
Northumbria 81.61 81.61
Severn Trent 115.07 78.99 194.06
South West 145.03 256.02 401.05
Southern 81.80 126.25 208.05
Thames 99.18 48.57 147.75
United Utilities 121.80 96.50 218.30
Wessex 148.25 133.96 282.21
Yorkshire 109.20 111.25 220.45

If you cant find your water company in the table above have a look at page 37 of the Ofwat report.