The Water Calculator

Frequently-Asked Questions

Who is the calculator aimed at?

The calculator is designed for anyone involved in the development of new homes—developers, Housing Associations and others—or major refurbishment. It is designed make it quicker and easier to calculate water use.

Can I use the results from the calculator in a planning application?

You can in the UK provided you have the calculation results verified and approved by a Building Control Officer.

Why should I want to use the calculator?

The calculator is designed to be used by anyone who is involved in the design and construction of new homes. It has been designed to be easy to use and results can be returned in 3 simple steps. Once you have your results you can print them, print with the product specified data or save them for later reference.

Can I save my results for later viewing?

Yes you can. You can save your results at the end of your calculation by clicking the save button. You will need to register with the calculator to do this, but don't worry, registration is just your e-mail address and a password so you can retrieve your data later, there are no personal details required.

The product I want to use isn't in the list. Can I enter my data/values manually?

Yes you can. Simply follow the instructions on the popup data input screen, clicking "Edit" to the right of an input box turns it into manual input. Remember to give your product a name so that when you print out your results you will know what you have specified.

What do Building Regulations mean for me?

If you are building a new home, you must ensure that it complies with Building Regulations, which may include restrictions on water usage. This site provides a working example of this and also contains information on water consumption for hundreds of products, enabling quick and easy specification without the hassle of collating data from several product manufacturers. You can use our example specifications if you need a quick starting point.

How do I calculate how much my greywater harvesting system will contribute to my development?

To calculate this, you will need to enter 3 values into the calculator, the water usage of the fittings supplying your greywater system, the percentage of water to be recycled and the greywater demand.

To calculate the water usage of the fittings supplying your greywater system you will need to add together the water consumption of the fittings that will supply your grey-water system e.g. shower, bath and taps excluding kitchen/utility rooms. To do this you can use the totals given in the water calculator in litres/person/day on the far right hand column.

The percentage of used water to be recycled should be available from the product manufacturer.

To calculate greywater demand you will need to add together the total water consumption of the fittings that will be supplied by your greywater system i.e. the toilet(s). If you have more than one toilet and your system does not supply all of them, you will need to ensure that you only add the correct proportion of water for that toilet.

Please note that if you are collecting greywater and rainwater, you must ensure that water demand is split equitably between the 2 calculations, so that there is no double-counting of water saved.

How do I calculate how much my rainwater harvesting system will contribute to my development?

Factors that will affect the volume of water harvested will include the average rainfall in the area, the size of the collection surface, the material used on the collection surface, the slope of the collection surface, the size of the collection tank and the efficiency of the filter. To calculate how much water this will supply per person, you also need to estimate the number of occupants (estimated at 2 people for the main bedroom and 1 person for all other bedrooms.) Our water calculator uses this information to make a simplified calculation of how much water your system will supply. The product manufacturer should be able to help you find this information.

To calculate rainwater demand you will need to add together the total water consumption of the fittings that will be supplied by your rainwater system i.e. the toilet(s) and/or washing machine. If you have more than one of these items and your system does not supply all of them, you will need to ensure that you only add the correct proportion of water for that item.

The Water Calculator guidance provides further information on how to calculate rainwater harvesting contribution to water consumption.

Please note that if you are collecting greywater and rainwater, you must ensure that water demand is split equitably between the 2 calculations, so that there is no double-counting of water saved.

I have used the calculator before and want to start a new calculation. How do I clear my old data?

If you have registered with the Calculator you can start a new calculation by logging in and clicking the 'Start a new Calculation' button. If you are not registered you will have to clear the cookies out of your browser to reset the system.