The Water Calculator Methodology

This online calculator uses the Government’s methodology for assessing the whole house water efficiency of new dwellings. The calculation method is to be used to assess compliance against the water performance targets in Building Regulations Part G. It is also used by United Utilities for their Developers Water Infrastructure Charge Scheme. It is not a design tool for water supply and drainage systems. It is also not capable of calculating the actual water consumption of a new dwelling. Behaviour and changing behaviour can also have an effect on the amount of potable water used throughout a home.

Performance target

Maximum Consumption (litres/person/day)

Part G Compliance



The calculation method requires the use of water consumption figures provided from manufacturers product details. Before the assessment can be carried out, figures will need to be collected from manufacturers’ product information to determine the consumption of each terminal fitting.

There is a separate tab for each terminal fitting. Figures from manufacturers’ product details should be entered into the table on the relevant appliance tab. These data will be used to populate the ‘Water Calculator’ table with the consumption of each fitting in litres per person per day. Specific data requirements and instructions are provided alongside each fitting’s data entry table.

Note, flow rates of external taps are not included in the calculation as a fixed allowance of fiver litres per person per day is assumed for external water use in Part G.