Understanding Part G Water Calculation: What, When, and How

by | Apr 2, 2024 | Last Articles | 0 comments

What is Part G Water Efficiency Calculation?

Part G Water Use and Efficiency Calculation refers to a set of regulations and standards established by building codes in the United Kingdom, specifically under Part G of the Building Regulations. These regulations primarily focus on the conservation and efficient use of water within buildings. Part G sets out requirements for the sizing, design, and installation of water supply systems, including fixtures, fittings, and appliances, to promote water efficiency and minimise waste.

A link to the current Building Regulation can be found by clicking here. The section on Water Efficiency can be found with G2, Page 15.

When is Part G Water Calculation Required?

Part G Water Calculation compliance is required for all new building constructions, renovations, and extensions in England, Wales, Scotland, and Northern Ireland. It applies to both residential and commercial properties. Compliance with Part G regulations is necessary to obtain building permits and approvals from local authorities.

How to Perform Part G Water Efficiency Calculation?

  1. Understanding Water Consumption Patterns: The first step in Part G Water Calculation is to assess the expected water consumption patterns within the building. This includes determining the number of occupants, their habits, and the types of fixtures and appliances that will be installed.
  2. Fixture Sizing and Selection: Part G specifies maximum flow rates for various fixtures and appliances such as taps, showers, toilets, and washing machines. These flow rates are measured in liters per minute (l/m) or liters per second (l/s). Select fixtures and appliances that comply with these flow rate requirements.
  3. Calculating Water Demand: Calculate the total water demand for the building by considering the number and type of fixtures/appliances and their corresponding flow rates. Part G provides guidance on calculating water demand based on occupancy and fixture types.
  4. Assessing Water Efficiency: Part G encourages the use of water-efficient fixtures and appliances to minimize water wastage. Choose products labeled with the Water Efficient Product Label (WEPL) or Waterwise Recommended Checkmark to ensure they meet water efficiency standards.
  5. Designing Water Supply Systems: Design the water supply system based on the calculated water demand and flow rates. Ensure that pipes, valves, and fittings are appropriately sized and installed to minimize pressure drops and optimize water distribution.
  6. Including Water-Saving Measures: Incorporate water-saving measures such as rainwater harvesting systems, greywater recycling, and dual-flush toilets to further reduce water consumption and promote sustainability.
  7. Documenting Compliance: Keep detailed records of the Part G Water Calculation process, including calculations, product specifications, and installation plans. These documents may be required for building permit applications and inspections.
  8. Verification and Testing: After construction, verify that the installed fixtures and appliances meet the specified flow rates and water efficiency standards. Conduct periodic inspections and maintenance to ensure continued compliance with Part G regulations.


Part G Water Efficiency Calculation is an essential aspect of building design and construction in the UK, aimed at promoting water efficiency and sustainability. By understanding the requirements of Part G regulations and following the prescribed calculation process, builders and developers can ensure compliance with building codes and contribute to the conservation of water resources. Additionally, implementing water-saving measures and using water-efficient fixtures and appliances not only reduces environmental impact but also offers long-term cost savings for building owners and occupants.

The Water Efficiency Calculation is first compiled during the project’s design phase when fixtures and appliances are specified. It’s then updated if there are changes during construction. While regulations only mandate stating potential consumption, Building Compliance Testing goes further. We provide a comprehensive report, empowering you to submit completed calculations to Building Control confidently. Learn more about our Part G Water Efficiency Calculation service by clicking here.