Water efficiency questions, answered simply.
Clear answers about Part G water efficiency calculations, the 125 and 110 litre targets, water fitting specifications, Building Control reports and using our online calculator.
If you're preparing a new dwelling for Building Regulations, this page covers the questions we're asked most often before, during and after the calculation.
What do most people want to know?
Do I need a Part G calculation? New dwellings need to demonstrate water efficiency compliance.
What's the normal target? 125 litres per person per day, or 110 where the optional target applies.
Can I calculate for free? Yes — complete the calculation and see the result before paying.
Part G water efficiency calculation FAQs
The key questions about when a calculation is required, what the result means and what information you'll need.
What is a Part G water efficiency calculation?
A Part G water efficiency calculation estimates the wholesome water consumption of a new dwelling in litres per person per day. The calculation uses the methodology set out in Appendix A of Approved Document G and assesses the proposed sanitary fittings, taps, showers, baths and relevant appliances.
Do I need a water efficiency calculation for a new dwelling?
New dwellings in England are required to demonstrate compliance with the water-efficiency requirement under Regulation 36. The calculated consumption is normally required to be no greater than 125 litres per person per day, or 110 litres where the optional requirement applies.
What does litres per person per day mean?
Litres per person per day, often written as L/person/day or l/p/d, is the calculated daily water consumption of the dwelling divided on a per-person basis. It is the figure compared against the applicable Part G water-efficiency target.
What information do I need for a Part G water calculation?
You will normally need the quantities and specifications of the toilets, wash-hand basin taps, kitchen and utility taps, showers, baths and relevant appliances proposed for the dwelling. Depending on the fitting, this may include flush volume, flow rate, bath capacity or appliance water consumption.
Can I do a Part G water calculation myself?
The calculation is based on the proposed fitting specifications and the Approved Document G methodology. Water Calculations guides you through the required information and performs the calculation automatically, so you do not need to build or maintain your own spreadsheet.
125 or 110 litres per person per day?
The target selected for the calculation matters. Most new dwellings are assessed against the standard 125 litre target, while the optional 110 litre requirement may apply where it has been imposed for the project.
The standard maximum calculated consumption under Regulation 36 for a new dwelling.
The tighter optional water-efficiency requirement where it applies to the development.
What is the 125 litres per person per day requirement?
Regulation 36 sets a maximum calculated wholesome water consumption of 125 litres per person per day for a new dwelling under the standard requirement.
What is the 110 litres per person per day target?
The 110 litres per person per day figure is the optional tighter water-efficiency requirement. It may apply where the relevant planning or project requirements require the enhanced standard.
How do I know whether I need to use 125 or 110 litres?
Check the planning permission, planning conditions and Building Regulations requirements for the project. If no enhanced requirement applies, the standard Part G target is generally 125 litres per person per day.
Does the calculation show actual water consumption?
No. The Part G methodology calculates estimated water consumption from the dwelling design and proposed fittings. Actual household water use can vary because of occupancy and behaviour.
What information goes into the water calculation?
Part G uses the proposed performance of the water-using fittings and appliances within the dwelling.
What WC flush volume do I enter?
Use the manufacturer's declared flush volume for the proposed toilet. For a dual-flush WC, the calculation uses both the full and reduced flush volumes.
What shower flow rate should I use?
Use the declared flow rate for the shower specification being installed, expressed in litres per minute. A higher shower flow rate generally increases the calculated water consumption.
What tap flow rate should I enter?
Enter the manufacturer's declared flow rate in litres per minute for the proposed fitting. Wash-hand basin taps and kitchen or utility taps are treated separately within the calculation methodology.
What bath capacity is used in a Part G calculation?
The calculation uses the proposed bath's capacity to overflow, expressed in litres. This information is normally available within the manufacturer's product specification.
Do washing machines and dishwashers affect the calculation?
Yes. Relevant white goods can form part of the water efficiency calculation. Where exact products are proposed, their declared water-consumption figures can be entered within the detailed calculation.
Can rainwater or greywater systems be included?
Yes. Alternative water sources such as rainwater or greywater can be taken into account where an appropriate system forms part of the dwelling design.
What happens after the calculation?
Your result is shown immediately. If the dwelling passes, you can proceed to generate the professional calculation documents if required.
If it fails, simply amend the proposed specification and run the calculation again — there is no charge for recalculating.
What happens if my water calculation fails?
You can amend the proposed water fitting specifications and recalculate for free. Reducing shower or tap flow rates, selecting more efficient WC flush volumes or changing other water-using fittings can reduce the calculated result.
Can I change the specification and calculate again?
Yes. You can edit the proposed specification and recalculate as many times as needed before purchasing reports.
Are the reports suitable for Building Control?
The Water Efficiency Report and Part G Calculation Schedule provide project-specific supporting calculation evidence suitable for submission to Building Control. Final acceptance remains with the relevant Building Control body.
What documents do I receive after purchase?
Purchased projects include a Water Efficiency Report and a detailed Part G Calculation Schedule. Your VAT receipt is also provided and the documents remain available through your Water Calculations account.
Accounts, pricing and multi-plot projects
A few practical questions about using the online calculator.
Is the Part G water calculator free?
Yes. The calculation itself is free and you can view the PASS or FAIL result without paying. You only pay if you choose to generate the professional reports.
How much does a Water Efficiency Report cost?
The first dwelling or plot costs £20 + VAT (£24 including VAT). Each additional plot within the same project costs £10 + VAT.
Can I calculate multiple dwellings or plots?
Yes. The calculator supports multi-plot residential developments and different house types within the same project.
Do I need to create an account?
No account is required to start or complete the free calculation. Account details are required when purchasing reports so your completed documents, receipts and order history can be accessed securely afterwards.
Do I need a subscription?
No. There is no monthly or annual subscription. You only pay for the professional reports you choose to generate.
The easiest way to find out is to calculate it.
Enter your proposed water fittings and see the Part G result instantly. It's free to calculate and no account is required to start.
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