Check a new home against the Part G water efficiency limit. Enter the fittings, see the litres per person per day, and if it fails, see the smallest change that gets it through.
Follows the national water efficiency calculation methodology used for Approved Document G
Checked against the fittings tables in Approved Document G
Covers the 125 and 110 litre targets
Reviewed 28 October 2026 by Andrew Chambi, accredited energy assessor
By Andrew Chambi, energy consultant · Last reviewed 28 October 2026
Part G water efficiency calculation
Project: - Dwelling: - Date: -
Target: litres per person per day Result: -
Calculated with the use factors, normalisation factor (0.91), external use allowance (5 litres) and multiple fittings rule of the national water efficiency calculation methodology referenced by Approved Document G (2015 edition, incorporating 2016 and 2024 amendments). Produced at part-g.co.uk. The designer is responsible for the fitting data entered.
The target
125 is the Building Regulations limit. 110 applies only where the planning permission has a condition requiring it, so check the decision notice.
Fittings
Use the manufacturer's figures. Where a home has more than one of the same fitting, add each one. The calculator applies the official rule for multiple fittings.
Toilets (WCs)
Basin taps
Flow rate in litres per minute. Bathroom and cloakroom basins, not the kitchen sink.
Showers
Flow rate in litres per minute at the outlet. Leave none if there's no shower.
Baths
Capacity to the overflow in litres, not the brochure's total capacity.
Kitchen and utility sink taps
Appliances
Pick the model being supplied and its energy label figures fill in for you. Not on the list? Choose "Enter figures from the energy label". If no appliance is being supplied, leave it on the default.
From the manufacturer's regeneration figures.
This calculator doesn't cover grey water or rainwater harvesting. If the home uses either, use the official calculator for that dwelling.
How this works
Part G sets a limit on the water a new home is designed to use: 125 litres per person per day, or 110 where planning requires it. It applies to new dwellings, including ones formed by a change of use.
Each fitting's figure is multiplied by a fixed use factor. The total is multiplied by 0.91, then 5 litres is added for outdoor use. Where there's more than one fitting of a type, the calculator uses whichever is higher: the average, or 70% of the highest. These steps follow the national calculation methodology that Approved Document G points to.
See the use factors
There's also a shortcut. If every fitting is within the limits in Table 2.1 (125) or Table 2.2 (110) of Approved Document G, you can use the "fittings approach" without a full calculation. This page tells you when that applies.
The full methodology, with the validation results and sources, is on the methodology page.
This tool follows the published methodology, but you are responsible for the figures you enter. Check with your building control body that they'll accept the output.