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Water Efficiency Calculations

The Water Efficiency Calculator for new dwellings is the Government's national calculation methodology for assessing water efficiency in new dwellings in support of both The Code for Sustainable Homes and The Building Regulations Part G, 17k.


The methodology is used to assess the whole house potable water consumption which is measured and compared to various benchmarks and is expressed in LPPPD (litres per person per day).

Performance Target
Maximum LPPPD
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 flow rate or capacity of each terminal fitting.
Part G. 17k Compliance
Code for Sustainable Homes Level 1/2
Code for Sustainable Homes Level 3/4
Code for Sustainable Homes Level 5/6

The following terminal fittings are assessed and in many cases if not present default flow rates or capacities are used.

Flow Rate or Capacity Assessed
WC Flushing capacity for both single & dual flush
Bidet Excluded from the calculator due to minimal water consumption
Taps Flow rate for each tap in litres per minute
Bath Total capacity of bath to overflow in litres
Dishwasher Litres per place setting based on the EU energy label - if not fitted 1.25 litres is assumed
Washing Machine Litres per kilogram of dry load - if not fitted 8.17 litres per kilogram is assumed
Shower Flow rate of shower in litres per minute
Water Softener If fitted - Percentage of total capacity used per regeneration cycle
Waste Disposal Unit If fitted - Standard assumption of 3.08 litres per person per day
External Tap Excluded as an assumption of 5 LPPPD is assumed (WAT 2 is used under CSH)
Greywater Assessed based on design
Rainwater Assessed in accordance with BS8515