What is a Water Calculator?
Reviewed by Matt Sutton, Energy Manager
What Is A Water Calculator?
In UK building compliance, a water efficiency calculator is the statutory assessment tool used to determine compliance with Part G of the Building Regulations. While consumer water calculators help households estimate utility bills, a building compliance water calculator proves to your Building Control Body that a new dwelling will not exceed mandatory daily water usage thresholds.
Approved Document G and Regulation 36 Requirements
Building Regulations 2010 Approved Document G (Sanitation, hot water safety and water efficiency, 2015 edition incorporating 2016 amendments) governs water consumption in new residential developments across England. The Code for Sustainable Homes, introduced in 2007, was formally withdrawn by the UK Government in March 2015. Water efficiency targets are now enforced directly through statutory Building Regulations under Regulation 36.
Regulation 36 specifies two distinct water efficiency tiers:
- National Baseline Target (Regulation 36(1)): 125 litres per person per day. This allowance comprises 120 litres per person per day for internal fittings plus a fixed statutory allowance of 5 litres per person per day for outdoor water use.
- Optional Requirement (Regulation 36(2)(b)): 110 litres per person per day (105 litres internal plus 5 litres external allowance). This stricter limit applies only when specified as a planning condition by a local planning authority, predominantly in regions classified as areas of serious water stress by the Environment Agency.
How the Water Efficiency Calculation Works
The official Part G water efficiency calculator calculates projected daily consumption based on sanitary fitting flow rates and standardised occupant use patterns. Rather than measuring actual occupant behaviour, the calculation models fixed consumption metrics based on manufacturer-tested performance data across seven primary sanitaryware categories:
- WCs: Effective flushing volumes for dual-flush and single-flush cisterns.
- Basin Taps: Maximum flow rates in litres per minute at 3 bar dynamic pressure (excluding external garden taps).
- Baths: Total capacity to the overflow in litres.
- Showers: Flow rates in litres per minute from mixer heads or electric units.
- Kitchen Sink Taps: Maximum flow rates in litres per minute.
- Washing Machines: Litres consumed per kilogram of dry load.
- Dishwashers: Litres consumed per place setting.
Calculation Method vs. The Fittings Approach
Approved Document G provides two routes for demonstrating compliance with Regulation 36. Developers may either follow the prescriptive Fittings Approach detailed in Table 2.2 or calculate consumption using the full calculation methodology.
Under the prescriptive Fittings Approach, every individual fitting must satisfy strict maximum thresholds (such as 4/2.6 litre dual-flush WCs, 5 l/min basin taps, and 10 l/min showers for the 125 l/p/d baseline; or 8 l/min showers and 160 litre baths for the 110 l/p/d standard). When sanitary specifications exceed Table 2.2 maximum rates, the water efficiency calculator allows trade-offs between higher-flow showers and lower-capacity baths, enabling architectural design flexibility while maintaining overall compliance.
Rainwater Harvesting and Greywater Recycling Offsets
Where local planning authorities apply planning conditions in water-stressed areas, the water efficiency calculator models greywater recycling or rainwater systems to meet the 110 litres per person per day target. When recycled water supplies non-potable demands such as toilet flushing or washing machines, the calculation deducts that volume from the internal potable consumption total, provided the system complies with BS EN 16941-1 or BS 8515.
Building Control Sign-Off and Notice of Compliance
Regulation 37 of the Building Regulations requires the person carrying out the work to provide a formal notice of water efficiency calculation to the Building Control Body (local authority or registered building control approver) no later than five days after completion of the work. This document must state the calculated consumption and confirm that the installed sanitary fittings match the design specification.
ATSPACE provides an Approved Document G water efficiency calculator service alongside design-stage SAP calculations to streamline Building Control submissions. Our energy assessors, managed by Matt Sutton and coordinated by Steve Xerri, review sanitary schedules and provide compliant calculation notices within 5 to 10 working days. Quote requests submitted between 8am and 4pm Monday to Friday receive same-day responses from your dedicated account manager.
