Water Use Calculator for Building Regulations (Part G)

An essential tool for developers, architects, and designers to ensure new dwellings meet UK water efficiency standards.

Compliance Calculator

0.00 Litres / Person / Day

Breakdown of Water Consumption:

WC Usage: 0.00 L/day

Shower Usage: 0.00 L/day

Bath Usage: 0.00 L/day

Tap Usage: 0.00 L/day

Appliance Usage: 0.00 L/day

Unsoftened Water Factor: 0.00 L/day

---

Sub-total Internal Use: 0.00 L/day

External Use Allowance: 5.00 L/day

What is a water use calculator for building regulations?

A water use calculator for building regulations is a tool used to estimate the potential water consumption of a new dwelling in litres per person per day. Its primary purpose is to demonstrate compliance with Part G of the UK Building Regulations, which sets standards for water efficiency. For all new builds and many conversions, it is a legal requirement to calculate the projected water use to ensure it does not exceed the maximum allowable limit, which is typically 125 litres per person per day. Some local planning authorities may impose even stricter targets, such as 110 litres per person per day, to promote greater water conservation.

This calculator is not meant to predict the exact water bills of future occupants, as individual habits vary greatly. Instead, it provides a standardized method based on the specifications of the installed fittings and appliances (like toilets, showers, and washing machines) to ensure the building itself is designed to be water-efficient. Developers, architects, and plumbers use the results to make informed decisions about which products to install to meet the required water use calculator building regulations target.

The Water Use Calculation Formula

The calculation methodology is defined by the official “Water Efficiency Calculator for New Dwellings”. It works by summing the projected water use from all micro-components (taps, toilets, etc.) based on manufacturer data and standard usage patterns. The total is then normalized for the number of occupants, which is estimated based on the number of bedrooms.

The basic formula for an individual component is:

Component Use (L/person/day) = (Flow Rate × Usage Frequency × Duration) / Occupancy

These individual values are summed, and then adjustments are made for factors like water softeners and rainwater harvesting before a final figure is produced. An allowance of 5 litres/person/day for external water use is also added for Part G compliance.

Variables in Water Efficiency Calculation

Variable	Meaning	Common Unit	Typical Range
WC Flush Volume	The effective volume of water used per flush for a toilet.	Litres (L)	4 – 6 L
Tap Flow Rate	The rate at which water flows from a tap.	Litres/minute	3 – 12 L/min
Shower Flow Rate	The rate at which water flows from a shower head.	Litres/minute	6 – 15 L/min
Bath Volume	The total capacity of the bath up to the overflow.	Litres (L)	120 – 200 L
Appliance Use	Water consumed by washing machines or dishwashers.	Litres/kg or Litres/place setting	Varies widely

Practical Examples

Example 1: Standard Family Home (3 Bedroom)

A typical new-build 3-bedroom house aiming for the standard 125 L/person/day target.

• Inputs:
  • Bedrooms: 3
  • WC Flush Volume: 4.5 L
  • Shower Flow Rate: 8 L/min
  • Bath Capacity: 160 L
  • Tap Flow Rate: 5 L/min
  • Washing Machine: 8.17 L/kg
  • Dishwasher: 1.25 L/place setting
• Results:
  • The calculated water use would likely fall just under the 125L limit, achieving a “Pass”.
  • This demonstrates how standard modern, water-efficient fittings can meet the water use calculator building regulations requirements.

Example 2: Eco-Home with Stricter Target (2 Bedroom)

A 2-bedroom apartment in London, where the planning authority requires a stricter target of 110 L/person/day.

• Inputs:
  • Bedrooms: 2
  • WC Flush Volume: 4.0 L (highly efficient dual flush)
  • Shower Flow Rate: 6 L/min (aerated showerhead)
  • Bath Capacity: 0 L (no bath installed)
  • Tap Flow Rate: 4 L/min (low-flow taps)
  • Washing Machine: 7 L/kg
  • Dishwasher: 1.0 L/place setting
  • Rainwater Harvesting: 5 L/person/day
• Results:
  • The combination of highly efficient fittings, the absence of a bath, and rainwater reuse would bring the total consumption below the 110L limit.
  • This highlights the need for careful specification to meet enhanced environmental standards. For more on this, see our guide on {related_keywords}.

How to Use This water use calculator building regulations Calculator

Follow these steps to accurately assess a dwelling’s water efficiency:

1. Gather Your Data: Collect the technical datasheets for all water-using fittings you plan to install. This includes toilets, taps, showers, baths, washing machines, and dishwashers. You need the specific flow rates (in litres/minute) or consumption figures (in litres per cycle/kg/place setting).
2. Enter Dwelling Details: Start by inputting the number of bedrooms. The calculator uses this to determine the assumed number of occupants as per the official methodology.
3. Input Fitting Data: Carefully enter the performance data for each component into the corresponding field. Use the average effective flush for dual-flush WCs and the average flow rate for all taps.
4. Add Water Reuse Systems: If you have installed a rainwater harvesting or greywater recycling system, enter the certified savings in the “Rainwater Harvesting Savings” field.
5. Review the Results: The calculator will instantly update, showing the total estimated consumption in Litres per Person per Day. The “Result Status” will clearly indicate “Pass” or “Fail” based on the standard 125L target.
6. Analyze the Breakdown: Use the breakdown and the chart to see which areas contribute most to water consumption. If you fail to meet the target, this is where you can identify opportunities for improvement, such as specifying a shower with a lower flow rate or a more efficient toilet. You might find our article on {related_keywords} useful for this.

Key Factors That Affect Water Use Calculations

Several key factors influence the final result of a water use calculator for building regulations assessment:

• Toilet Flush Volume: As toilets are used frequently, the difference between a 6-litre single flush and a 4-litre effective dual flush is significant over a day.
• Shower Flow Rate: This is a major contributor. An inefficient 15 L/min power shower uses more than double the water of a 7 L/min aerated showerhead in the same amount of time.
• Presence and Size of Baths: The methodology attributes a significant amount of water usage to baths. Designing a dwelling with only showers is a common strategy to meet stricter targets.
• Tap Flow Rates: While less impactful than showers, the cumulative use of kitchen and bathroom taps adds up. Installing flow regulators or aerated taps is a simple fix.
• White Goods Efficiency: The efficiency of washing machines and dishwashers has improved dramatically. Always use the manufacturer’s stated consumption figures. If you need to know more about {related_keywords}, follow this link.
• Water Recycling: Systems that collect and reuse rainwater or greywater provide a direct reduction in the final calculated potable water consumption, making them very effective for achieving low targets.

Frequently Asked Questions (FAQ)

1. Is this calculation a legal requirement?

Yes, for all new-build dwellings in the UK, and for many types of conversions, a water efficiency calculation must be submitted to building control to demonstrate compliance with Approved Document G of the Building Regulations.

2. What happens if my design fails the calculation?

If the calculated water use exceeds the target (e.g., 125 L/person/day), you will not achieve building control sign-off. You must revise your design by specifying more water-efficient fittings or appliances and then re-calculate until you pass.

3. What’s the difference between the 125L and 110L targets?

The 125L/person/day target is the national standard set by Part G. The stricter 110L target is an optional requirement that can be enforced by a local planning authority as a condition of planning permission, often in areas of water stress. Check out our guide to {related_keywords}.

4. Do I need to know the exact make and model of the fittings?

Yes, for an official submission, you must have the manufacturer’s technical data for each fitting to prove its consumption rate. Generic values are not sufficient for final sign-off.

5. Does this calculator account for external water use?

Yes, the methodology specified in the regulations includes a fixed allowance of 5 litres per person per day for outdoor water use (like watering plants or washing a car). This is automatically added in our calculation.

6. What if a dwelling has no bath?

Simply enter ‘0’ for the bath capacity. This will reduce the overall water consumption figure, making it easier to pass the calculation. The methodology accounts for shower-only dwellings.

7. How are water softeners handled?

The official calculation methodology adds a small amount of water usage to account for the regeneration process of water softeners. Our calculator includes this as the “Unsoftened Water Factor” based on the number of occupants.

8. Where can I find the official government methodology?

The methodology is detailed in “The Water Efficiency Calculator for New Dwellings”, published by the government. Many water companies and building control bodies also provide access to it.