Building Regulations Part G Water Calculator

Estimate internal potable water use in litres per person per day and compare your design against the common Part G target of 125 l/p/d and the tighter optional requirement of 110 l/p/d.

Project and occupancy

Fittings and appliances

Expert guide to the Building Regulations Part G water calculator

The Building Regulations Part G water calculator is used to estimate how much wholesome water a dwelling is expected to consume. In practice, designers, architects, developers, building control professionals, and sustainability consultants use it to assess whether a proposed home is likely to comply with the water efficiency standards that apply in England. If you are trying to understand how close a house design is to the usual 125 litres per person per day threshold, or whether it can reach the tighter 110 litres per person per day level often required by planning policy, a water calculator is the clearest place to start.

Part G of the Building Regulations deals with sanitation, hot water safety, and water efficiency. For new dwellings, the most commonly referenced water efficiency limit is 125 litres per person per day. Some local planning authorities also require the optional tighter standard of 110 litres per person per day. Those figures matter because water efficiency has become a core part of resilient housing design, especially in areas facing water stress, drought risk, population growth, and infrastructure pressure. A compliant design is no longer just about passing regulation; it also affects utility bills, long-term asset performance, and environmental impact.

The calculator above gives a practical design-stage estimate based on the main internal water uses in a home: WCs, showers, baths, basin taps, kitchen taps, dishwashers, and washing machines. It is intended as a fast planning tool, not a substitute for a formal assessment using the exact regulatory methodology where that is required for sign-off.

What Part G water efficiency is trying to achieve

The policy goal behind Part G is straightforward: reduce unnecessary potable water consumption in homes while preserving usability, hygiene, comfort, and occupant satisfaction. Unlike commercial buildings, homes have a fairly predictable mix of water uses. Most of the demand comes from a small group of fittings and appliances, so selecting efficient products can produce a large compliance benefit without radically changing the layout of a house or flat.

From a design perspective, the biggest contributors are usually showers, toilets, baths, and taps. In many schemes, the difference between passing and failing comes down to one or two product decisions, such as specifying a lower-flow shower, reducing WC flush volume, or selecting more efficient white goods. The purpose of a water calculator is to reveal exactly where those opportunities sit.

Core Part G thresholds you should know

There are two benchmark figures most professionals discuss when they refer to the Part G water calculator:

• 125 litres per person per day – the mainstream Building Regulations water efficiency requirement for new dwellings.
• 110 litres per person per day – the optional tighter requirement that can be imposed where a local planning authority adopts it through planning policy.

These numbers may sound close together, but achieving 110 l/p/d can materially affect specification choices. At 125 l/p/d, many standard efficient fixtures may be enough. At 110 l/p/d, the margin narrows, and the cumulative impact of every litre becomes more important. That is why an early-stage calculator is valuable: it helps teams identify the risk before products are procured.

Benchmark or statistic	Value	Why it matters for design
Part G baseline water efficiency target	125 litres/person/day	This is the commonly applied regulatory benchmark for new homes in England.
Optional tighter planning standard	110 litres/person/day	Often used by local authorities in areas with stronger water efficiency policy objectives.
Typical personal water use often cited in the UK	About 142 litres/person/day	Shows why many existing patterns of use sit above new-build performance expectations.
Illustrative reduction from 142 to 110 l/p/d	32 litres/person/day	Represents a reduction of roughly 22.5%, which usually requires deliberate specification choices.

The often-cited personal consumption figure of around 142 litres per person per day is useful context because it demonstrates the gap between ordinary household behaviour and what high-performing new homes are expected to achieve. If a scheme is aiming for 110 l/p/d, it is not enough to rely on average products. Instead, the whole internal water strategy needs to be coordinated.

How a building regulations Part G water calculator works

A water calculator converts the performance of each water-using component into daily water demand. The basic principle is simple: identify how much water each activity consumes, estimate how often it occurs, total the litres used across the dwelling, and then divide by the number of occupants to arrive at litres per person per day. In more formal assessments, occupancy assumptions and methodology are set out in guidance, but the logic is consistent: demand comes from fixtures and usage patterns.

For example, if a shower runs at 8 litres per minute and the average person showers for 6 minutes per day, that one activity contributes 48 litres per person per day. If the WC flush volume is 4.5 litres and the occupant flushes 4 times per day, that adds another 18 litres. A basin tap running at 5 litres per minute for 3 minutes contributes 15 litres. Before you even account for the kitchen tap, baths, washing machine, or dishwasher, the total can already be substantial.

The calculator on this page uses the following internal demand model:

1. Shower use = shower flow rate × shower minutes per person per day
2. WC use = flush volume × flushes per person per day
3. Bath use = bath volume × baths per person per day
4. Basin use = basin tap flow rate × basin minutes per person per day
5. Kitchen use = kitchen tap flow rate × kitchen minutes per person per day
6. Appliance use = dishwasher litres/person/day + washing machine litres/person/day
7. Total household demand = litres/person/day × occupants

This structure is useful because it mirrors the way practitioners think about specification. Every change in a product schedule has a visible numerical effect. Lowering shower flow from 10 l/min to 7 l/min can be more influential than reducing basin tap flow by one litre, simply because showers tend to represent a larger share of daily water use.

Which fixtures usually have the greatest impact?

In most homes, showers and WCs are among the largest controllable demand components, followed by baths and taps. Appliances matter too, but they are often easier to optimize because manufacturers already publish water consumption figures. The challenge tends to be balancing product performance, occupant experience, maintenance considerations, pressure conditions, and compliance targets all at once.

Fixture or use	Illustrative specification	Estimated daily use per person	Design implication
Shower	8 l/min for 6 minutes	48 litres	Often the single largest daily component in a water-efficient home.
WC	4.5 litres per flush, 4 flushes/day	18 litres	High-frequency use means small flush savings accumulate quickly.
Basin tap	5 l/min for 3 minutes	15 litres	Flow controls and aerators can significantly reduce demand.
Kitchen tap	6 l/min for 2 minutes	12 litres	Useful secondary saving area, especially in compact homes.
Bath	80 litres, 0.1 baths/day	8 litres	Bath frequency assumptions can materially shift outcomes.
Washing machine	Efficient model allowance	8 litres	Specification can support compliance without affecting user comfort much.
Dishwasher	Efficient model allowance	3 litres	Usually a smaller contributor, but still relevant near the compliance margin.

The data above illustrates a critical design lesson: when a scheme misses its target, the fastest route to compliance is usually to focus on the largest contributors first. If the design is at 129 l/p/d, trying to save half a litre in one minor component will not be enough. A more meaningful intervention, such as reducing shower flow or selecting a more efficient WC, is typically needed.

How to use the calculator properly

To get a useful result, enter realistic assumptions rather than optimistic guesses. If the intended shower is a standard mixer with a flow rate of 10 to 12 litres per minute, do not type 6 litres simply because it helps the design pass. The purpose of a water calculator is to predict probable performance. Unrealistic inputs may produce a paper pass but can lead to specification risk later in the project.

• Use manufacturer data where available for taps, showers, and appliances.
• Check whether stated flow rates apply at the actual operating pressure.
• Confirm whether WC figures refer to full flush only or dual flush performance.
• Be consistent about whether bath assumptions are occasional or frequent.
• Review the result category by category rather than looking only at the final total.

For early concept design, the calculator is excellent for option testing. You can compare several fixture schedules in minutes. For example, one shower model at 8 l/min and another at 6 l/min may look similar on a spec sheet, but the lower-flow option can reduce daily demand by 12 litres per person when used for a 6-minute shower. In a four-person household, that is 48 litres per day saved across the dwelling.

Why the optional 110 litres standard is more demanding

Moving from 125 l/p/d down to 110 l/p/d means cutting an extra 15 litres per person per day. On paper that may seem modest, but in a practical specification exercise it can be a major shift. The easiest savings may already have been captured to meet 125 l/p/d. To reach 110 l/p/d, teams often need a coordinated package of measures: efficient showers, carefully selected taps, low-flush WCs, efficient appliances, and sensible assumptions about baths.

That is also why local planning policy can influence product procurement much earlier than some teams expect. If the site sits in a local authority area that uses the optional tighter standard, it is wise to check planning requirements before the sanitaryware schedule is finalized. Retrofitting water efficiency late in the design can be more expensive and can narrow product choice.

Common mistakes when assessing Part G water use

• Ignoring flow pressure: a quoted flow rate may not reflect real site conditions.
• Overestimating appliance efficiency: always verify actual certified figures.
• Underestimating shower duration: small time increases can have a large effect on water use.
• Assuming baths are rare: family homes may have more frequent bath use than apartments aimed at single occupants.
• Leaving calculations too late: compliance risk rises when water efficiency is checked only after specifications are mostly fixed.

Practical ways to reduce litres per person per day

If your result is above target, begin with the highest-volume categories. In many cases, these strategies are the most effective:

1. Select a lower-flow shower with proven performance and acceptable user comfort.
2. Use efficient WC cisterns with reduced full-flush volumes.
3. Specify basin mixers or taps with lower regulated flow rates.
4. Choose efficient washing machines and dishwashers with strong water performance data.
5. Limit oversized baths where project type and market positioning allow.
6. Coordinate planning policy, building regulations strategy, and product schedules early.

It is also worth remembering that compliance should not come at the expense of usability. Poorly performing fittings may encourage occupants to use more water for longer, which undermines the design intent. The best specification is one that combines verified low consumption with acceptable pressure, spray pattern, rinse performance, and durability.

How this calculator should be used in a project workflow

This page works best as a quick feasibility and option comparison tool. At concept stage, it helps establish whether a scheme is likely to pass comfortably, narrowly, or fail. At developed design stage, it helps compare supplier alternatives. At pre-construction stage, it can act as a sense-check before the formal evidence pack is assembled. In all cases, the breakdown chart is important because it shows exactly where the water demand is concentrated.

If the chart reveals that showers account for the largest share, that is the first category to review. If toilets or taps are disproportionately high, those become the priority. This is much more actionable than a single pass or fail number because it gives the design team a clear route to improvement.

Authoritative sources and further reading

For official guidance and statutory context, consult these sources:

• Approved Document G: sanitation, hot water safety and water efficiency
• The Building Regulations 2010, Part G
• National planning policy framework and related planning context

This guide is for educational and design-planning purposes. Where a formal Part G compliance submission is required, always use the exact methodology, assumptions, and evidence standards accepted by the relevant authority or building control body.