Part G Water Calculations Uk

Estimate litres per person per day (l/p/d) for a dwelling and check compliance with UK Part G water efficiency targets.

Expert Guide to Part G Water Calculations in the UK

Part G water calculations are one of the most important compliance checks for new homes in England. If you are involved in design, planning, SAP support, MEP coordination, building control submissions, or developer-side technical management, understanding how Part G water efficiency is assessed can save time, prevent redesign, and protect project margins. This guide explains what Part G means in practice, how the litres per person per day metric is formed, which fittings influence outcomes most, and how to prepare evidence that stands up at approval stage.

What Part G actually requires

Approved Document G covers sanitation, hot water safety, and water efficiency. For new dwellings in England, the core national requirement is generally interpreted as a maximum potable water consumption of 125 litres per person per day. In many local authority areas, planning conditions impose a stricter optional requirement of 110 litres per person per day. This means design teams cannot treat water calculations as a late-stage formality. The specification of taps, showers, WCs, baths, and appliances directly affects whether the dwelling can be signed off without changes.

In practical terms, the calculation combines estimated usage for key internal end uses, then divides by occupancy to produce a normalised l/p/d result. Occupancy assumptions matter. Product data quality matters. Coordination between architect, employer’s agent, and MEP consultant matters. If one component is entered using non-compliant values or unsupported datasheets, the final figure can fail even when the built intent is good.

Where teams make mistakes

• Using brochure marketing flow rates instead of certified test figures.
• Mixing nominal and regulated flow values without clear basis.
• Assuming planning needs 125 when local policy requires 110.
• Changing bathroom product schedules after planning without recalculating.
• Not controlling substitutions during procurement and value engineering.

The result is often a late scramble to swap fittings or add compensating reductions elsewhere. A better approach is to set a design water budget early and keep it live through procurement.

Regulatory and policy benchmarks

Benchmark	Typical figure	What it means for design teams	Primary source
Part G baseline target	125 l/p/d	Default compliance level for many new dwellings in England.	UK Government Approved Document G
Optional tighter target	110 l/p/d	Frequently required by local plan policy in water-stressed authorities.	UK Government Approved Document G and local planning policy
Observed household consumption in England and Wales	Commonly reported around 130 to 145 l/p/d in recent years	Shows why efficient fittings are often needed to achieve 110 consistently.	Ofwat and water company reporting datasets

Figures above combine statutory limits and widely reported sector consumption ranges. Always confirm project-specific policy wording and the latest utility reporting cycle.

How the Part G calculation is formed

At its core, the methodology is a demand model. Each fixture or appliance contributes a daily or weekly volume. Weekly values are converted to daily values. The whole-dwelling daily demand is divided by occupancy. The output is compared against the applicable threshold. If the result is less than or equal to target, the design is compliant from a water efficiency perspective, subject to accepted evidence and product data.

1. Define occupancy basis for the dwelling.
2. Enter WC full and reduced flush data with expected use frequency.
3. Enter shower flow and duration assumptions.
4. Enter bath volume and use frequency.
5. Enter tap flow and expected usage duration.
6. Add appliance consumption for washing machine and dishwasher.
7. Add a small allowance for other demand where applicable.
8. Total, divide by occupants, compare to target.

This calculator on the page provides a transparent engineering estimate that is useful for early-stage design optioneering. For formal submissions, always align with the exact method expected by your building control body or approved inspector and ensure model inputs are backed by acceptable documentation.

End-use split and why shower and WC data dominate outcomes

Not all inputs have equal influence. In most modern UK homes, showers and toilet flushing are major contributors, with taps and appliances following. If your calculation is close to threshold, reducing shower flow by 1 to 2 litres per minute can create meaningful headroom. Similarly, selecting efficient dual-flush WCs with robust in-use performance can materially lower overall l/p/d.

Typical indoor end use category	Indicative share of household use	Approximate litres per person per day at 140 l/p/d baseline	Design action
Showers and baths	About 30% to 40%	42 to 56 l/p/d	Specify low-flow showers, optimise bath strategy, and reduce run times.
Toilet flushing	About 20% to 30%	28 to 42 l/p/d	Use verified dual-flush products and realistic flush use assumptions.
Taps	About 10% to 20%	14 to 28 l/p/d	Control tap flow rates while preserving user comfort.
Appliances and other internal use	About 15% to 25%	21 to 35 l/p/d	Choose efficient white goods and monitor occupancy-driven behavior.

Ranges are representative of UK domestic studies and utility evidence over recent years. Actual split depends on occupancy profile, dwelling type, and product choices.

Specification strategy for developers and design teams

A robust compliance process starts before tender. Leading teams maintain a water efficiency schedule aligned with project stages. At concept stage, set a target with contingency, for example aiming for 105 to 115 when policy demands 110. At developed design stage, lock core sanitaryware and tap performance data. At technical design stage, ensure every plotted dwelling type has an auditable input set. During procurement, substitutions should trigger automatic recalculation before approval. This avoids late-stage surprises and protects programme certainty.

• Set clear performance caps: shower flow, tap flow, flush volumes, appliance limits.
• Standardise product families: fewer variants make calculations and evidence easier.
• Control substitutions: require equivalent certified performance, not nominal claims.
• Integrate with planning matrix: map each plot or block to its policy target.
• Keep an approval trail: issue calculation revision logs with product data references.

Occupancy assumptions and confidence in your numbers

One of the most misunderstood aspects is occupancy. Because the metric is per person per day, occupancy assumptions directly influence outcomes. If occupancy is understated, l/p/d may appear artificially high or low depending on how total demand is distributed. Best practice is to follow the accepted method for your submission route and ensure consistency across drawings, schedules, and worksheets. For mixed-unit schemes, run type-specific calculations rather than relying on one generic number.

Planning versus building regulations

Many teams treat Part G as a building control item only, but local planning policy can impose the optional 110 standard through planning conditions. When this happens, evidence of compliance may be requested at discharge stage, before final building control completion. Always check decision notices and condition wording. If a scheme has multiple phases or reserved matters approvals, verify whether policy updates apply to later plots.

Risk management and quality assurance checklist

1. Confirm required target per phase and plot type.
2. Issue an approved water input schedule with product references.
3. Require evidence for each fitting category before order placement.
4. Recalculate after any design development or VE proposal.
5. Record final as-built products and archive compliance pack.

Teams that use this workflow reduce rework, avoid late replacement costs, and improve audit resilience. It also helps handover teams explain to residents how efficient fittings should be used for best performance.

How to interpret calculator outputs on this page

The calculator provides total daily demand, l/p/d, and a breakdown by end-use category. Use it to test scenarios quickly:

• What happens if shower flow drops from 8 to 6 l/min?
• How sensitive is compliance to bath frequency assumptions?
• Can appliance upgrades offset a higher tap flow?
• What occupancy margin exists if household size is larger than assumed?

Scenario testing is especially valuable in early design and procurement negotiations. You can estimate which product changes create the biggest compliance gains per pound spent.

Key authoritative references

Use these sources when preparing formal compliance documentation and policy checks:

• UK Government: Approved Document G (Sanitation, hot water safety and water efficiency)
• UK Government: Water Efficiency Calculator for New Dwellings
• Ofwat: Water company performance reporting and household consumption context

Final professional takeaway

Part G water calculations are not just a compliance tick box. They are a design-control tool that affects planning discharge, procurement choices, and final handover confidence. If you manage the process proactively, achieving 125 is straightforward and achieving 110 is usually feasible with disciplined specification and data management. Use the calculator above to build a defensible water strategy early, then lock evidence quality so that what is designed is exactly what gets built and signed off.