Part G Water Efficiency Calculator

Have the fittings schedule and the manufacturers’ product data: WC flush volumes, bath capacity to overflow, shower and tap flow rates, and the white goods figures from their energy labels. Settle the target first, under Target & route in the toolbar, because everything else is measured against it.

It is not one figure but four, and the tool asks which applies. In England, 125 litres per person per day under regulation 36(2)(a) for every new dwelling, or 110 under 36(2)(b) where planning permission imposes the optional requirement — and where it does, you must tell the building control body. Wales is not England and is not the same way round: the Welsh regulation was substituted by the Building (Amendment) (Wales) Regulations 2018 and has read, since 1 November 2018, 110 for a dwelling erected and 125 for one formed by a material change of use.

A dwelling is rarely held to the regulation alone, so a second target sits beside it and both are answered. Three are built in. The London Plan 2021, Policy SI 5, asks for mains water consumption of 105 litres or less per head per day excluding an allowance of up to five litres for external use — which matters, because it is measured at line (9) of Appendix A, after the normalisation factor and before the external allowance is added, and the tool measures it there. The other two are the 105 and 100 litre figures consulted on between September and December 2025; they are not in force and are labelled as proposals, but they are worth designing against.

Choose "A figure of your own" for anything else: a planning condition, a section 106 agreement, a client brief, an assessment scheme, or a legacy Code for Sustainable Homes level still cited in an old agreement. Give it the figure, a name if you want one on the report, and — this is the part worth reading the condition for — whether it includes the five litre external use allowance. Most do. The London Plan is the common exception, and getting it the wrong way round moves the answer by five litres.

Regulation 36 can be met either through the water efficiency calculator in Appendix A or through the fittings approach in section 2, where every fitting simply sits at or below the maximum in Table 2.1 or Table 2.2 and no calculation is needed at all. Both are answered from the same schedule and both verdicts sit beside the result, because a schedule that passes the fittings approach needs no further arithmetic. Paragraph 2.6 closes that route where no shower is provided, and where a waste disposal unit, water softener or water re-use system is installed. Both tables belong to the England approved document, so the route is not offered against a Welsh requirement or a proposed figure either. The tool says which of those applies and why.

A dual-flush WC contributes through two separate rows of Table A1, with different use factors: 1.46 on the full flush and 2.96 on the part flush, because the part flush is assumed to be used about two thirds of the time. The effective flushing volume shown under each WC is (full × 0.33) + (part × 0.67), so a 6/4 litre WC has an effective volume of 4.66 litres. Add another WC specification where the dwelling has more than one: the tool then uses the average effective flushing volume, because paragraphs A8(e) and A8(f) exclude WCs from the 70%-of-the-highest rule that applies to everything else.

Whether a bath and a shower are both present changes which rows apply, and the use factors differ a lot: a bath with a shower present is 0.11 against 0.50 for a bath alone, and a shower with a bath present is 4.37 against 5.60 for a shower alone. Bath capacity is to overflow — do not subtract for displacement, because the use factor already allows for it. Bath taps are excluded entirely, since their consumption sits inside the use factor for baths.

Where a dwelling has more than one specification of the same fitting, Add another lists them with quantities and paragraphs A8 to A10 do the rest: the average is the total divided by the quantity, and the greater of that average and 70% of the highest governs. That second step exists so a single ultra-low fitting cannot drag an average down, and the tool says when it is the one biting. WCs are the exception and their average stands.

Washing machine consumption is litres per kilogram of dry load, from the energy label: annual water use divided by 220. Dishwasher consumption is litres per place setting: annual water use divided by 280. Where no dishwasher is provided, Appendix A still requires 1.25 litres per place setting to be assumed, and the tool does that automatically. Only water softener consumption beyond the 4% good-practice benchmark counts, and both a softener and a waste disposal unit close the fittings approach.

Greywater and rainwater contributions come from Tables 4.6 and 5.5 of Approved Document G. They are deducted before the 0.91 normalisation factor is applied, which is why a litre saved here is worth slightly less than a litre saved at a fitting. Using either also rules the fittings approach out.

Both routes are shown side by side, then the change that would move the total most. That last one is worth reading: use factors differ by an order of magnitude between rows, so the largest contributor is rarely the fitting people expect, and the chart puts every fitting on the same scale. Below it sit the fittings-approach table and the whole Table A1 build-up, both of which the PDF reproduces along with the line-by-line arithmetic from total calculated use to the final figure.

Flow rate tools in the toolbar, or Measure beside any flow-rate field, works out any one of volume, time and flow rate from the other two. The usual reason is the measuring-jug method: use a container of known volume, open the outlet fully and time how long it takes to fill. Run it for a few seconds first so the flow settles, and for a mixer tap measure with both hot and cold fully open. The result is converted to every common unit, checked against the Part G limit for each fitting, and can be sent straight to the field you opened it from. A fill-time box answers the other question a flow rate raises — how long a bath, cylinder or cistern takes.

Approved Document G wants flow rates from manufacturers’ product data, measured at a dynamic pressure of 3 bar for a high-pressure supply or 0.1 bar for a low-pressure one, and for click taps the full flow rate rather than the flow at the break point. A measurement taken on site tells you what that installation delivers today at whatever pressure was available. It is evidence about the installation as built, not a substitute for the rated figure.

The PDF needs your name and company under Project details, because it records who produced it. It carries the schedule, both routes, the whole Table A1 working, the drawn contributions and any flow rate you measured. It is a design-stage calculation on the fittings you specify, and not the notice required under regulation 37, which must state the calculated consumption for the dwelling as constructed and normally reaches the building control body within five days of completion. Save inputs writes a .briarywater file that reopens the whole workspace.

Check the units on every figure: litres for flush volumes and bath capacity, litres per minute for taps and showers, litres per kilogram and litres per place setting for the white goods. Appendix A is explicit that this methodology is not a design tool for water supply and drainage systems and cannot calculate a dwelling’s actual consumption. For problems, email toolsuite@briary.co.uk with the browser and what you entered.