What a water efficiency calculation is
A water efficiency calculation works out the potential consumption of a new dwelling from the fittings you specify — taps, showers, baths, WCs, white goods — and compares it with the target set by the Building Regulations. It is a fixed methodology, not an opinion: every fitting is scored against a published use factor, the lines are summed, and two adjustments are applied at the end.
It is required by Regulation 36 and guided by Approved Document G, and it is the evidence Building Control keeps on file for the dwelling. It has nothing to do with metered usage after handover — it is a design-stage calculation on the specification.
What Building Control asks for
In practice, a Building Control body wants three things, and rejections are almost always a missing one rather than a wrong number:
- The calculation itself, showing each fitting, its capacity or flow rate, the use factor applied and its contribution in litres per person per day.
- The fittings schedule it was based on — specific enough that what arrives on site can be checked against it.
- A stated result against a stated target, dated, with the dwelling or dwelling type it applies to.
What we send is all three in one document, laid out the way Building Control expects to read it. Where a scheme has several house types, each type gets its own calculation and its own result.
How a Part G water calculation works
Each fitting is scored as capacity × use factor + fixed use, with the use factors from Table 1 of the methodology: a WC by its flush volumes, taps and showers by their flow rate in litres a minute, a bath by its capacity to overflow, white goods by their rated consumption or a default where they are not specified. The lines are added up, the total is multiplied by the 0.91 normalisation factor, and 5 litres per person per day of external use is added at the end. Where several showers or taps of different flow rates are fitted, the methodology takes the higher of their average and 70% of the highest-flow fitting, so one powerful master shower cannot be averaged away. An ordinary specification lands within a litre or two of 125, which is why a late change to a bath or a shower so often turns a pass into a fail. The full arithmetic, fitting by fitting, is in the water calculation guide and the no-spreadsheet walkthrough.
When it is due
Regulation 37 sets the backstop — notice of the calculated consumption to the building control authority no later than five days after the work is completed (the building control authority, not the local authority, since 1 October 2023) — but almost every Building Control body asks for the calculation with the plans or at first fix, and the sensible time to run it is before the sanitaryware order goes in, while a shower head is still a decision. We turn it round in a working day or two, so it is rarely the item that holds up the plans check.
The targets — and where they apply
| Where | Target for a new dwelling | Calculation needed? |
|---|---|---|
| England | 125 l/p/d, or 110 l/p/d where planning imposes the optional requirement | Yes |
| Wales | 110 l/p/d (125 for a material change of use) | Yes |
| Scotland | No consumption target — Standard 3.27 sets fitting limits instead | No |
Two things worth knowing. Wales is tighter than England by default: since 2018 the mandatory figure for a newly erected dwelling has been 110 l/p/d, and specifications drawn up for an English site will not always pass there. Scotland does not use this calculation at all — Standard 3.27 of the Technical Handbook sets performance limits on the fittings themselves (dual and single flush WC cisterns to an average of 4.5 litres, basin taps to 6 litres a minute) rather than a whole-dwelling figure. Northern Ireland works to its own Technical Booklets; ask us if you have a scheme there.
When it gets tricky
125 l/p/d is comfortable with modern sanitaryware — but large baths and powerful showers can push you over, and 110 leaves very little room. The fix is offsetting: low-flow fittings elsewhere, dual-flush WCs, or alternative water sources such as rainwater or greywater harvesting. We’ll tell you the cheapest combination that works for your spec rather than asking you to downgrade the master shower.
The single most common late failure is a specification change after the calculation was signed off — a bigger bath, a pumped shower, a waste disposal unit added at fit-out. An ordinary compliant spec often lands within a litre or two of the target, so it does not take much.
Prefer to run the numbers yourself?
Our free in-browser calculator, WaterMonkey, runs the full official methodology with live PASS/FAIL at 125 or 110 l/p/d — model your fittings for free, and the Building Control-ready PDF is just £25 +VAT when you need it.
Open the free Part G calculator →Do it yourself, or hand it to us
For a straightforward house type on mains water, WaterMonkey — our free Part G water calculator — is all most people need: the same methodology, live PASS/FAIL, and a Building Control-ready PDF for £25 +VAT that covers every identical plot. We do the calculation ourselves where that is the better use of your time or the safer bet — several house types on one scheme, a planning condition at 110 (or London’s 105 excluding external use) where the fittings have to be found rather than checked, rainwater harvesting or greywater reuse to be credited, a site in Wales, a calculation being run alongside the SAP so the shower flow rates match, or simply a Building Control body that wants a consultant’s name on it. Either way it is the same arithmetic; what you are buying from us is the schedule that passes and the report that is not sent back.
What we need, and what you get back
Send the sanitaryware schedule if you have one. If the fittings aren’t picked yet, drawings are enough — we will tell you what specification hits the target, which is far cheaper than finding out after the order has gone in.
Back comes the calculation and the compliance report for each dwelling type, ready to go straight to Building Control, normally within a working day or two. If a type fails, you get the options to fix it in the same email rather than a bill for a second run.
Beyond Building Control
We also produce water calculations for planning and other purposes — legacy CSH level 4, the G2 optional requirement, and regulation 36 notices. Since SAP 10 now uses shower flow rates too, it pays to have one assessor doing both calculations with consistent fittings data — see our Compliance Pack.
Common questions
Is a water efficiency calculation a legal requirement?
Yes, for a new dwelling. Regulation 36 of the Building Regulations requires the potential consumption of wholesome water to be within the target, and Building Control will ask for the calculation as evidence before signing off.
What is the difference between 125 and 110 litres per person per day?
125 l/p/d is the standard requirement in England. 110 l/p/d is the tighter optional requirement, which a local planning authority can impose by condition in a water-stressed area — and which is the default mandatory figure in Wales. The methodology is identical; only the target moves. See the 110 l/p/d planning condition guide.
Do I need a separate calculation for every plot?
No — one calculation covers every plot built to the same fittings specification. You need one per *dwelling type*, not per plot, so a forty-plot scheme built to two specifications needs two calculations.
Can you do the calculation before the fittings are chosen?
Yes, and it is usually the better order. We can work backwards from the target and tell you the flow rates and capacities that will pass, so the schedule is written to something that already complies.
What happens if the dwelling fails?
Nothing dramatic — it is a design-stage calculation, so it is fixed on paper. We show which lines are costing the most litres and what the cheapest change is. Shower flow rate is almost always the biggest single number.
How much do Part G water calculations cost?
It depends on how many dwelling types there are, not how many plots — one calculation covers every plot built to the same specification. Send the schedule or the drawings and we will quote the same day; a single house type is a small job and is priced accordingly. If you would rather run it yourself, WaterMonkey is free to use and its Building Control-ready PDF is £25 +VAT.
Is a water calculation the same as a water efficiency calculation?
Yes. Building Control bodies, planners and the regulations use both names for the same thing: the potential-consumption calculation required by regulation 36 and set out in the Water Efficiency Calculator for New Dwellings methodology. You will also see it called a Part G calculation or a G2 calculation, after the part of the Building Regulations and the requirement it comes from.