Water Efficiency Calculator 

A water efficiency calculator is used to estimate how much water a new dwelling is expected to use, usually expressed in litres per person per day. For Building Regulations purposes, the calculation looks at the water-using fittings specified for the property, such as WCs, taps, showers, baths, dishwashers and washing machines. The result is then checked against the relevant Part G water efficiency target.

The calculation is not a water meter reading and it is not based on how a future occupant might personally behave. It is a design-stage assessment based on the fittings and fixed appliances that are being installed. That makes it useful before completion, because a developer, self-builder, architect or contractor can see whether the specification is likely to meet the required standard before Building Control asks for evidence.

A Part G water efficiency calculator is most commonly used for new-build houses, new flats and some conversions where a new dwelling is being created. The current GOV.UK Approved Document G page covers sanitation, hot water safety and water efficiency for buildings in England, and is the best official starting point for checking the wider guidance: GOV.UK Approved Document G.

In practical terms, the calculator helps turn a list of fittings into a clear pass/fail-style result. A good report should show the inputs, the calculated litres-per-person-per-day figure, the target used and the assumptions behind the result.

You usually need a Part G water efficiency calculation where the work creates a new dwelling. That includes many new-build houses, new flats, and some conversions or material change-of-use projects where a building or part of a building becomes residential accommodation. Building Control may ask for the calculation as evidence that the dwelling meets the water efficiency requirement.

You do not normally need a whole-dwelling Part G water calculation for every piece of plumbing work. A bathroom refit, replacement kitchen, ordinary extension or refurbishment to an existing home may still need to comply with relevant plumbing, hot water safety and sanitary requirements, but it will not always require a new whole-house water efficiency calculation. The key question is whether a new dwelling is being created or whether Building Control has specifically requested the calculation.

The safest approach is to check the Building Control application, approval notes and any planning conditions. If the documents mention water efficiency, Part G, 125 litres per person per day, 110 litres per person per day, or a requirement to provide a water calculation, then you should prepare one before sign-off is needed.

A Part G water calculation is also useful earlier in the design process. If the result is too high, you still have time to change the taps, showers, WCs or appliances before they are ordered and installed. Leaving it until completion can create avoidable delays if the selected fittings do not meet the required target.

Before using an online Part G water calculator, gather the details of the fittings and appliances that will be installed in the dwelling. The calculation is only as reliable as the information entered, so it is better to use manufacturer data, product schedules or specification sheets rather than rough guesses.

For WCs, you will usually need the flush volume. For dual-flush WCs, record both the full and reduced flush where available. For basin taps, kitchen taps and showers, you will normally need flow rates, often shown in litres per minute. For baths, you may need the capacity to overflow, sometimes adjusted to reflect normal usable volume. For dishwashers and washing machines, product data may show water consumption per cycle or per place setting, depending on the appliance type.

You should also know whether the property includes water softeners, waste disposal units, rainwater harvesting or greywater reuse. These can affect how the calculation is completed, although more complex reuse systems may need separate design information rather than a simple online input.

It also helps to have the project basics ready: plot number, address, dwelling type, number of fittings, and which target you are checking against. If the planning consent includes a 110 litres-per-person-per-day condition, use that target. If not, the standard 125 litres-per-person-per-day target is more commonly used.

Do the calculation against the actual intended specification. If fittings change later, the calculation should be updated so the certificate or report matches what is being installed.

The standard Part G water efficiency target for new dwellings is commonly 125 litres per person per day. In some areas, a tighter optional requirement of 110 litres per person per day may apply. The lower target is usually triggered through planning policy or a planning condition, often in areas where water stress is a concern.

The practical way to check is to read the planning decision notice and any attached conditions. Look for wording such as “water efficiency”, “Part G”, “optional requirement”, “110 litres per person per day” or “110 l/p/d”. The condition may also refer to evidence being submitted before occupation or prior to completion. If a 110 target is listed, your Part G water efficiency calculator should be run against that lower figure, not just the standard 125 figure.

If there is no planning condition and Building Control has not asked for the optional requirement, the standard target is more likely to apply. However, it is worth checking early because the difference can affect the fittings you choose. A specification that passes at 125 litres per person per day may fail at 110, especially if it includes high-flow showers, large baths or higher-flush WCs.

GOV.UK’s Approved Document G guidance is the official reference point for Part G water efficiency, but the target for an individual project often depends on the planning approval as well as the Building Regulations: GOV.UK Approved Document G.

A Part G water efficiency calculator looks at the main fittings and fixed appliances that affect day-to-day water use in a new dwelling. These usually include WCs, bathroom taps, kitchen taps, showers, baths, dishwashers and washing machines. The calculator uses the performance of those fittings to estimate the overall water consumption for the dwelling.

The important point is that the calculation is based on the whole specification, not one product in isolation. A very efficient basin tap will not necessarily compensate for a high-flow shower, a large bath and older-style WC flush volumes. Similarly, a dwelling can sometimes pass without every fitting being the lowest possible flow rate, provided the overall combination stays within the target.

Flow rates and capacities should come from reliable sources wherever possible. Product data sheets, sanitaryware schedules, manufacturer websites and plumbing specifications are all useful. If the exact model has not yet been chosen, use realistic provisional figures and update the calculation once the final products are confirmed.

Rainwater harvesting and greywater reuse may also be relevant, but they should be treated carefully. A simple online Part G water calculator may allow an allowance for these systems, but the system itself may need proper design information to show that it will work as intended. It is better not to rely on a notional water reuse system just to make a calculation pass if there is no firm design behind it.

The finished report should make it clear which fittings were included, what figures were used and how the final litres-per-person-per-day result was reached.

People often use the terms “Part G water efficiency certificate”, “water efficiency certificate” and “water efficiency calculation” to mean the same thing, but there is a useful distinction. The calculation is the actual assessment of the dwelling’s predicted water use. The certificate or report is the document that records the result and provides evidence for Building Control, the client, the developer or the project file.

A good Part G water efficiency certificate should not simply say “pass”. It should show the project details, plot or dwelling reference, the target used, the final litres-per-person-per-day figure, the fittings included and the assumptions behind the calculation. That makes the document easier to check and easier to update if the specification changes.

It is also worth understanding that a water efficiency certificate is not normally a government-issued certificate. In most cases, it is a professional record produced from the calculation and submitted as supporting evidence. Building Control will be interested in whether the calculation is suitable, clear and based on the correct information.

This is why the quality of the inputs matters. If the wrong shower flow rate or WC flush volume is entered, the certificate may look tidy but still be inaccurate. The certificate should reflect the actual design or installed fittings, not a best-case version of the specification.

For that reason, keep the report with the rest of the Building Regulations evidence and reissue it if fittings are changed before completion.

Yes, many straightforward Part G water calculations can be completed online without using a consultant, provided you know the fittings being installed and the project does not involve unusual water systems. An online Part G water calculator can be a practical option for self-builders, small developers, architects, contractors and plumbers who need a clear calculation report for a new dwelling.

The benefit of completing a water efficiency certificate online is structure. Instead of building a spreadsheet from scratch, the calculator prompts you for the relevant inputs and turns the fitting data into a litres-per-person-per-day result. This can reduce simple mistakes, especially where several fittings need to be checked together.

However, an online Part G water calculation service still depends on accurate information. You need the correct flow rates, flush volumes, bath capacities and appliance data. If the figures are guessed, copied from the wrong model or changed later on site, the certificate may need to be updated.

A consultant may still be useful where the project is more complex. Examples include large developments with varied specifications, unusual water reuse systems, unclear planning conditions, disagreement with Building Control, or a design that is struggling to meet the target without more detailed advice.

For a simple new dwelling with standard fittings, an online calculation is often enough to produce a clear report. The key is to treat it as compliance evidence, not just admin. Check the target, enter the real specification, keep the report and update it if anything changes.

If a Part G water calculation fails, it means the predicted water use is above the target you selected. That does not necessarily mean the whole design is wrong. It usually means one or more fittings are using too much water for the dwelling to meet the required litres-per-person-per-day figure.

Start by checking the inputs. Mistakes are common: a shower flow rate may have been entered too high, a bath capacity may be based on total capacity rather than the correct usable figure, or a WC may have been entered as single-flush when it is actually dual-flush. Check the product data and make sure every fitting has been entered consistently.

If the calculation is still over the limit, look at the fittings with the biggest effect. Showers, baths and WCs often make a noticeable difference. Lower-flow showers, reduced-flow taps, dual-flush WCs, smaller bath capacities and more efficient appliances can all help. The right change depends on the design; the aim is to reduce the overall figure without specifying fittings that will be unsuitable for users.

Be careful with quick fixes. Very low-flow fittings may look good in a calculation but perform poorly if they are not matched to the plumbing system or user expectations. It is better to choose realistic, available products that can actually be installed.

Once the specification changes, rerun the calculation and reissue the water efficiency certificate or report. The final document should match the fittings that will be installed, not an earlier failed version.

A Part G water calculation is normally carried out for each dwelling, because the result depends on the fittings and appliances in that specific home. On a development with several identical plots, the same specification may produce the same result, but each dwelling should still be clearly identified in the project records.

For example, if ten flats all use the same bathroom layout, kitchen tap, shower, WC, bath and appliance specification, the underlying calculation may be identical. In that situation, it is sensible to use the same fitting data across those units, but the certificate or report should still make clear which plot, flat or dwelling it relates to. This avoids confusion at Building Control sign-off and helps the project team track which homes have been covered.

If the plots are not identical, do not rely on one calculation for all of them. A flat with only a shower may calculate differently from a house with a bath and separate shower. A change in WC type, bath capacity, shower flow rate or appliance specification can change the litres-per-person-per-day result.

Multi-plot schemes are also where version control matters. Developers often swap fittings during procurement, and a small change across many plots can affect compliance. Keep a record of the specification used for each plot, and update the relevant calculations if products change.

The safest rule is simple: one dwelling should have one clear record. Where several dwellings share the same design, you can work efficiently from the same data, but the evidence should still be traceable.

A water efficiency calculation should be updated whenever the information behind it changes. The certificate or report is only useful if it reflects the fittings and appliances that are actually specified or installed. If the design changes but the report does not, Building Control may be looking at evidence that no longer matches the dwelling.

Common reasons to update a Part G water efficiency certificate include changing the shower model, choosing a different bath, swapping WCs, altering tap specifications, adding or removing a dishwasher or washing machine, or changing a rainwater or greywater proposal. Even small changes can matter if the original calculation was close to the target.

It is usually best to run the first calculation during design or specification, before products are ordered. That gives the project team time to fix any issues. The calculation can then be checked again before completion, especially if substitutions were made on site.

The current GOV.UK page for the water efficiency calculator explains that the updated water efficiency methodology is now included in Approved Document G, so the calculation should be treated as part of the wider Part G compliance evidence rather than a separate box-ticking exercise: GOV.UK water efficiency calculator for new dwellings.

Keep the latest report with the Building Regulations file, product schedules and any Building Control correspondence. If there are multiple versions, make sure the final issued certificate is the one that reflects the completed dwelling.