Free to use, nothing to download. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser and autosaves. Installs as an app if you want it offline.
Is there a Part G water efficiency calculator spreadsheet?
Yes — spreadsheet versions of the water efficiency calculation have been passed around since the methodology was published in 2009, and plenty of practices keep one in a shared drive. There is nothing proprietary inside them. They hold the Table 1 use factors, one averaging rule and two adjustments at the end, and that is the entire calculation. All of it is set out below, so you can work it by hand, rebuild it in a sheet of your own, or let the browser do it.
So the useful question isn't where to find a spreadsheet — it's whether you want to own one. A sheet has to be version-controlled, protected from a stray row insert, and re-checked every time somebody copies it for the next job. The arithmetic is fixed and public either way, which is why we put it in a browser instead: same factors, same rules, live PASS/FAIL, nothing to maintain.
What a Part G spreadsheet actually contains
Less than people expect. Each fitting is scored as capacity × use factor + fixed use, the lines are added up, the total is multiplied by 0.91, and 5 litres per person per day of external use is added. The result has to come in at or below your target — normally 125 l/p/d, or 110 l/p/d where planning imposes the tighter optional standard.
The use factors are published in Table 1 of The Water Efficiency Calculator for New Dwellings, the methodology Approved Document G and Building Regulation 36 require. They never change, which is exactly why this doesn't need to be a document you keep a copy of:
| Fitting | What you enter | Use factor | Fixed use |
|---|---|---|---|
| WC, dual flush — full | Full flush volume (l) | 1.46 | 0.00 |
| WC, dual flush — part | Part flush volume (l) | 2.96 | 0.00 |
| WC, single flush | Flush volume (l) | 4.42 | 0.00 |
| Taps, excl. kitchen | Flow rate (l/min) | 1.58 | 1.58 |
| Bath, shower also present | Capacity to overflow (l) | 0.11 | 0.00 |
| Shower, bath also present | Flow rate (l/min) | 4.37 | 0.00 |
| Bath only | Capacity to overflow (l) | 0.50 | 0.00 |
| Shower only | Flow rate (l/min) | 5.60 | 0.00 |
| Kitchen / utility taps | Flow rate (l/min) | 0.44 | 10.36 |
| Washing machine | Litres per kg dry load | 2.1 | 0.00 |
| Dishwasher | Litres per place setting | 3.6 | 0.00 |
| Waste disposal unit | Fitted or not | 1 | 3.08 |
Where a machine isn't specified, defaults of 8.17 litres/kg and 1.25 litres per place setting apply. Want these constants machine-readable? They're published as open CSV/JSON. Want to see what each line is worth? Start with shower flow rate — it is usually the biggest number on the page.
A worked example, start to finish
An ordinary specification: a 6/4 dual-flush WC, 6 l/min basin taps, a 180 litre bath with a 10 l/min shower over it, an 8 l/min kitchen tap, and unspecified white goods.
| Line | Arithmetic | l/p/d |
|---|---|---|
| WC full flush | 6 × 1.46 | 8.76 |
| WC part flush | 4 × 2.96 | 11.84 |
| Basin taps | (6 × 1.58) + 1.58 | 11.06 |
| Bath | 180 × 0.11 | 19.80 |
| Shower | 10 × 4.37 | 43.70 |
| Kitchen taps | (8 × 0.44) + 10.36 | 13.88 |
| Washing machine | 8.17 × 2.1 | 17.16 |
| Dishwasher | 1.25 × 3.6 | 4.50 |
| Total | sum of the lines | 130.70 |
| Normalised | 130.70 × 0.91 | 118.9 |
| Plus external use | 118.9 + 5.0 | 123.9 |
123.9 against a 125 target — a pass, with 1.1 litres to spare. That is how tight an ordinary specification is, and it is why the number is worth checking before the sanitaryware is ordered rather than after. It also explains why a late change — a bigger bath, a pumped shower, a waste disposal unit — so often turns a pass into a fail.
Where spreadsheet calculations go wrong
- The averaging rule. Where several taps or showers of different flow rates are fitted, the methodology takes the higher of the arithmetic average and 70% of the highest-flow fitting. Done as a plain average, a high-flow master shower quietly disappears.
- Effective flush volume. For multiple WCs it is one third of the full flush plus two thirds of the part flush, not the mean of the two numbers.
- The two adjustments at the end. Forgetting the 0.91 normalisation or the 5 l/p/d external allowance moves the answer by more than most specification changes do.
- Recycled water double-counting. Rainwater and greywater can both be set against WC demand — but not the same demand twice, and neither can save more than the demand it displaces.
- Version drift. A copy of a copy, with a row inserted and a formula range left behind, is the classic way a compliant scheme turns out not to be.
None of these are hard. They are just easy to get wrong once, quietly, in a file nobody re-checks — and that file is the evidence Building Control keeps on record. A spreadsheet will happily carry the mistake forward into every job it is copied into.
What you get instead
- Live PASS/FAIL. The figure and the verdict update as you type, against 125, 110 or a custom target.
- A visible breakdown. Every line, its use factor and its contribution, so you can see where the litres actually are.
- Nothing to maintain. No download, no version, no formulas to protect. It also installs as an app and works offline.
- Dwelling types, not calculations. One type covers every identical plot; extra types cost £15 +VAT rather than a whole new fee.
- A certificate when you want one. A dated, branded, Building Control-ready PDF — £25 +VAT (£30 inc VAT), and only if you need it.
Fitting by fitting
If you want to understand a particular line before you change it:
Frequently asked questions
Is there an Excel spreadsheet for the Part G water calculation?
Spreadsheet versions of the water efficiency calculation have circulated since the methodology was published in 2009, and plenty of practices keep one. There is nothing proprietary in them: the Table 1 use factors, the multiple-fittings averaging rule, the 0.91 normalisation and the 5 l/p/d external allowance. All of it is set out in full on this page, so you can build your own sheet from it — or use the free browser calculator and skip the maintenance.
Where can I download a water efficiency calculator spreadsheet?
We do not publish one, deliberately. A downloaded sheet is a copy that ages: a stray row insert or a formula range left behind is the classic way a scheme that looked compliant turns out not to be. If you want the numbers machine-readable, the Table 1 factors and constants are published as open CSV and JSON at energycount.co.uk/data/, free to reuse with attribution. If you want the calculation done, WaterMonkey is free in the browser with live PASS/FAIL.
Do I need a spreadsheet to do a Part G water calculation?
No. The calculation is a fixed set of arithmetic defined by the official methodology, and it runs perfectly well in a browser. WaterMonkey applies the same Table 1 use factors, the same multiple-fittings rule, the same 0.91 normalisation and the same 5 l/p/d external allowance — with a live PASS/FAIL as you type instead of a formula you have to trust.
Is WaterMonkey free?
Yes. Building the fittings schedule, changing specifications and checking PASS/FAIL against 125, 110 or a custom target is free in your browser, with no sign-up. You only pay £25 +VAT (£30 inc VAT) when you export the Building Control-ready PDF, and that one fee covers every identical plot on the site.
Will Building Control accept it?
The export is a dated, branded PDF showing the fittings schedule, the calculated litres-per-person-per-day figure, the target and the PASS result — the same information a spreadsheet-based calculation presents, laid out for a compliance file. The methodology is The Water Efficiency Calculator for New Dwellings, as required by Approved Document G and Building Regulation 36.
Does it handle rainwater and greywater?
Yes — both, using the BS 8515 intermediate calculations or a direct daily figure, with savings capped against the demand they can actually displace and protection against counting the same WC demand twice across two systems. That double-count guard is one of the easiest things to get wrong by hand.
Can I work on it offline?
Yes. WaterMonkey installs like an app from your browser and then works fully offline, autosaving as you go. You only need a connection to export the paid PDF.
What about a site with several house types?
One dwelling type covers every identical plot at no extra cost, and each additional type with a different fittings spec is £15 +VAT. A 40-plot site built to one specification is still £25 +VAT.
Background reading: what Building Control is asking for, the 125 l/p/d standard, and the 110 l/p/d planning condition. Rather hand it over entirely? See our Part G service.