Free to calculate. Building Control-ready PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Change this fitting and watch the l/p/d figure move.
The short answer
A dual-flush WC is scored as two separate lines: the full flush at a use factor of 1.46 and the part flush at 2.96. That is the whole trick of the WC — the methodology assumes two part flushes for every full one, so a litre off the part flush is worth 2.69 l/p/d while a litre off the full flush is worth only 1.33. A single-flush WC carries the whole 4.42 factor on one volume.
What the methodology does with it
Consumption for each fitting is capacity × use factor + fixed use. These are the Table 1 figures that apply here, from The Water Efficiency Calculator for New Dwellings — the methodology Approved Document G and Building Regulation 36 require:
| Fitting line | What you enter | Use factor | Fixed use (l/p/d) |
|---|---|---|---|
| WC, dual flush — full flush | Full flush volume, litres | 1.46 | 0.00 |
| WC, dual flush — part flush | Part flush volume, litres | 2.96 | 0.00 |
| WC, single flush | Flush volume, litres | 4.42 | 0.00 |
Every fitting line is added together, the total is multiplied by the 0.91 normalisation factor, and a fixed 5 l/p/d of external water use is added. That final number is the one that has to come in at or below your target.
What one unit is worth
A litre off the part flush moves the final figure by 2.69 l/p/d. A litre off the full flush moves it by 1.33. And switching a 6 litre single-flush WC to a 6/4 dual saves 5.4 l/p/d for no design compromise at all — which makes it the cheapest compliance decision in the whole calculation.
Worked example
- A 6/4 dual-flush WC: (6 × 1.46) + (4 × 2.96) = 8.76 + 11.84 = 20.60 l/p/d before normalisation.
- The same cistern as a 6 litre single flush: 6 × 4.42 = 26.52 — nearly 6 litres/person/day worse.
- Specify a 4.5/3 dual flush instead: (4.5 × 1.46) + (3 × 2.96) = 6.57 + 8.88 = 15.45.
- Against the 6/4, that is 5.15 before normalisation, or 4.69 l/p/d on the submitted figure.
What you will meet in practice
Indicative ranges only — always use the manufacturer's declared figure for the product you are actually specifying, at the pressure your system will deliver.
| Specification | Typical figure | Why it matters |
|---|---|---|
| Older single-flush cistern | 6 litres | Scored on the full 4.42 factor — avoid in a new dwelling if the number is tight. |
| Standard dual flush | 6 / 4 litres | The default assumption in most specifications. |
| Water-efficient dual flush | 4.5 / 3 or 4 / 2.6 litres | Widely available, and the part flush is where the saving lands. |
Getting it right
Where several WCs are fitted with different volumes, the calculation uses the average effective flushing volume across them — effective flush being one third of the full volume plus two thirds of the part volume. So an efficient cloakroom WC genuinely does pull the average down, unlike showers, where the highest-flow fitting dominates.
The mistake to avoid: Entering the cistern's nominal capacity rather than its actual flush volumes, and entering a dual-flush WC as a single flush "to be safe". The second one is not safe — it costs you around 6 l/p/d and it is not what you are installing.
Free to calculate — pay only for the certificate
WaterMonkey is free to use: build the fittings schedule, change this fitting and watch the result move, and check against 125 l/p/d or the tighter 110 l/p/d target. You pay £25 +VAT (£30 inc VAT), once, for the Building Control-ready PDF — and one fee covers every identical plot on the site.
Frequently asked questions
How does wc flush volume affect a Part G water calculation?
A dual-flush WC is scored as two separate lines: the full flush at a use factor of 1.46 and the part flush at 2.96. That is the whole trick of the WC — the methodology assumes two part flushes for every full one, so a litre off the part flush is worth 2.69 l/p/d while a litre off the full flush is worth only 1.33. A single-flush WC carries the whole 4.42 factor on one volume.
How much difference does changing it actually make?
A litre off the part flush moves the final figure by 2.69 l/p/d. A litre off the full flush moves it by 1.33. And switching a 6 litre single-flush WC to a 6/4 dual saves 5.4 l/p/d for no design compromise at all — which makes it the cheapest compliance decision in the whole calculation.
Why does the part flush have a bigger use factor than the full flush?
Because the methodology assumes people use the part flush about twice as often as the full flush. The two factors, 1.46 and 2.96, are the full and part shares of the same 4.42 uses per person per day. It is why specifying a lower part flush is the most efficient change you can make to a WC.
Is the calculator free?
Yes — building your fittings schedule and checking PASS/FAIL against 125 or 110 l/p/d is free in the browser. 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.
The other fittings
Every line in the calculation behaves differently. Compare another:
Or back to the main Part G water calculator, read what Building Control is actually asking for, or do the whole thing without a spreadsheet.