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Runs in your browser. Change this fitting and watch the l/p/d figure move.
The short answer
Where a shower is also present, the bath carries a use factor of only 0.11, so a 180 litre bath contributes 19.8 l/p/d before normalisation and every 10 litres of capacity is worth about 1 l/p/d. Take the shower away and the factor jumps to 0.50 — the same bath then contributes 90 l/p/d, and a bath-only dwelling becomes very difficult to pass.
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) |
|---|---|---|---|
| Bath (shower also present) | Capacity to overflow, litres | 0.11 | 0.00 |
| Bath only (no shower) | Capacity to overflow, litres | 0.50 | 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
With a shower present, 10 litres of bath capacity is worth 1.00 l/p/d — so specifying a smaller bath is rarely the answer to a failing calculation. Without a shower, the same 10 litres is worth 4.55 l/p/d, and the bath alone can account for more than half the allowance.
Worked example
- A 180 litre bath in a bathroom that also has a shower: 180 × 0.11 = 19.80 l/p/d before normalisation, or 18.02 after.
- A 160 litre bath instead: 160 × 0.11 = 17.60 — a saving of just 2.00 l/p/d on the submitted figure.
- The same 180 litre bath in a dwelling with no shower at all: 180 × 0.50 = 90.00 before normalisation, 81.90 after.
- That is nearly two thirds of a 125 l/p/d allowance spent on one fitting — which is why adding a shower over the bath is usually the single most effective thing you can do to such a dwelling.
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 |
|---|---|---|
| Compact / space-saver bath | around 140–160 litres to overflow | Useful, but the saving is small when a shower is present. |
| Standard 1700mm bath | commonly 170–190 litres to overflow | The usual assumption where no figure is given. |
| Freestanding or double-ended bath | often 200 litres or more | Worth checking, particularly on a tight 110 l/p/d target. |
Getting it right
The figure the methodology wants is capacity to overflow, measured with no one in the bath — not the volume of water someone would actually run. Manufacturers publish it, and it is usually noticeably larger than the "usable capacity" quoted in marketing.
The mistake to avoid: Using the marketing "capacity" figure rather than capacity to overflow, and forgetting that adding a shower over the bath moves the bath onto the much kinder 0.11 factor. On a dwelling designed with a bath and no shower, that one change is transformative.
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 bath capacity affect a Part G water calculation?
Where a shower is also present, the bath carries a use factor of only 0.11, so a 180 litre bath contributes 19.8 l/p/d before normalisation and every 10 litres of capacity is worth about 1 l/p/d. Take the shower away and the factor jumps to 0.50 — the same bath then contributes 90 l/p/d, and a bath-only dwelling becomes very difficult to pass.
How much difference does changing it actually make?
With a shower present, 10 litres of bath capacity is worth 1.00 l/p/d — so specifying a smaller bath is rarely the answer to a failing calculation. Without a shower, the same 10 litres is worth 4.55 l/p/d, and the bath alone can account for more than half the allowance.
Is it better to leave the bath out of the design entirely?
On the numbers, a shower-only dwelling is easier to pass — but the shower then moves onto the higher 5.60 factor, so it is not a free win, and most house types need a bath to be saleable. A bath plus a shower is usually both the more marketable and the more forgiving combination.
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.