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Runs in your browser. Change this fitting and watch the l/p/d figure move.
The short answer
In a dwelling with both a bath and a shower, the shower carries a use factor of 4.37, so every litre per minute of shower flow adds 3.98 l/p/d to the final figure. A 10 l/min shower is contributing about 40 l/p/d on its own — roughly a third of the 125 l/p/d allowance. In a shower-only dwelling the factor rises to 5.60, or 5.10 l/p/d per litre per minute.
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) |
|---|---|---|---|
| Shower (bath also present) | Flow rate, litres/minute | 4.37 | 0.00 |
| Shower only (no bath) | Flow rate, litres/minute | 5.60 | 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
Each 1 litre/minute you take off the shower is worth 3.98 l/p/d on the result where there is also a bath, or 5.10 l/p/d in a shower-only dwelling. Nothing else in the calculation responds that strongly to a specification change, which is why the shower is almost always the first place to look when a dwelling fails.
Worked example
- A 10 litres/minute shower in a bathroom that also has a bath: 10 × 4.37 = 43.70 l/p/d before normalisation.
- Specify 8 litres/minute instead: 8 × 4.37 = 34.96 l/p/d — a saving of 8.74 before normalisation.
- The whole total is then multiplied by 0.91, so that change is worth 8.74 × 0.91 = 7.95 l/p/d on the figure you submit.
- On a specification sitting at 123.9 l/p/d — a pass, but with barely any headroom — that one change takes the dwelling to about 116 and puts a 110 l/p/d condition within reach.
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 |
|---|---|---|
| Electric shower | often 6–8 l/min | Flow is limited by the heating element, so these are usually the easiest to pass with. |
| Mixer shower on a combi | commonly 8–12 l/min | The middle of the range, and where most specifications land. |
| Pumped or thermostatic shower | can exceed 12–15 l/min | The usual culprit when a dwelling fails; a flow regulator is often the cheapest fix. |
| Aerated or flow-regulated head | typically 6–9 l/min | Same experience, lower measured flow — check the manufacturer's figure at your pressure. |
Getting it right
Use the manufacturer's stated flow rate at the working pressure your design will actually deliver — a head quoted at 3 bar behaves very differently on a gravity system. Where more than one shower is fitted, the methodology takes the higher of the arithmetic average and 70% of the highest-flow fitting, so a single high-flow master shower cannot be averaged away by a modest en-suite.
The mistake to avoid: Quoting a flow rate from the shower head alone when a pump or a high-pressure supply will drive it harder. Building Control is entitled to the figure that matches the installed system, and a shower is the one line where an optimistic number changes the answer.
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 shower flow rate affect a Part G water calculation?
In a dwelling with both a bath and a shower, the shower carries a use factor of 4.37, so every litre per minute of shower flow adds 3.98 l/p/d to the final figure. A 10 l/min shower is contributing about 40 l/p/d on its own — roughly a third of the 125 l/p/d allowance. In a shower-only dwelling the factor rises to 5.60, or 5.10 l/p/d per litre per minute.
How much difference does changing it actually make?
Each 1 litre/minute you take off the shower is worth 3.98 l/p/d on the result where there is also a bath, or 5.10 l/p/d in a shower-only dwelling. Nothing else in the calculation responds that strongly to a specification change, which is why the shower is almost always the first place to look when a dwelling fails.
Does fitting a flow restrictor count for the Part G calculation?
Yes, provided the restricted flow rate is the specification being installed and you can evidence it — a manufacturer's figure for the head-and-regulator combination, or a stated flow-regulated valve. What you cannot do is claim a restricted figure for an unrestricted fitting.
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.