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
Taps are split in two. Basin and other non-kitchen taps carry a use factor of 1.58 plus a fixed 1.58 l/p/d, so every litre per minute is worth 1.44 l/p/d on the final figure. Kitchen and utility taps carry a factor of only 0.44 but a fixed 10.36 l/p/d — meaning about 9.4 l/p/d of your allowance is spent on the kitchen tap before you have specified anything at all.
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) |
|---|---|---|---|
| Taps, excluding kitchen and utility | Flow rate, litres/minute | 1.58 | 1.58 |
| Kitchen and utility sink taps | Flow rate, litres/minute | 0.44 | 10.36 |
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 per minute off the basin taps is worth 1.44 l/p/d. The same litre off the kitchen tap is worth 0.40. If the number is tight, aerators on the basin taps are worth roughly three and a half times as much as the same money spent at the kitchen sink — and the fixed 10.36 l/p/d on the kitchen line cannot be designed out at all.
Worked example
- Basin taps at 6 litres/minute: (6 × 1.58) + 1.58 = 11.06 l/p/d before normalisation.
- Fit 4 litres/minute aerated taps: (4 × 1.58) + 1.58 = 7.90 — a saving of 3.16, or 2.88 l/p/d on the submitted figure.
- Kitchen taps at 8 litres/minute: (8 × 0.44) + 10.36 = 13.88 l/p/d before normalisation.
- Drop the kitchen tap to 6 litres/minute and you save just 0.88, or 0.80 l/p/d — the fixed component dominates.
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 |
|---|---|---|
| Aerated basin tap | often 4–6 l/min | The most cost-effective single change on a marginal spec. |
| Standard basin mixer | commonly 6–9 l/min | What most schedules assume by default. |
| Kitchen mixer | commonly 8–12 l/min | Worth specifying honestly, but not worth agonising over. |
Getting it right
Where several taps of different flow rates serve the same dwelling, the same multiple-fittings rule applies as for showers: the higher of the arithmetic average and 70% of the highest-flow fitting. Two efficient cloakroom taps will not fully offset one high-flow basin mixer.
The mistake to avoid: Assuming the kitchen tap is where the savings are because it feels like the busiest tap in the house. In this calculation it is the least responsive fitting you have.
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 tap flow rates affect a Part G water calculation?
Taps are split in two. Basin and other non-kitchen taps carry a use factor of 1.58 plus a fixed 1.58 l/p/d, so every litre per minute is worth 1.44 l/p/d on the final figure. Kitchen and utility taps carry a factor of only 0.44 but a fixed 10.36 l/p/d — meaning about 9.4 l/p/d of your allowance is spent on the kitchen tap before you have specified anything at all.
How much difference does changing it actually make?
A litre per minute off the basin taps is worth 1.44 l/p/d. The same litre off the kitchen tap is worth 0.40. If the number is tight, aerators on the basin taps are worth roughly three and a half times as much as the same money spent at the kitchen sink — and the fixed 10.36 l/p/d on the kitchen line cannot be designed out at all.
Do I have to include the outside tap?
No. External water use is handled separately: a flat 5 l/p/d is added to every dwelling at the end of the calculation, regardless of what is fitted outside. That figure is fixed and there is nothing you can specify to reduce it.
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.