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 washing machine is scored on litres per kilogram of dry load at a use factor of 2.1, and a dishwasher on litres per place setting at 3.6. If you do not specify a machine, the methodology applies defaults of 8.17 l/kg and 1.25 l/place setting — together about 19.7 l/p/d on the submitted figure. Most modern appliances comfortably beat both, so declaring the real specification is free headroom where the white goods are actually being supplied.
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) |
|---|---|---|---|
| Washing machine | Litres per kg of dry load | 2.1 | 0.00 |
| Dishwasher | Litres per place setting | 3.6 | 0.00 |
| Waste disposal unit, if fitted | Present or not | 1 | 3.08 |
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 litre per kilogram off the washing machine is worth 1.91 l/p/d, and each litre per place setting off the dishwasher is worth 3.28. A waste disposal unit works the other way: fitting one adds a flat 2.80 l/p/d to the result, which is often enough on its own to turn a marginal pass into a fail.
Worked example
- Default washing machine: 8.17 × 2.1 = 17.16 l/p/d before normalisation.
- A machine specified at 6.0 l/kg: 6.0 × 2.1 = 12.60 — a saving of 4.56 before normalisation, or 4.15 l/p/d on the submitted figure.
- Default dishwasher: 1.25 × 3.6 = 4.50 before normalisation.
- A machine specified at 0.9 l/place setting: 0.9 × 3.6 = 3.24 — a further 1.26 before normalisation, or 1.15 l/p/d on the figure.
- Together that is 5.3 l/p/d, for appliances the buyer would probably have chosen anyway.
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 |
|---|---|---|
| Unspecified washing machine | 8.17 l/kg (the default) | Applied automatically when no appliance is specified. |
| Unspecified dishwasher | 1.25 l/place setting (the default) | Same — the calculation assumes it whether or not one is fitted. |
| Specified modern appliance | commonly better than both defaults | Use the manufacturer's declared figure from the product datasheet. |
Getting it right
The catch is evidence. You can only claim a specific appliance figure if that appliance is genuinely part of the specification being handed over — a fitted machine in a kitchen package, for example. On a shell handover where the buyer brings their own white goods, the defaults are the honest answer, and Building Control may reasonably ask which it is.
The mistake to avoid: Fitting a waste disposal unit late in the design and never revisiting the calculation. It is a flat 2.80 l/p/d penalty on the final figure with no offsetting benefit anywhere in the methodology.
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 washing machine & dishwasher affect a Part G water calculation?
A washing machine is scored on litres per kilogram of dry load at a use factor of 2.1, and a dishwasher on litres per place setting at 3.6. If you do not specify a machine, the methodology applies defaults of 8.17 l/kg and 1.25 l/place setting — together about 19.7 l/p/d on the submitted figure. Most modern appliances comfortably beat both, so declaring the real specification is free headroom where the white goods are actually being supplied.
How much difference does changing it actually make?
Each litre per kilogram off the washing machine is worth 1.91 l/p/d, and each litre per place setting off the dishwasher is worth 3.28. A waste disposal unit works the other way: fitting one adds a flat 2.80 l/p/d to the result, which is often enough on its own to turn a marginal pass into a fail.
What if the dwelling has no dishwasher at all?
The calculation still includes one at the default 1.25 litres per place setting. The methodology assumes a dishwasher will be installed at some point in the dwelling's life, so leaving it out of the kitchen design does not remove it from the number.
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.