Part G calculator methodology
The calculator follows the national water efficiency calculation methodology that Approved Document G uses to set the limits for new dwellings. This page sets out every step, every factor and how the calculator has been checked.
The requirement
Regulation 36 of the Building Regulations limits the potential water consumption of new dwellings in England to 125 litres per person per day. Where a planning condition requires it, the tighter optional requirement of 110 litres applies instead. Both figures include a fixed 5 litres per person per day for outdoor use (Approved Document G, paragraph 2.3).
Step 1: each fitting
Each fitting's figure from the manufacturer is multiplied by a use factor, and in some cases a fixed amount is added. This gives litres per person per day for that fitting (methodology source).
| Fitting | Input | Use factor | Fixed amount |
|---|---|---|---|
| WC, single flush | Flush volume (litres) | 4.42 | |
| WC, dual flush | Full and part flush (litres) | Full 1.46, part 2.96 | |
| Basin taps | Flow rate (l/min) | 1.58 | 1.58 |
| Bath, with a shower also present | Capacity to overflow (litres) | 0.11 | |
| Bath, no shower | Capacity to overflow (litres) | 0.50 | |
| Shower, with a bath also present | Flow rate (l/min) | 4.37 | |
| Shower, no bath | Flow rate (l/min) | 5.60 | |
| Kitchen and utility sink taps | Flow rate (l/min) | 0.44 | 10.36 |
| Washing machine | Litres per kg of dry load | 2.1 | |
| Dishwasher | Litres per place setting | 3.6 | |
| Waste disposal unit | Fitted or not | 3.08 | |
| Water softener | Litres per person per day (manufacturer) | 1.0 |
Where appliances aren't being supplied, the calculator uses the default figures from Approved Document G's fittings tables: 8.17 litres per kg for a washing machine and 1.25 litres per place setting for a dishwasher.
Choosing a washing machine or dishwasher model
The methodology says the washing machine figure is litres per kg of dry load, and the dishwasher figure is litres per place setting, both "derived from the figure quoted on the EU Energy Label" (The Water Efficiency Calculator for new dwellings, WRC guidance). The current energy label gives water use per cycle for the eco programme, and the rated capacity. So the calculator works out:
Washing machine (l/kg) = litres per cycle (eco 40-60) / rated capacity (kg) Dishwasher (l/place setting) = litres per cycle (eco) / place settings
Results are rounded up to two decimal places, so they never flatter the result. A typical 8kg integrated washing machine at 45 litres a cycle gives 5.63 l/kg, against the 8.17 default. That alone takes about 4.9 litres per person per day off the result.
The figures below come from retailer and manufacturer listings of each model's energy label data, checked 29 October 2026. Models get updated, so check the label of the exact model being supplied, and keep it with the calculation for building control. If a model isn't listed, choose "Enter figures from the energy label" and type in the two numbers.
| Model | Water per cycle | Capacity | Calculator figure | Source |
|---|---|---|---|---|
| Bosch WIW28501GB | 45 litres | 8 kg | 5.63 l/kg | Mark's Electrical |
| Bosch WIW28302GB | 45 litres | 8 kg | 5.63 l/kg | Mark's Electrical |
| Neff W543BX2GB | 45 litres | 8 kg | 5.63 l/kg | Mark's Electrical |
| Neff W544BX2GB | 45 litres | 8 kg | 5.63 l/kg | Euronics |
| Siemens WI14W302GB | 45 litres | 8 kg | 5.63 l/kg | Euronics |
| AEG LF7C8636BI | 45 litres | 8 kg | 5.63 l/kg | Mark's Electrical |
| Hotpoint BIWMHG81485UK | 48 litres | 8 kg | 6.00 l/kg | Mark's Electrical |
| Indesit BIWMIL81485UK | 48 litres | 8 kg | 6.00 l/kg | Mark's Electrical |
| Zanussi ZW84PCBI (discontinued) | 45 litres | 8 kg | 5.63 l/kg | Mark's Electrical |
| Beko WTIK74151F (discontinued) | 41 litres | 7 kg | 5.86 l/kg | Mark's Electrical |
| Bosch SMV2HTX02G | 9 litres | 13 place settings | 0.70 l/place setting | Bosch |
| Bosch SMV4HTX00G (no longer sold by bosch) | 9 litres | 13 place settings | 0.70 l/place setting | Bosch |
| Neff S153HTX02G | 9 litres | 13 place settings | 0.70 l/place setting | Mark's Electrical |
| AEG FSS53637Z | 9.9 litres | 13 place settings | 0.77 l/place setting | Euronics |
| Hotpoint HIC3C33CWEUK | 9.5 litres | 14 place settings | 0.68 l/place setting | Euronics |
| Hotpoint HSIC3M19CUKN | 11.5 litres | 10 place settings | 1.15 l/place setting | Mark's Electrical |
| Indesit D2IHL326UK | 9.5 litres | 14 place settings | 0.68 l/place setting | Euronics |
| Beko DIN15C20 | 12.9 litres | 14 place settings | 0.93 l/place setting | Euronics |
Where a development has more than one model, the methodology uses the higher of the average and 70% of the highest figure, as for other fittings. Enter the figure for the worst-case model if they differ.
Step 2: more than one of the same fitting
Where a dwelling has several fittings of one type, the methodology uses whichever is higher: the average of their figures, or 70% of the highest figure. For example, showers of 8, 2 and 5 l/min average 5, but 70% of the highest is 5.6, so 5.6 is used. For dual flush WCs, each WC is first turned into an average effective flush using the same weightings as the use factors.
Step 3: the total
Total A = sum of all fittings Result = (Total A x 0.91) + 5
The 0.91 is the normalisation factor in the methodology, and the 5 litres is the allowance for outdoor use. The result is compared with 125 or 110.
Validation
Approved Document G gives two "fittings approach" tables. A dwelling where every fitting is within the table can comply without a full calculation, so a correct calculator must show each table's specification passing its target. Here are the results from this calculator:
| Specification | Target | This calculator | Outcome |
|---|---|---|---|
| Table 2.1: 6/4 dual flush WC, 10 l/min shower, 185 litre bath, 6 l/min basin taps, 8 l/min sink taps, 1.25 l/place dishwasher, 8.17 l/kg washing machine | 125 | 124.4 | Passes |
| Table 2.2: 4/2.6 dual flush WC, 8 l/min shower, 170 litre bath, 5 l/min basin taps, 6 l/min sink taps, 1.25 l/place dishwasher, 8.17 l/kg washing machine | 110 | 106.3 | Passes |
The Table 2.1 result lands just under 125, which is what you'd expect from a table designed around the limit. The individual fitting figures also match the worked figures in the methodology source, for example 13.54 litres for a 4/2.6 litre dual flush WC and 39.33 litres for a 9 l/min shower with a bath.
The fittings approach
If every fitting is within the limits in Table 2.1 (for 125) or Table 2.2 (for 110), Approved Document G allows the fittings approach instead of a full calculation. The full calculation must be used where any fitting is above the table, where there's no shower, or where a waste disposal unit, water softener or water reuse system is installed. The calculator tells you which applies.
What it doesn't cover
- Grey water and rainwater reuse. These need their own steps in the methodology. Use the official calculator for dwellings that have them.
- The calculator doesn't check that the figures entered are right. Use manufacturers' data, and for baths use the capacity to the overflow, not the total capacity.
- Building control bodies decide what evidence they accept. Check with yours.