Failed your Part G water calculation? How to get it to pass
The fittings that move the result most, with worked figures, so you can fix a fail with the smallest change to the specification.
At a glance
A fail is rarely a big problem. The Part G calculation weights each fitting by how often it is used, so a couple of targeted changes usually bring a dwelling under the limit. The key is knowing which fittings carry the most weight.
Where the litres come from
Appendix A of Approved Document G multiplies each fitting's flow rate or volume by a usage factor. The main ones are:
| Fitting | How it is counted |
|---|---|
| Shower (no bath in the home) | Flow rate × 5.60 |
| Shower (home also has a bath) | Flow rate × 4.37 |
| Bath (home also has a shower) | Capacity to overflow × 0.11 |
| Bath (no shower) | Capacity to overflow × 0.50 |
| Dual-flush WC | Full flush × 1.46 + reduced flush × 2.96 |
| Wash-hand basin taps | Flow rate × 1.58 + 1.58 |
| Kitchen and utility taps | Flow rate × 0.44 + 10.36 |
The total is then multiplied by 0.91 and 5 litres is added for outdoor use. Each litre per minute of shower flow is therefore worth about 4 to 5 litres on the final result, while each litre per minute at the kitchen tap is worth less than half a litre.
The changes that work, in order
1. Reduce the shower flow rate
This is almost always the biggest lever. Going from a 10 L/min to an 8 L/min shower in a home without a bath saves about 10 litres per person per day. A flow-limiting shower head or a flow regulator in the supply is a low-cost way to do it. Use the flow rate at the pressure the shower will actually run at.
2. Fit a lower-flush WC
Changing a 6/4 litre dual-flush WC to a 4/2.6 litre one saves about 6.4 litres per person per day.
3. Basin and kitchen taps
Aerated basin taps at 5 L/min instead of 6 save about 1.4 litres. The kitchen tap has a small factor, so cutting it from 8 to 6 L/min saves under a litre.
4. Appliances
If the washing machine and dishwasher are not known, the calculation uses the official defaults (8.17 litres per kg and 1.25 litres per place setting). If efficient models are being supplied, entering their actual figures can help.
A worked example
| Specification | Result |
|---|---|
| WC 6/4 L, basin 6 L/min, kitchen 8 L/min, shower 10 L/min, no bath | 117.1 L |
| As above with an 8 L/min shower | 106.9 L |
| As above with a 4/2.6 L WC | 100.5 L |
Recalculate as often as you like
Our calculator is free to use as many times as you need. Change a fitting, recalculate, and see the new result straight away. Its fail guidance points to the fittings having the biggest effect. You only pay for the report once the specification passes.
Frequently asked questions
What is the quickest way to reduce a Part G water calculation result?
Reduce the shower flow rate. In a home without a bath each litre per minute of shower flow adds about 5 litres per person per day to the result, so a flow-limited shower head often turns a fail into a pass.
Does a bath make it harder to pass Part G?
It depends. Where the home also has a shower, the bath is weighted lightly. Where a bath is the only bathing option, its capacity counts much more heavily, so a large bath can push the result over the limit.
Can I recalculate as many times as I like?
Yes. Our calculator is free to use as often as you need. You only pay if you want the Building Control report once the specification passes.
Test your changes now
Enter your current specification, then try a lower-flow shower or WC and watch the result change.
Open the calculator Free to calculate. No account or payment needed to start.