Where the number comes from
Part G of the Building Regulations caps a new dwelling’s calculated water use at 125 litres per person per day. Nobody meters your actual usage — the figure comes from the official methodology, which multiplies each fitting’s consumption by a standardised “use factor” (how often a typical person uses it daily) and adds a fixed 5 litres for outdoor use.
So a 9 litres/minute shower, a 180-litre bath, a 4/2.6-litre dual-flush WC and so on each contribute their share, and the shares must total ≤125. (Some areas use the tighter optional 110 l/p/d standard — check your planning conditions.)
Where the litres actually go
The big three are showers, WCs and baths — together usually two-thirds of the total. Taps and white goods matter less than people expect. Which leads to the practical rules of thumb:
- A powerful shower (12+ l/min) is the most common budget-killer. Dropping to ~9 l/min usually fixes a fail on its own.
- Dual-flush WCs are near-universal now and cheap litres saved.
- A huge bath hurts, but only the largest bath counts a partial factor — it’s rarely fatal alone.
- Offsetting is allowed: splash out on the shower, claw it back with efficient taps and WCs.
- Rainwater or greywater systems can offset heavy use, though they’re rarely needed just to hit 125.
See your own numbers
The fastest way to understand it is to try it: our free WaterMonkey calculator shows a live breakdown of where your litres go and a PASS/FAIL that updates as you change fittings. Experiment freely; the Building Control-ready report is £25 when you’re done. Or we’ll do it for you.