The one property that separates them
A household that suddenly uses more water uses it in bursts, at human hours, and stops when the house is empty or asleep. A leak is indifferent to any of that. It runs at the same rate at four in the morning as at seven in the evening, every day, including the fortnight you were away.
Every test below is a way of exploiting that one difference. None of them requires equipment.
Test one: the overnight baseline
Take a full meter reading last thing at night, including the red digits, with every tap, appliance and irrigation timer off. Take another before anyone runs a tap in the morning. Note the times.
Work out the rate:
- Subtract the first reading from the second, in cubic metres.
- Multiply by 1,000 to get litres.
- Divide by the hours between the readings to get litres per hour.
- Multiply by 24 to see what that rate does over a full day.
Zero over eight hours of genuine non-use is a clean result: whatever is raising the bill is being used, not lost. Any consistent figure over a period when nothing should be flowing is the leak's signature, and the litres-per-hour number is roughly the size of it.
One caveat that invalidates more of these tests than anything else: appliances that run on timers. An overnight dishwasher cycle, a softener that regenerates at two in the morning or a garden system on a controller will all register, and none of them is a leak. Turn them off at the socket or at the valve before you start.
Test two: the thirty-minute test, and the stop valve
For a faster answer, use Thames Water's version: everything off, read the meter, wait thirty minutes using no water, read it again. Same reading, no leak. Different readings, a loss somewhere between the meter and the inside stop valve.
Then close the inside stop valve, usually under the kitchen sink, and repeat. If the meter stops, the loss is inside the house. If it keeps turning, it is outside, on the supply pipe you own. CCW puts it the same way: a meter dial still spinning with the inside stop valve closed points to a leak on the outside supply pipe. The full method and the arithmetic are worth reading before you start, because the common mistakes are all avoidable.
Test three: compare cubic metres, not pounds
Bills confuse the comparison, because the rate changes every April and the periods are rarely the same length. Meter readings do not. Take the readings printed on your last few bills, work out cubic metres per day for each period, and put them side by side.
What the shape tells you:
| Pattern in m³ per day | What it points to |
|---|
| A step up on one date, flat before and flat after at the higher level | Something started and did not stop. Usually a leak, or an appliance stuck on. |
| A gradual climb over several periods | A growing leak, or creeping usage. Needs the overnight test to separate. |
| A single high period that falls back on its own | Usage. A summer of watering, a pool, visitors, or building work. |
| High, and it did not fall while the house was empty | A leak. Nothing else behaves this way. |
That last row is the strongest single piece of evidence most households can produce, and it costs nothing. If you went away and the meter kept counting at the same rate, you are not arguing about usage any more.
What genuine usage changes look like
Before concluding the worst, it helps to know how much ordinary activity moves the needle. A cubic metre is 1,000 litres, which Thames Water describes as roughly 25 quick showers. That scale is useful: a household adding one person taking daily showers is adding a measurable but modest amount, whereas a hosepipe left running, a filling pool or a continuously passing WC operates in a different league entirely.
Common honest causes, roughly in the order we find them:
- A WC passing into the pan. It is technically a leak, it is inside the house, and it is usually a worn flush valve seal you can replace yourself for the cost of the part.
- An additional occupant, or someone newly working from home.
- Garden watering, a new lawn, or a pressure washer used across a dry spell.
- A water softener regenerating too often after a settings change.
- A neighbour or contractor drawing from your outside tap.
The leaking WC deserves its own check because it is so easily missed. Dry the porcelain behind the pan, hold a dry strip of toilet paper against it below the cistern outlet, and look for a wet line after a few minutes. Nothing about that job needs us.
When the answer is a leak
If the overnight test shows a steady rate with everything isolated and the meter keeps moving with the stop valve shut, you have a supply-pipe leak and the ground has been taking the water. Under a lawn it may never surface. Under a drive or a solid floor it will track along the pipe and appear somewhere unrelated, which is why guessing where to dig is expensive.
What a detection visit adds is confirmation and a location: an isolation and pressure test to size the loss, then acoustic tracing, thermal imaging and, where the run is buried under screed or slab, tracer gas. Our rate is £150 per hour agreed before we attend, and if we do not find the leak you do not pay for the time spent looking. We also repair the pipe and reinstate the surface, rather than handing you a report and a hole.
Two things worth doing in parallel. Keep the readings, because your water company will want them when you claim the lost water back, and the leakage allowance can only be claimed after the repair. And if water has damaged the building, that is a separate route: see the insurance claims hub for how trace and access cover works and what a report needs to contain. If the pipe in question runs outside your boundary, the Thames Water hub sets out whose problem it is.
