My bill doubled and I cannot see anything wrong. What is most likely?

A catch-up after estimated readings, if any recent bill was marked as estimated. An estimate is a prediction from your past use, so it cannot show a change, and the correction all arrives at once when somebody finally reads the meter. That produces a large bill without anything having changed in the house at all.

Which leaks are genuinely invisible?

Three kinds. An underground supply pipe leak, which soaks away and leaves nothing to see indoors. A leak under a solid floor, which can take months to show. And a toilet passing silently into the pan, which is the most common of the three and makes no noise and leaves no mark.

How big does a leak have to be to show on a bill?

Larger than most people expect. A loss of 0.5 litres per hour adds about 4.4 cubic metres a year, which is roughly two per cent of a household of four and would be invisible in ordinary variation. A loss of 11 litres per hour adds about 100 cubic metres a year, which is unmissable.

Is my meter reading it wrong?

Possible and uncommon, and it is not the first thing to suspect. The check is the same test done properly: with the internal stop tap closed and nothing connected, a correctly working meter does not move. If it does, that is worth reporting to your supplier, because the meter is theirs.

My supplier says it is my responsibility. Is it?

Usually, from the boundary to the property. That is the stretch you own and cannot see, which is also the case the reasoning behind leak allowances addresses, so a measured rate and the dates you took it are worth having before that conversation.

High water bill with no visible leak

Most unexpectedly high water bills are not leaks. The commonest causes are a catch-up bill after a period of estimated readings, a price change, a season of garden watering, or more people in the property, and all four are free to check. A leak that is invisible is usually underground, under a floor, or a toilet passing silently, and one overnight meter test separates it from everything else.

Last reviewed 14 August 2026

The short answer

Work through the causes in order of how easy they are to rule out, not in order of how alarming they are. Most of these are free to check and most high bills stop here.

Check	What it looks like	Cost to rule out
Estimated readings	A recent bill marked estimated, then a large catch-up. Nothing changed in the house.	Free. Look at the bill.
A price change	Similar volume, higher total. The volume is the number to compare, not the pounds.	Free. Compare volumes.
Seasonal use	A summer quarter with a garden, a paddling pool or hot weather in it.	Free. Compare a year against a year.
More people, or more time at home	A change in the household that nobody thought of as a water event.	Free.
A silently passing toilet	Nothing visible or audible at all.	Half an hour and some food colouring.
An underground or under-floor leak	Nothing visible indoors. Possibly a damp patch or unusually green ground outside.	One night on a meter test.

Causes in elimination order

The order matters. Opening with "you probably have a leak" sends a reader looking under floorboards when the answer was usually on the first line of the bill.

How this works in practice

Five steps, and most bills are explained before the fourth.

Check whether any recent reading was estimated. If so, this is very likely a catch-up and nothing is wrong.
Compare volumes rather than totals, and a year against a year rather than a quarter against a quarter. Two totals differ by the annual price change as well as by use.
Account for the household honestly: a garden, a pool, visitors, somebody newly at home during the day, a new appliance.
Dye-test every toilet. It is the most common invisible loss, it costs nothing, and it takes half an hour.
Run an overnight meter test with everything closed. That measures the property directly instead of inferring anything from a bill.

That last step is the one that settles it, and it is worth doing even if you think you have found the answer. measure the current rate of loss at the meter takes one night and produces a rate rather than a theory.

A clean overnight test after a high bill is a real result rather than a failure to find something. It means the water was used rather than lost, which redirects the question from the plumbing to the household and saves a great deal of pointless searching.

What changes the answer

Three things, and the first is the reason a small leak can hide indefinitely.

The size of the loss. A leak of 0.5 litres per hour is about two per cent of a four-person household's annual use, which disappears entirely into ordinary variation. It can run for years without a bill looking wrong.
Whether anything runs automatically at night. An irrigation timer or an automatic pool top-up produces exactly the pattern of a leak on an overnight test, so both have to be isolated before testing.
Where the loss is. An underground supply pipe leak never reaches the sewer, so it should not carry a sewerage charge, and getting that wrong can nearly double the apparent cost of it.

That third point is worth settling before any conversation about money. how the sewerage half of the bill is calculated covers when the sewerage rate applies to a leak and when it does not.

Assumptions and sources

Water resources 2024 to 2025: analysis of the water industry's annual water resources performance

Environment Agency · primary source

Household per capita consumption: 136.5 l/person/day
Household per capita consumption, prior year: 137 l/person/day
Household per capita consumption, dry-year adjusted: 140.3 l/person/day
National leakage: 2617 Ml/day
National leakage as share of water put into supply: 19 %

PR24 common performance commitments: per capita consumption (PCC), version 2.1

Ofwat · primary-standard source

PCC formula: (measured household consumption + unmeasured household consumption) / total household population l/person/day

Litres in a cubic metre

Exact by definition

SI definition. The litre is defined as one cubic decimetre, so one cubic metre is exactly 1,000 litres.

Days used for an annual figure

Editorial convention

Annual figures on this site are a daily rate multiplied by 365. Leap years are not modelled, so a leap year is understated by one day, which is 0.27 per cent.

Where the first severity band ends

Editorial convention

A practical threshold set just above the 0.4 litres per hour starting flow quoted for a DN15 meter with the TVM register. It is not a meter accuracy tolerance. Against the STANDARD it sits about 31 times below Q1, the 15.6 litres per hour at which OIML R 49-1 first defines any permitted error. Against THIS MANUFACTURER it sits about 6 times below 3 litres per hour, the lowest flow at which Itron volunteers a typical accuracy that the standard does not require. Neither figure reaches down to 0.5, so nothing quantifies a reading here on either footing. Meters with other registers start higher, so this threshold sits above the most sensitive case rather than above every meter.

The elimination order on this page is a judgement about what is cheapest to check first rather than a ranking of how often each cause occurs. No survey of the causes of high UK water bills was found, and this site does not claim one.

The figures used are the site's own severity threshold and the published England consumption average, both of which appear with their sources in the table above. The percentages are arithmetic on those.

What to do next

Two things, in this order, and the first is five minutes at a kitchen table.

Work down the elimination table. Four of the six causes are free to rule out and between them they explain most high bills.
If all four are ruled out, dye-test the toilets and then run an overnight meter test. That sequence goes from cheapest to most involved and stops as soon as it finds something.

And whatever the outcome, keep the readings and the dates. If it does turn out to be a leak, that record is what a leak allowance claim is assessed on, and whether a leak allowance can reduce that charge covers what one covers.

Common questions

My bill doubled and I cannot see anything wrong. What is most likely?: A catch-up after estimated readings, if any recent bill was marked as estimated. An estimate is a prediction from your past use, so it cannot show a change, and the correction all arrives at once when somebody finally reads the meter. That produces a large bill without anything having changed in the house at all.
Which leaks are genuinely invisible?: Three kinds. An underground supply pipe leak, which soaks away and leaves nothing to see indoors. A leak under a solid floor, which can take months to show. And a toilet passing silently into the pan, which is the most common of the three and makes no noise and leaves no mark.
How big does a leak have to be to show on a bill?: Larger than most people expect. A loss of 0.5 litres per hour adds about 4.4 cubic metres a year, which is roughly two per cent of a household of four and would be invisible in ordinary variation. A loss of 11 litres per hour adds about 100 cubic metres a year, which is unmissable.
Is my meter reading it wrong?: Possible and uncommon, and it is not the first thing to suspect. The check is the same test done properly: with the internal stop tap closed and nothing connected, a correctly working meter does not move. If it does, that is worth reporting to your supplier, because the meter is theirs.
My supplier says it is my responsibility. Is it?: Usually, from the boundary to the property. That is the stretch you own and cannot see, which is also the case the reasoning behind leak allowances addresses, so a measured rate and the dates you took it are worth having before that conversation.