How a water leak is actually found
A meter test tells you how fast you are losing water. It never tells you where. Finding the position is a separate job done by three methods: listening for the sound the escaping water makes, putting a traceable gas into the drained pipe and detecting where it surfaces, and looking for the temperature difference the water creates. Each fails in specific conditions, and those conditions are things about your property you already know, so you can work out which is likely to suit yours before anybody visits.
Last reviewed 14 August 2026
What this section covers
The three methods used to locate a water leak, how each one works, and the conditions under which each one stops working.
It exists because every calculator on this site produces a rate and none of them produces a position. That is not a limitation of the tools, it is what a meter measures: volume passing a point, over time. Nothing about a rate contains a location, so at some point quantification hands over to location and this is where that happens.
The failure modes get as much room as the capabilities, deliberately. A page listing only what a method can do is an advertisement with a methodology section, and this site is funded by a company that sells all three of these services. Where they fail is the part you cannot get anywhere else.
How it works in the UK
All three methods answer the same question, which is where along a pipe the water is leaving. They differ in what they detect and therefore in what defeats them.
| Method | What it actually detects | Where it fails |
|---|---|---|
| Acoustic | The noise water makes escaping from a pipe under pressure, listened for at the surface or along the pipe. | Plastic pipe, which carries the sound far less far than metal. Noisy sites. Very low flow rates, which make little noise. A pipe that is not under pressure makes none at all. |
| Tracer gas | Hydrogen, in a hydrogen and nitrogen mix, put into the drained pipe and detected where it rises to the surface. | It needs the pipe drained and isolated, which is not always practical. Sealed surfaces can stop the gas surfacing where the leak is, or send it somewhere else. |
| Thermal imaging | A temperature difference at a surface the camera can see. | Any situation with no temperature difference to see: cold water in a cold floor, thick or insulating coverings, or a surface the camera cannot get a view of. |
Read that table as a selection guide rather than as a list of caveats. What decides which method suits a property is a handful of things you already know about your own house, so the choice is largely made before anybody arrives.
And none of it is worth doing until there is a measured rate to justify it. quantify the loss at the meter first, because a survey is a paid search and it should be looking for something you have established is there.
The figures that matter
There are none, and that is the honest answer rather than an omission. No sourced accuracy figure, success rate or price exists for any of these methods, so this site publishes none.
If somebody quotes you a detection accuracy percentage, ask what it is measured against. An accuracy claim needs a known answer to compare a result with, which means somebody dug up a set of leaks to check. That work would be publishable and this site could not find it published.
What you can establish for nothing, before any of this, is which side of the internal stop tap the loss is on. Close the stop tap, leave everything off, and read the meter twice. If it still moves, the loss is on the underground supply pipe. If it stops, it is inside. That single test halves the area anybody has to search.
It is worth doing carefully rather than approximately, because it is the last narrowing available to you at no cost. Everything after it is a paid search over whatever area you hand over.
How this changes your loss figure
It does not change it at all. Locating a leak does not alter how fast it is losing water, and this is the one page here that changes none of the six figures a result reports.
What it changes is what you can do about the figure. A rate you cannot locate is a number you can only watch. A rate with a position attached is a repair quote.
| After | You know | What it lets you do |
|---|---|---|
| A meter test | How fast you are losing water, and roughly what that costs. | Decide whether it is worth pursuing at all. |
| The stop tap test | Which side of the internal stop tap it is on. | Halve the search, and know whether a leak allowance is likely to apply. |
| A survey | Where along the pipe, within whatever tolerance the method managed. | Get a repair quote for a specific job rather than an open-ended one. |
The middle row is worth more than it looks, because a supply pipe leak is the case most allowance policies exist for. whether a leak allowance can reduce that charge depends on which side of that stop tap the answer landed.
Assumptions and sources
Drip Calculator: how much water does a leaking faucet waste?
United States Geological Survey, Water Science School · primary source
- Faucet drip volume, adopted: 0.25 ml
- Faucet drip volume, measured range: 0.2 to 0.33 ml
- Bath tap drip volume: 0.5 ml
- Drips per US gallon: 15140 drips
- Drips per litre: 4000 drips
The pharmacopoeial metric drop, 20 drops to 1 millilitre
United States Pharmacopeia, British Pharmacopoeia and European Pharmacopoeia convention · primary-standard source
- Standard drop volume: 0.05 ml
Tate's law: the weight of a falling drop
Thomas Tate, Philosophical Magazine, 1864, with the later drop-weight literature · primary source
- Drop weight: W = 2 * pi * r * gamma equation
Plumbers shine the spotlight on dripping taps during Water Saving Week
WaterSafe, with Waterwise · untraceable source
- Claimed annual waste from a dripping tap: 5500 litres per year
International Organization of Legal Metrology · primary-standard source
- MPE, accuracy class 2, lower zone Q1 to Q2: 5 %
- MPE, accuracy class 2, upper zone Q2 to Q4: 2 %
- MPE, accuracy class 1, lower zone: 3 %
- MPE, accuracy class 1, upper zone: 1 %
- In-service MPE multiplier: 2 ratio
- Q2 / Q1: 1.6 ratio
- Q4 / Q3: 1.25 ratio
Aquadis+ DN15 and DN20 specification sheet
Itron · manufacturer source
- Q1 minimum flow rate, DN15, Q3 2.5, R160: 15.6 l/h
- Q2 transitional flow rate, DN15, Q3 2.5, R160: 25 l/h
- Q3 permanent flow rate, DN15: 2500 l/h
- Typical starting flow rate, DN15: 0.4 l/h
- Typical starting flow rate, DN20: 2 l/h
- Q1 minimum flow rate, DN20, Q3 4.0, R160: 25 l/h
- Flow rate at which accuracy is within +/- 5%, DN15: 3 l/h
- Flow rate at which accuracy is within +/- 2%, DN15: 5 l/h
- Minimum scale interval, DN15 and DN20: 0.02 l
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
Leaky loos, why it's not as simple as faulty flush valves
ech2o, reporting research attributed to Thames Water · secondary source
- Toilets inspected: 58551 toilets
- Toilets found leaking: 4854 toilets
- Prevalence: 8.3 %
- Average residential leak rate: 400 l/day
- Average commercial leak rate: 2100 l/day
- UK-wide average across all WCs, leaking and sound: 20 l/day
- Share of leaking toilets with flush valves rather than siphons: 81 %
Waterwise · untraceable source
- Single leaking toilet: 215 to 400 l/day
- Share of toilets leaking: 5 to 8 %
- UK total from leaking toilets: 400000000 l/day
Methods for Calculation of Evaporation from Swimming Pools and Other Water Surfaces
M. M. Shah, ASHRAE Transactions SE-14-001 · primary source
- Outdoor unoccupied pool evaporation: E0 = (C1 + C2 * u) * (pw - pa) / ifg kg/(m2*h)
- C1, SI: 235 constant
- C2, SI: 206 constant
Location-specific long-term averages, Wisley, Surrey
Met Office · primary source
- Annual mean daily maximum temperature: 15.41 degrees C
- Annual mean daily minimum temperature: 6.66 degrees C
- Annual rainfall: 667.92 mm
- Annual mean wind speed at 10 m: 5.06 knots
Discover Water, the water industry transparency site run with Water UK, Ofwat, CCW, the Drinking Water Inspectorate and Defra · primary source
- Average annual household bill, combined water and sewerage: 639 GBP
- Average annual household bill, water only: 309 GBP
- Average annual household bill, sewerage only: 330 GBP
SI Brochure: The International System of Units (SI), 9th edition (2019), version 4.01, June 2026
Bureau International des Poids et Mesures · primary-standard source
- cubic metre: coherent derived unit of the SI, Table 5 m3
- litre: non-SI unit, Table 8. 1 l = 1 L = 1 dm3 = 10^-3 m3 l or L
- bar: non-SI unit, Table 8. 1 bar = 0.1 MPa = 10^5 Pa bar
Authority URLs for the named entities in content/entities.ts
Various, see per-entity notes · primary source
WC flush volume: three UK instruments, not one
UK Statutory Instruments, Northern Ireland Statutory Rules, and Scottish Water · primary-standard source
- Maximum single flush, all three jurisdictions: 6 litres
- Dual flush, lesser flush maximum: two-thirds of the largest flush volume ratio
- England and Wales transitional maximum, 1 Jul 1999 to 1 Jan 2001: 7.5 litres
Per-event central heating top-up volume, derived
Derived by Q3 from Boyle's law and typical UK domestic expansion vessel specifications · derived source
- Water held in an expansion vessel: V_water = V * (1 - P0 / P), pressures absolute equation
- Typical top-up, 8 l vessel at 1.0 bar pre-charge, 0.5 to 1.5 bar: 1.6 litres
- Typical top-up, 8 l vessel at 0.75 bar pre-charge, 0.5 to 1.5 bar: 2.4 litres
- Typical top-up, 12 l vessel at 1.0 bar pre-charge, 0.8 to 1.5 bar: 2.4 litres
- Typical top-up, 8 l vessel at 1.0 bar pre-charge, 1.2 to 1.5 bar: 0.9 litres
- Realistic range: 0.9 to 2.5 litres
Monodispersed Bubble Generation Using Hydrophobic Orifices: The Extended Tate’s Law
Bo Liu, Hao Zhang and co-authors, ACS Omega 2024, 9(17), 18854 to 18861 · primary source
Everything on this page is mechanism rather than measurement. How sound travels through a pipe wall, why hydrogen surfaces through soil, and why a camera sees a temperature difference rather than water are all descriptions of how a method works, and none of them is a claim about how often it succeeds.
- No accuracy figure appears for any method, because none was found published with a method behind it.
- No price appears for any method, for the same reason, and because it would vary with the property in any case.
- No claim is made about which method is used most often, or which finds the most leaks. Those would be facts about the industry and no source for them was found.
The sponsor of this site sells all three of these services. That is disclosed on every page, and it is the reason this page was written to the standard it was: the failure modes are given as much space as the capabilities, no method page ends by telling you to call anybody, and the one question this page most wants you to ask a supplier is what happens if they do not find it.
What to do next
Four things, and the first three cost nothing and make the fourth cheaper.
- Confirm there is a loss and how big it is, by meter test, before paying anybody to look for it.
- Do the stop tap test properly and write down both readings. It halves the search area and it is the last narrowing you get for free.
- Work out what you already know that decides which method suits: whether your supply pipe is plastic or metal, whether the suspect run is under a solid floor, whether the water in it is hot or cold, and how much noise there is around the property.
- Ask what happens if it is not found, before booking. That establishes whether you are buying a result or an attempt.
Take the readings and the dates with you. A survey that starts from a measured rate and a known side of the stop tap is a smaller job than one that starts from a suspicion, and you are the only person who can supply either.
And check your own policy before you weigh the cost, because the cost may not be yours. check your policy for trace and access cover is worth doing first: those three words are the standard name for cover that pays for the search and the access rather than for the repair, so they are a phrase you can search a document for. Whether your policy carries it is a question for your policy, and this site has read none.
Common questions
- Can I find the leak myself?
- You can narrow it down a long way for nothing, and you should, but you cannot usually locate it. Closing the internal stop tap and repeating the meter test tells you whether the loss is on the underground supply pipe or inside the house, which halves the search. Past that point every method needs equipment, and none of the household substitutes people suggest for it work reliably.
- Which method is best?
- None of them, in general, and that is not a dodge. Acoustic is the usual first method on a pressurised pipe and it struggles on plastic, in noise and at very low flow rates. Tracer gas reaches what acoustic cannot but needs the pipe drained. Thermal imaging finds a temperature difference, which is not the same as finding a leak. The right one depends on your pipe, your floor and your water.
- Is tracer gas anything to do with the fuel coming into my house?
- No, and the name confuses people reasonably. Tracer gas leak detection puts a harmless mixture of hydrogen and nitrogen into a water pipe that has been drained and isolated, then detects the hydrogen where it escapes and rises to the surface. It is a water leak method. Nothing on this site concerns fuel, appliances or their safety.
- How accurate is leak detection?
- This site does not publish a figure, because no sourced one exists for any of these methods. Anybody quoting you an accuracy percentage should be asked where it comes from. What can be said honestly is what each method depends on and what defeats it, which is on this page and is more useful for deciding anyway.
- What should I ask before booking a survey?
- What happens if it is not found. That single question tells you more than any accuracy claim, because it establishes whether you are paying for a result or for an attempt, and both are legitimate as long as you know which one you have bought. Ask it before, not after.
- Does the survey include the repair?
- Usually not, and they are separate jobs with separate costs. Locating a leak and excavating or lifting a floor to fix it are different work, and a quote for one is not a quote for the other. Establish which you are being quoted for.