Sewerage charges on leaked water
Water lost from an underground supply pipe never reaches the sewer, so no sewerage charge should be applied to it at all. For water that does reach the sewer, all six of the schemes read for this site assume some of it does not, and across those six the allowance runs from 5 to 10 per cent. What differs between companies is not the size of that allowance so much as where they apply it.
Last reviewed 14 August 2026. Rates correct as of 14 August 2026
The short answer
Two questions decide whether a sewerage charge belongs on a leak, and they are asked in this order.
- Did the water reach the sewer? An underground supply pipe loss does not, so the sewerage rate does not apply to it. An internal leak that drains away does.
- If it did, what share of the volume does your company actually bill for wastewater? Not all of it, and how they arrive at that varies.
Getting the first question wrong is the larger error by far, because sewerage rates are often comparable to water rates, so applying one where it does not belong can come close to doubling the cost of a leak.
How this works in practice
Every company assumes some of the water it supplies never comes back as wastewater, for the obvious reasons: garden watering, cooking, evaporation. That assumption is called a non-return-to-sewer allowance, and there are two entirely different places to apply it.
| Mechanism | Company | Allowance | What the scheme says |
|---|---|---|---|
| Built into the rate. 100 per cent of volume billed. | Thames Water | up to 10 per cent | Bills the same volume for wastewater as for water supplied, because the wastewater charge already assumes up to 10 per cent is not returned. Clause 6.1.3. |
| Built into the rate. | Northumbrian Water | 5 per cent | A standard 5 per cent allowance is made within the calculation of the volumetric rate, which is then applied to the total metered quantity. Paragraph 84. |
| Built into the rate. | United Utilities | 5 per cent | When the volumetric charge for household sewerage is set, an average of 5 per cent of metered water is assumed not to be returned. Section 9.5. |
| Applied to the volume. 95 per cent of volume billed. | Yorkshire Water | 5 per cent | The volume item is applied to 95 per cent of the volume of water received. Septic tank discharge reduces that assumption to 70 per cent. |
| Applied to the volume. | South West Water | 5 per cent | Sewerage volume charges are based on the metered amount, with a 5 per cent standard allowance. Clause 4.19, with a worked example billing 30 cubic metres at 95 per cent. |
| Applied to the volume. | Welsh Water | 5 per cent | Billed on 95 per cent of the metered volume. |
The two mechanisms reach a similar place by different routes, and that is exactly why the distinction matters rather than being a technicality.
Applying a 95 per cent factor to a company that has already priced its allowance into the rate counts the same allowance twice and understates the bill. Not applying it to a company that takes it off the volume overstates the bill. The percentage on its own is not enough information: you have to know which of the two mechanisms your company uses, and that is why this site records the mechanism per company rather than applying one number to everybody. what that rate costs over a year is where the whole multiplication is set out.
What changes the answer
One thing changes it more than the mechanism does, and one widely published comparison gets it backwards.
The larger factor is where the water went. A supply pipe leak underground never reaches the sewer at all, so the whole sewerage element comes off rather than a few per cent of it. That single question is worth more than every mechanism in the table put together.
The comparison that gets it backwards is ranking companies by published sewerage rate. Thames Water bills 100 per cent of the volume, which reads as the least generous treatment on the list, and it is the opposite: its assumption is up to 10 per cent, the largest of the six, and the 100 per cent is the volume basis rather than the allowance. A difference of mechanism has been presented elsewhere as a difference of generosity.
- Septic tank discharge, where the company recognises it. Yorkshire Water reduces its assumption from 95 to 70 per cent.
- Whether your company has been read at all. Five of the eleven have not, and this site says so rather than filling the gap.
- Whether you are a customer of a water-only company, in which case the sewerage half of your bill, and its mechanism, belong to a different company entirely.
That last one applies to nine of the twenty England and Wales supply areas. how UK water is priced has the whole picture, and the water-only leaf has the pairings.
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
Every figure in the mechanism table was read from the company's own 2026 to 2027 household charges scheme, and the clause or paragraph number is recorded against each. Nothing in that table came from a comparison site, a summary or a press release, which matters here more than usual: the claim this page corrects originated in exactly that kind of secondary source.
Six of the eleven water and sewerage companies in England and Wales have been read. The other five have not, because their sites refuse automated requests, and their mechanism is recorded as unknown. It is not defaulted, because both 1.0 and 0.95 are the correct value for real companies on this list and a default would pick a side. Whether other companies match Yorkshire on septic tank discharge has not been checked and is not assumed.
What to do next
Three checks, and the first one decides whether the rest matter.
- Establish where the water went. Close the internal stop tap and repeat the meter test: if the meter still moves, the loss is on the supply pipe and the sewerage charge does not belong on it at all.
- Find the sewerage section of your own company's current charges scheme and read how the volume is calculated. It is one clause, it is usually plain English, and it tells you which of the two mechanisms applies to you.
- If your company is one of the five not yet read here, that clause is the thing to send us. It is a single sentence and it closes a real gap.
Common questions
- Should the sewerage rate be added to my leak at all?
- Only if the leaking water reaches the sewer. A leak on the underground supply pipe soaks into the ground, so it never enters the sewerage system and no sewerage charge belongs on it. An internal leak that runs to a drain usually does reach the sewer.
- Which company gives the biggest allowance?
- Of the six read directly, Thames Water, at up to 10 per cent. That is the opposite of what a comparison of published sewerage rates suggests, because Thames bills 100 per cent of the volume and takes its allowance off the rate instead.
- Why can I not just compare sewerage rates between companies?
- Because the two groups are quoting different things. A company that takes the allowance off the volume publishes a rate with no allowance in it. A company that builds the allowance into the rate publishes a rate that already has one. Ranking them side by side compares two different quantities.
- What if my company is not in the table?
- Then this site records its mechanism as unknown rather than guessing one. Five of the eleven water and sewerage companies could not be read, because their sites refuse automated requests. A default would have to be either 1.0 or 0.95, and both of those are correct for real companies, so picking one would silently take a side.
- Does a septic tank change anything?
- It can. Yorkshire Water reduces its assumption from 95 per cent to 70 per cent where discharge is to a septic tank, on the basis that far less of the supplied water reaches the public sewer. Whether other companies do the same has not been checked.