How much evaporation is normal for a UK pool
An uncovered UK outdoor pool loses roughly 0.8 mm of depth a day in midwinter and about 3 mm a day in July. On a 32 m² pool that July figure is 95 litres a day going nowhere but the sky, and it is entirely normal.
Last reviewed 14 August 2026
The short answer
Roughly one millimetre a day in winter and three in high summer, for an uncovered unheated pool in southern England. Heated pools are a different and more surprising story.
| Month | Unheated | Heated to 28 °C |
|---|---|---|
| January | 0.88 | 12.64 |
| April | 1.70 | 11.27 |
| July | 2.97 | 8.73 |
| October | 1.34 | 9.99 |
One millimetre of depth is one litre per square metre, so these convert straight into a volume for your own pool by multiplying by its surface area. No other arithmetic is involved.
How this works in practice
Two things fall out of that table that are worth more than the figures themselves, because both are checkable and both are counter-intuitive.
First, a UK outdoor pool roughly breaks even on rain. The annual unheated evaporation total is 614 mm and the annual rainfall at the same weather station is 668 mm. Those are two independent figures and they land within about eight per cent of each other, which is a useful anchor: an uncovered unheated pool in southern England is close to self-topping-up over a year, and a pool that needs constant topping up is telling you something.
Second, and this is the one that looks like a bug: a heated pool loses more water in January than in July. It is right, and the reason is that evaporation is driven by the difference in vapour pressure between the water surface and the air above it, rather than by the temperature you feel. Hold the water at 28 degrees in cold dry January air and that difference is at its widest. In July the air is warm and damp and the gap narrows.
That is why a pool cover saves the most in the months a pool is least used, which is the opposite of when most people put one on.
None of this is needed to run the test. the bucket test, which measures your evaporation rather than estimating it subtracts it in your pool, on the day, with no figure from this page involved.
What changes the answer
Four things, all of them our judgements rather than measurements, and they do not matter equally. Here is how far the July unheated figure moves across the plausible range of each.
| Assumption | Across its plausible range | Moves the figure by |
|---|---|---|
| Wind shelter. How much a fence, a hedge or a building cuts the open-site wind. | sheltered to fully exposed | about 90 per cent |
| Water temperature. How far an unheated pool sits above mean air temperature. | 0 to 5 °C above | about 68 per cent |
| Relative humidity, the monthly array. | 61 to 81 per cent | about 26 per cent |
| Where you are. The climate inputs are for inland southern England. | not quantified here | Aberdeen less, exposed coast more |
The useful thing that table says is one you can act on. If your pool sits in the open rather than behind a fence or a hedge, these figures are too LOW for you, by up to about ninety per cent. That is the direction that matters on a leak site: our baseline assumes shelter, so an exposed pool loses more than the baseline says it should, and a reader comparing against it could conclude they have a leak when they have a windy garden.
Note that humidity, which is the one input on this page with no source at all behind it, moves the answer least of the three that have been measured. Those are two different questions and it is worth keeping them apart: where a number came from, and how much the answer depends on it. A weak number that barely moves the result is a smaller problem than a judgement that dominates it.
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
The equation is measured and citable: a correlation fitted to readings taken on an actual outdoor swimming pool rather than to wind tunnel tests, which is why it was chosen over the alternatives. The climate inputs are Met Office long-term averages for an inland southern England station over 1991 to 2020. Both of those are real sources and both are named in full on the sources page.
Four of the inputs are not sourced and are our judgements: the wind shelter factor, the unheated water temperature offset, the heated pool set point, and the monthly relative humidity. On provenance the humidity array is the weakest of them, because the Met Office location averages used for everything else here do not publish humidity at all, so it has no source whatever rather than a weak one.
On influence it is the mildest of the three that have been measured. The wind shelter factor moves the July unheated figure nearly four times as far as humidity does, and it is a judgement about one specific garden that nobody but you can make. This page previously named humidity as the assumption to watch, which conflated the two axes and put the emphasis in the wrong place.
The replacement has been found and not yet used. HadUK-Grid, published by the Met Office through CEDA under the Open Government Licence, carries monthly relative humidity from 1961 with UK regional averages, which is exactly the series this needs. It requires a free registration to download, and that is the only reason these figures are still a judgement. A named gap with a named replacement is a different thing from a number somebody made up, and this site would rather say which one it has.
What to do next
Two things, and the first is the only one that answers your actual question.
- Run the bucket test. It measures your evaporation rather than estimating it, so it answers "is this a leak" definitively where this page can only tell you whether a figure is plausible.
- If your measured drop is far above the row for the month you are in, that is worth knowing before you start the test, because it means the answer is likely to be yes.
And if the pool is heated, look at the winter row again before deciding a cover is a summer accessory. the bucket test, which measures your evaporation rather than estimating it turns millimetres into litres for your own pool.
Common questions
- Is 3 mm a day a leak?
- Not in July on an uncovered pool. That is ordinary summer evaporation. On a 32 m² pool it is about 95 litres a day, which sounds alarming written as a volume and is simply what an open body of warm water does.
- Why does a heated pool lose more in January than in July?
- Because evaporation is driven by the difference in vapour pressure between the water and the air, not by how warm it feels outside. A pool held at 28 degrees Celsius in January sits in cold dry air, and that gap is at its widest. In July the air is warmer and wetter, so the gap narrows. It looks like a mistake and it is not, and it is the reason a cover earns its keep in winter far more than in summer.
- Do I need these figures to run the bucket test?
- No, and that is the point of the bucket. It sits in the same pool in the same weather and evaporates at the same rate, so the test subtracts evaporation empirically rather than estimating it. These figures are a sanity check for a reader who wants to know whether their measured drop is plausible, and nothing on this site calculates with them.
- How reliable are these numbers?
- The equation behind them was fitted to measurements on a real outdoor pool, and the climate inputs are Met Office long-term averages. But four inputs are our own judgements. The humidity array has no source at all behind it, which makes it the weakest by provenance, and separately the wind shelter factor is the one that moves the answer furthest. Treat these as an order of magnitude rather than a prediction for your pool.
- Does a cover make much difference?
- A great deal, and most of it in the months people least expect. Evaporation is what a cover stops, and for a heated pool the evaporation is highest in winter, so a cover saves most when the pool looks least used.