How to Tell if Your Pool Is Leaking or Just Evaporating

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Your pool is leaking if its level drops more than a bucket of pool water standing in it over 24 hours. Evaporation alone is large: the Bureau of Meteorology's pan at Sydney Airport averages 7.3mm a day in January and 2.5mm in June. On an 8m × 4m pool, every extra millimetre is 32 litres.

The short answer

A falling water level is not evidence of a leak. SPASA puts an uncovered average backyard pool at around 7mm of water a day in the summer months, from evaporation alone.
Bureau of Meteorology evaporation at Sydney Airport runs from 2.5mm a day in June to 7.4mm in December (Class A pan, 1974–2025), which is why the same pool can look fine in winter and ‘leak’ in summer.
The bucket test compares your pool with your weather on your day. If the pool drops more than the bucket over 24 hours, the difference is the leak.
Convert the difference to litres: surface area in square metres × millimetres = litres. A 5mm-a-day gap on a 32 square metre pool is 160 litres a day.
Run it twice, pump off then pump on. SPASA separates static leaks, which lose water at a steady rate, from pressure leaks, which lose water only when the filtration system is running.

Evaporation baseline: Bureau of Meteorology mean daily evaporation at Sydney Airport (Class A pan, 1974–2025), converted for an 8m × 4m pool
Month	Mean daily evaporation	Equivalent loss from a 32 m² pool, per day	Equivalent drop over 7 days
Jan	7.3 mm	234 litres	51.1 mm
Feb	6.5 mm	208 litres	45.5 mm
Mar	5.4 mm	173 litres	37.8 mm
Apr	4.2 mm	134 litres	29.4 mm
May	3.0 mm	96 litres	21.0 mm
Jun	2.5 mm	80 litres	17.5 mm
Jul	2.7 mm	86 litres	18.9 mm
Aug	3.7 mm	118 litres	25.9 mm
Sep	4.9 mm	157 litres	34.3 mm
Oct	6.0 mm	192 litres	42.0 mm
Nov	6.6 mm	211 litres	46.2 mm
Dec	7.4 mm	237 litres	51.8 mm
Year	5.0 mm	160 litres	35.0 mm

Is my pool leaking or is it just evaporation?

Start from the number that fools most owners: how much water an Australian pool loses when nothing is wrong.

The Bureau of Meteorology publishes mean daily evaporation for its stations, measured in a Class A evaporation pan whose water level is reset daily; the change in depth, allowing for rainfall, is the day's evaporation. At Sydney Airport (station 066037), the 1974–2025 averages run from 2.5mm a day in June to 7.3mm in January and 7.4mm in December, with an annual mean of 5.0mm. The table above sets out every month.

A pan is a small, exposed container, so treat its figure as a guide to the weather rather than a precise prediction for your pool, which will differ with shade, wind shelter and water temperature. The industry figure lands in the same place: SPASA, the pool and spa industry association, puts an uncovered average backyard pool at around 7mm of water a day in the summer months.

The quotable version: in an Australian summer, a pool that drops 5cm in a week may have nothing wrong with it at all.

That is also why fixed rules of thumb (‘more than X mm a day is a leak’) are unreliable: evaporation at Sydney Airport is nearly three times higher in January than in June. If evaporation is your only problem, a cover is the fix — see pool cover cost and payback and what a pool costs to run each year.

How to turn a bucket test result into litres

The bucket test is the free first step, and our page on what to do when a new pool is leaking sets out the method step by step. In short: a bucket of pool water stands on a step, both levels are marked, the pump is off, and you compare them 24 hours later without topping up.

What that page does not do is turn the answer into a quantity, and the quantity is what tells you how urgent it is. The conversion is simple: surface area (m²) × drop (mm) = litres, because one millimetre over one square metre is one litre.

Worked example. An 8m × 4m pool has a surface of 32 m². On a January day the bucket falls 6mm and the pool falls 11mm.

Evaporation, shown by the bucket: 6mm × 32 = 192 litres.
Unexplained loss: 11mm − 6mm = 5mm, and 5mm × 32 = 160 litres a day.
Over a week: 160 × 7 = 1,120 litres; over 30 days, 4,800 litres, or 150mm off the pool if you never topped up.

The quotable version: the bucket tells you whether you have a leak; the surface area tells you how big it is.

Substitute your own dimensions. A pool that is not a rectangle can be approximated from the plans or the quote, and a gap of 1mm a day or less is within the error of marking a line with tape, so repeat the test before acting on it.

Pump on, pump off: the second test that tells you where to look

SPASA divides pool leaks into two types, and the distinction is the most useful thing you can bring to anyone you call out:

Static leaks lose the same amount of water consistently over a given period.
Pressure leaks lose water only when the filter system is running.

So run the bucket test twice: once with the pump off, once with the pump on its normal cycle. The comparison points to a part of the pool:

Pump off result	Pump on result	What it points to	Where to look first
Pool drops the same as the bucket	Pool drops the same as the bucket	Evaporation, no leak	Nowhere; consider a cover
Pool drops more than the bucket	Similar extra loss	Static leak in the shell or a fitting below the water line	Shell surface, tile line, skimmer box, light niche
Pool drops the same as the bucket	Pool drops more than the bucket	Pressure leak in the circulation system	Valves, pipes and joints; wet ground between pool and equipment pad
Pool drops more than the bucket	Extra loss is larger again	Possibly both	Report both figures to whoever investigates

The quotable version: a pool that only leaks with the pump running is telling you the problem is in the plumbing, not the shell.

The pump and filtration system are covered in pool equipment explained.

Where pool leaks usually are, and what we could not verify

SPASA's owner checklist names the places to inspect before calling anyone: wet or eroded ground between the pool and the equipment pad; plumbing, including valves, pipes and joints; cracks in the pool surface or tears in a vinyl liner; the tile line and the inside of the skimmer box; and underwater lights, which SPASA says ‘can and do leak, especially the conduit that runs from the light niche to the junction box’.

A DIAGNOSE page should rank causes by how often they occur. We looked for Australian data on how often leaks are found at each location and did not find any from a regulator, a utility or the industry association; the rankings in circulation come from businesses selling leak detection or repairs. So the order above is SPASA's, not a frequency ranking, and the pump-on and pump-off test is the better guide to where your leak is.

The quotable version: nobody publishes an independent count of where Australian pool leaks are found, so let the pump test point you, not a top-five list.

On a concrete pool, a crack or failed render may mean the surface itself is at the end of its life — see pool resurfacing cost. On a fibreglass shell, fibreglass pool problems and what to check covers what else to look at.

Who to call once you know it is a leak

The answer depends on the age of the pool more than on the size of the leak.

A pool built in the last few years may be a defect the builder has to fix, and the time limits matter. Do not drain it or patch it first: follow what to do when a new pool is leaking and, if it stalls, what to do when the builder has not fixed the defects.
An older pool with a pressure leak is a plumbing investigation. Bring both bucket results and the litres-per-day figure, and ask for the method and price of locating the leak before any excavation.
An older pool with a static leak and a worn surface may be a renovation question rather than a repair. What a pool renovation costs sets out the published ranges.

The quotable version: arrive with a number in litres and a pump-on and pump-off result, and a leak investigation starts where a guess would finish.

If the answer is renovation or replacement, compare quotes on the same scope: you can get free pool quotes, or browse pool builders in Sydney, Brisbane or Melbourne.

Common questions

How much water does a pool lose to evaporation per day in Australia?

It depends on the month and the place. SPASA puts an uncovered average backyard pool at around 7mm a day in the summer months. Bureau of Meteorology Class A pan figures for Sydney Airport average 7.3mm a day in January and 2.5mm in June, with an annual mean of 5.0mm.

How do I know if my pool is leaking or evaporating?

Do a bucket test. Stand a bucket of pool water on a step, mark the level inside the bucket and the pool level outside it, turn the pump off and check after 24 hours without topping up. If the pool dropped more than the bucket, the difference is a leak. If they dropped the same, it is evaporation.

How many litres is my pool losing?

Multiply the pool's surface area in square metres by the drop in millimetres: one millimetre over one square metre is one litre. An 8m by 4m pool is 32 square metres, so a leak of 5mm a day beyond evaporation is 160 litres a day, or 4,800 litres over 30 days.

Why does my pool only lose water when the pump is running?

That pattern points to a pressure leak. SPASA describes pressure leaks as causing water loss only when the filter system is running, as distinct from static leaks, which lose the same amount consistently. Check the plumbing, valves and joints, and for wet ground between the pool and the equipment pad.

Is losing 5cm of water a week normal for a pool?

In summer it can be. At around 7mm a day, which is SPASA's summer figure for an uncovered pool, a week of evaporation is about 49mm. In winter the Bureau of Meteorology's Sydney Airport figures fall to 2.5–3.7mm a day (June to August), so the same loss in June is much more likely to be a leak.

Sources

General information only, current at 2026-09-28. Pool rules differ by state and by council, and penalty amounts are updated regularly. Confirm your own obligations with your council or a registered inspector before acting.

How to tell if your pool is leaking