To tell if your pool is leaking, do the bucket test: float a bucket of pool water on a step, mark both levels, wait 24 hours and compare. If the pool drops noticeably more than the bucket, it's a leak, not evaporation. Pools most often leak at the skimmer, liner, light fitting or underground pipework. Losing more than about a quarter of an inch a day beyond the bucket points to a real leak.
A pool that needs topping up more than usual is one of those problems that's easy to shrug off, at least until the water bill climbs, the ground around it stays soggy, or the chemicals never seem to balance. The tricky part is that every pool loses water to evaporation, so a falling level doesn't automatically mean a leak. This guide shows you how to separate normal evaporation from a genuine leak with a simple test, where pools tend to leak, and when it's worth bringing in a specialist.
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Is it a leak, or just evaporation?
This is the question to settle first, because it changes everything you do next. Every uncovered pool loses water to evaporation, and the rate goes up in warm, dry, breezy weather. That is exactly the conditions a Cornish or Devon summer brings. A pool can quite normally drop a noticeable amount over a week of sunshine and wind without anything being wrong.
A leak, by contrast, keeps losing water regardless of the weather, and usually faster than evaporation alone can explain. The widely used industry rule of thumb is that losing more than around a quarter of an inch of water a day, over and above what evaporation accounts for, points to a leak. The cleanest way to measure that "over and above" is the bucket test below, which holds evaporation constant so you can see what's left.
How to do the pool bucket test
The bucket test is the simplest, most reliable do-it-yourself check for a pool leak. It works on a neat principle: a bucket of pool water and the pool itself sit in the same sun and air, so they lose water to evaporation at the same rate. Anything the pool loses beyond the bucket is a leak. Here's how to run it.
- Top the pool up to its normal level and turn off the pump and any auto-fill device, so nothing is adding or moving water during the test.
- Fill a bucket about three-quarters full with pool water. A 9 to 19 litre (2 to 5 gallon) bucket works well; smaller containers are less accurate.
- Stand the bucket on the first or second step of the pool so it's partly submerged. This keeps the water inside it at the same temperature and exposure as the pool.
- Mark both water levels. Use a waterproof marker or tape to mark the water line inside the bucket and the pool's water line outside it.
- Wait 24 hours, then compare how far each level has dropped. Don't run it over rain or a big weather change, which will skew the result. Repeat it if conditions turn.
If you normally run the pump, it's worth doing the test twice: once with the pump off and once with it on. A difference between the two runs is a useful clue about where the leak sits, which we come back to below.
Reading the result
Once the 24 hours are up, line up the two drops:
| What you see | What it usually means |
|---|---|
| ✔ Pool and bucket dropped by roughly the same amount | Normal evaporation, no leak indicated |
| ✘ Pool dropped noticeably more than the bucket | A leak: water is escaping beyond evaporation |
| ✘ Pool loses more with the pump running | Points to the pressure side (return pipework) |
| ✘ Pool loses more with the pump off | Points to the suction side or skimmer |
If the two levels are close, you've ruled a leak out and saved yourself a lot of worry. If the pool has clearly outpaced the bucket, the next job is working out where the water is going.
Where pools commonly leak
Pool leaks tend to cluster in a handful of predictable places. Knowing the usual suspects helps you check the right spots first.
The skimmer
The skimmer is the single most common culprit. Leaks develop where the skimmer's plastic body meets the pool wall, or in cracks in the skimmer itself. Constant exposure to sun, rain and temperature swings makes the plastic brittle over time, and shifting ground stresses both the body and the pipework joined to it.
The liner
On vinyl-liner pools, the liner is a frequent source of leaks: small tears, separated seams, or failed seals around the points where the liner is cut for fittings such as lights, steps, returns and the skimmer. Liners most often fail where they're stretched (walls, corners, steps) or gasketed (around those fittings).
The pool light
A pool light sits in a housing with a conduit running out of the back to carry the cable to a junction box. That conduit isn't watertight and can crack as the surrounding ground settles, letting water track away behind the wall. Light leaks are easy to miss because the loss happens out of sight.
The pipework
Underground plumbing (the suction lines from the skimmer and main drain, and the pressurised return lines feeding water back in) is a major hidden source. A leak here often shows up as soft, wet or sunken ground near the pipe run, and it's the type that benefits most from professional water leak detection rather than digging on a hunch.
The shell or structure
Concrete and rendered pools can crack as the ground beneath them moves, and those cracks may weep slowly. Structural leaks are less common than fitting and pipework leaks, but they're worth keeping in mind if everything else checks out.
Other signs your pool is leaking
Beyond a falling water level, a few telltale signs point to a leak rather than evaporation or splash-out:
- Soggy or eroding ground around the pool, the equipment pad, or along a pipe run.
- Air bubbles from the return jets, which can indicate air being drawn in through a leak on the suction side.
- Chemicals and water needing constant topping up, because fresh water keeps diluting your balance.
- Cracks in the deck or pool surround, or tiles that have started to come loose.
- An unexplained rise in your water bill, the same red flag that often signals a hidden leak indoors. Our guide on a high water bill with no visible leak covers how to read those signs.
How to narrow down the leak yourself
Once the bucket test confirms a leak, a couple of simple checks can help you (or your specialist) zero in faster:
- Watch where the level settles. If the water stops dropping at the skimmer mouth, the leak is likely at or around the skimmer. If it stops at a return or light, suspect that fitting. If it keeps falling well below all the fittings, the leak is probably in the floor, main drain or shell.
- Try the dye test. With the pump off and the water perfectly still, release a little leak-detection dye (or food colouring) from a syringe right next to a suspected crack or fitting. If there's a leak, the dye gets drawn into it; if not, the colour just hangs and disperses. It's fiddly but genuinely useful around the skimmer throat and visible cracks.
- Note the pump-on versus pump-off difference from your bucket tests, as above, to tell pressure-side from suction-side leaks.
These checks are safe, cheap and informative. What they can't do is pinpoint a leak buried in a pipe run or behind a solid wall without guesswork. That's the point where digging blind starts to cost more than it saves.
When to call a leak detection specialist
If you've confirmed a leak but can't see where it's going, or the trail leads to the underground pipework, that's the moment for professional help. A specialist locates the leak without excavating on guesswork, using methods such as acoustic listening, pressure-testing the lines and tracer gas to find the exact point, so any repair is targeted and the disruption is kept to a minimum.
Across Cornwall and Devon, our swimming pool leak detection service does exactly this: we find the leak fast, with the least possible digging, and tell you precisely what needs repairing. If the loss has driven up a water bill or caused damage you may need to claim for, the same non-invasive approach produces the documentation insurers look for. Our guide to what trace and access cover is explains how that side works.
Frequently asked questions
How much water loss is normal for a pool?
A typical uncovered pool loses water to evaporation every day, and the amount rises in hot, dry, windy weather. As a rule of thumb, losing more than around a quarter of an inch a day, beyond what the bucket test shows, points to a leak rather than evaporation. The bucket test is the reliable way to tell the two apart.
What is the pool bucket test?
The bucket test compares how fast your pool loses water against a bucket of pool water sitting in the same conditions. Both lose water to evaporation at the same rate, so if the pool drops noticeably more than the bucket over 24 hours, you have a leak. It's the simplest do-it-yourself check for pool leaks.
Where do swimming pools most commonly leak?
The most common leak points are the skimmer (where it meets the pool wall), the liner (tears or failed seals, especially around fittings), the pool light conduit, and the underground pipework, including return and suction lines. The structure itself can also crack as the ground shifts.
Can a pool leak just be evaporation?
Yes, and it's the first thing to rule out. Pools lose water to evaporation all the time, more so in warm, breezy conditions. The bucket test separates normal evaporation from a genuine leak: if the pool and the bucket lose roughly the same amount, it's evaporation, not a leak.
Does the pump being on or off change where a pool leaks?
It can help pinpoint the leak. If the water level drops faster with the pump running, the leak is more likely on the pressure side (return plumbing). If it drops faster with the pump off, suspect the suction side or the skimmer. Noticing when the loss is worst gives a specialist a useful head start.
Should I find the pool leak myself or call a specialist?
The bucket test and a visual check are well worth doing yourself, as they confirm whether you have a leak and roughly where. Pinpointing a hidden leak in the pipework or shell without digging needs professional equipment, such as acoustic listening and pressure testing, which is where a leak detection specialist saves you time, water and unnecessary excavation.
Confirmed a leak but can't find it? We'll trace it, without the digging
We pinpoint swimming pool leaks across Cornwall & Devon with non-invasive equipment, so any repair is targeted and the disruption is kept to a minimum. Fast response, no call-out fees.
Call Dickie on 07822 025 911 No Find, No Fee on residential leak detection (subject to terms)