Liner and structure loss, and what a stopping level tells you
If the water level falls and then stops at a particular height, the leak is at that height, because a hole cannot let water out once the water is below it. That single observation costs nothing and narrows a structural loss further than another day of testing will.
Last reviewed 14 August 2026
The short answer
Let the pool find its own level, with the pump off and nothing topping it up, and watch where it stops. Water cannot leave through a hole it is no longer touching.
| The level stops at | Which points to |
|---|---|
| The skimmer mouth | The skimmer throat or its seal. The most common single place a pool leaks. |
| A return inlet | That fitting, or the pipe union immediately behind it. |
| A light housing | The niche seal or the conduit behind it. |
| A step or a ledge | The joint where that feature meets the shell or the liner. |
| It does not stop | The floor, the lower wall, or pipework running below the pool. |
This costs nothing and needs no equipment. It is geometry rather than measurement: the leak is at the height the water stops at, because below that height the hole is above the water.
How this works in practice
A structural loss is one the bucket test has already told you does not care about the pump. That leaves the shell, the liner, or a fitting through them, and the stopping level splits those three further.
- Turn the pump off, disable any auto-fill device, and stop topping up. Any of those three will hide the stopping level completely.
- Let it fall for as long as it takes. A slow loss can take a week to find its level, and stopping early tells you nothing.
- Mark the level each day. A rate that slows as the level drops is also informative, because it means the head of water above the fault is falling.
- Note where it settles. That height is the height of the fault.
A loss that never stops is not a failure of the method. It is a result: it says the fault is below the lowest fitting in the pool, which rules out every skimmer, return and light in one observation and points at the floor, the lower wall or buried pipework.
If the loss turned out to be pump-dependent rather than structural, this page is the wrong one. whether the loss depends on the pump separates the circulation from the structure first.
What changes the answer
Three things, and the first two will simply hide the answer if you let them.
- An auto-fill device. It holds the level constant, so the pool never finds its stopping point and the whole method produces nothing. Isolate it first.
- Rain. A wet week can refill a pool faster than a slow leak empties it, which reads as no loss at all. Note the weather alongside the levels.
- How far the loss is below the surface. The deeper the fault, the more head of water is pushing through it, so the same size of hole leaks faster when the pool is full and slows as the level drops. A rate that changes as the level falls is consistent with a single fault rather than several.
Letting a pool drop a long way has consequences of its own on some constructions, particularly with a vinyl liner, which can shift or crease once the water is no longer holding it in place. That is a reason to watch it rather than to leave it indefinitely.
Assumptions and sources
Litres in a cubic metre
Exact by definition
SI definition. The litre is defined as one cubic decimetre, so one cubic metre is exactly 1,000 litres.
Millimetres in a metre
Exact by definition
SI prefix definition. Milli is a factor of one thousandth.
Days used for an annual figure
Editorial convention
Annual figures on this site are a daily rate multiplied by 365. Leap years are not modelled, so a leap year is understated by one day, which is 0.27 per cent.
Nothing on this page rests on a constant or a citation. That water cannot escape through a hole above its own surface is geometry, and the rest follows from it.
The list of fittings is what a UK domestic pool typically has rather than a surveyed set. If your pool has a feature not listed, the same rule applies to it: if the level stops there, look there.
What to do next
Three things, in order, and the first is free.
- Let the pool find its level with the pump off and nothing topping it up. Note the height and how long it took.
- Look hard at whatever sits at that height. A skimmer throat, a return, a light niche or a step joint is a small area to examine compared with a whole pool.
- If it never stops, note that too and treat it as the result it is. It rules out every fitting above the lowest point in one observation.
And check the volume against the meter if the pool is on a metered supply, because a pool loss is a household water loss. confirm the rate of loss at the meter measures it independently of anything happening in the pool.
Common questions
- The level stopped at the skimmer. What does that mean?
- That the loss is at or just below the skimmer, which is the most common place for a pool to leak. The skimmer throat is a joint between two different materials that expand at different rates, and it sits at the waterline where the pool is worked hardest.
- The level keeps falling and does not stop. What then?
- Then the fault is below every fitting it could have stopped at, which usually means the floor, the lower wall, or a pipe running under the pool. That is the harder case, and it is the one where the pump comparison matters most, because it separates pipework from structure.
- How is a liner leak different from a structural one?
- In how it is found and fixed rather than in how it behaves during the test. A vinyl liner leak is often a small puncture or a failed seal at a fitting, and it can sometimes be located by watching dye drift. A leak in a concrete shell is usually a crack or a failed joint. Both are structural for the purposes of the bucket test, because neither cares whether the pump is running.
- Can I find a liner leak myself?
- Sometimes. With the pump off and the water completely still, a small amount of dye released near a suspected point will drift towards a hole rather than dispersing evenly, because water is moving that way. It takes patience and stillness and it does not work while anything is circulating.
- Is a slow structural loss worth fixing?
- That is a cost decision rather than something this site can answer, but the volume makes it concrete. A pool losing 10 mm a day at 32 m² is 320 litres a day, which is 117 cubic metres a year on top of evaporation.