If the pool is dropping in level and you do not know whether that is normal, the bucket test answers one question: is the pool losing more water than evaporates? Over 24 hours it compares the drop in the water inside a bucket placed in the pool with the drop in the pool itself. Both are exposed to the same sun, wind and temperature, so the bucket shows how much evaporates. If the pool drops clearly more than the bucket, that difference is not evaporation: the water is going somewhere.
The test does not locate the leak or say whether it is in the basin or in the pipes. It helps you decide with data whether a diagnosis is needed and reach it with useful information.
What you need
- A plastic bucket (any one of 10 to 20 litres will do) without cracks.
- Waterproof adhesive tape or a permanent marker.
- A ruler or tape measure that reads in millimetres.
- 24 hours without swimming and without adding water.
How to do the bucket test step by step
- Fill the pool to its usual level, normally halfway up the skimmer opening. If it has automatic filling, switch it off: this is the mistake that most often invalidates the test.
- Fill the bucket with pool water to a few centimetres from the rim and rest it on the first or second step of the ladder, so that it is partly submerged and stable. This way the water in the bucket is at the same temperature as the pool. If it is light and floats, put a stone or a brick inside before filling it.
- Mark the two levels: the water level inside the bucket (on the inner face) and the pool level on the outer wall of the bucket or on a tile of the basin. What matters is that both marks are made at the same moment.
- Leave the pump running on its normal cycle during this first test. Turn off waterfalls, fountains, jets and the robotic cleaner.
- Wait 24 hours without swimming, without topping up and without covering the pool or the bucket. If it rains or there is unusually strong wind, discard the test and repeat it another day.
- Measure how much each level has dropped in millimetres and compare.
How to interpret the result
| Result | What it means |
|---|---|
| The bucket and the pool have dropped practically the same | The loss is evaporation. There are no signs of a leak under those conditions. |
| The pool has dropped several millimetres more than the bucket | There is water loss that is not evaporation. Repeat the test to confirm it and note the daily difference. |
| The pool has dropped much more (centimetres) | Significant loss. Move on to diagnosis without waiting for it to get worse. |
| The bucket has dropped more than the pool | Something went wrong: the bucket had a crack, water entered the pool, or the marks moved. Repeat it. |
A difference of one or two millimetres is within the margin of measurement error. That is why the test gains a lot if you repeat it on two or three days and note the values: a small but constant difference is more meaningful than a single reading.
Convert millimetres into litres
Multiply the surface area of the water by the difference in level. In a pool of 8 × 4 metres (32 m²), each millimetre of level is 32 litres, and each centimetre is 320 litres. A pool that drops one centimetre a day more than the bucket is losing more than 2,000 litres a week on top of what evaporates.
The second test: with the pump switched off
Repeating the test with the pump stopped for the 24 hours gives a clue that saves time in the diagnosis, because the circuits of a pool behave differently depending on whether the pump is running or stopped. It is only that, a clue: it points to where to start looking, but it does not replace the watertightness tests of each circuit or the inspection of the basin.
- It loses more with the pump running than stopped: this usually points to the pressure side of the circuit, that is, the delivery pipes that return water to the basin through the jets. With the pump running those pipes are pressurised and a fissure expels water; with the pump stopped they barely lose any. But note: "with the pump running" does not always mean a broken pipe. In a pool in the Region of Murcia that only lost water while the filter was running, the circuits, the basin and the connections were sound: it was the multiport valve, which did not close fully and sent water to the drain while filtering. Before blaming the delivery pipe, check whether water comes out of the drain in the plant room with the pump running.
- It loses more with the pump stopped than running: this usually points to the suction side (skimmer, main drain, cleaner intake). With the pump stopped the water leaves by gravity through the break; with the pump running those pipes are under suction and, instead of losing water, they tend to draw in air, which sometimes shows up as bubbles at the jets or a prefilter that loses its prime.
- It loses the same in both tests: the loss does not seem to depend on the pump, so it is more likely to be in the basin itself or in some element built into it (lights, wall penetrations, skimmer or jet mouths, joints, cracks in the lining). It does not rule out a pipe either: a pool can lose water from more than one place at once.
With these three readings you decide where to start; what confirms where the leak is is checking each circuit separately and, if they hold, inspecting the basin.
If the level stops at one height
When a pool drops and then always stabilises at the same height, that height is a direct clue: the loss is at something located at that level, because below it no water leaves. It is usually the skimmer mouth, a light or a jet outlet. If you have the chance to observe it, note it down.
What can alter the comparison
- Pool filled to the brim: if you fill it up to the coping, it is normal for it to drop to its working level. We once checked a pool in the province of Alicante whose circuits and basin were all watertight: it was simply being filled up to the edge. Do the test with the water at mid-skimmer height.
- Automatic filling or overflow: an active fill valve hides any loss. In overflow pools the basin level stays constant by design and the bucket test is of no use; in that case monitor the balance tank.
- Bathing, waterfalls and jets: water splashing out and surface agitation increase the pool's loss but not the bucket's.
- Rain: it enters the bucket and the pool in different proportions relative to what each one loses. It invalidates the test.
- Cover or thermal blanket: if you cover the pool and leave the bucket uncovered (or the other way round), the comparison no longer holds. Both must be under the same conditions.
- Bucket badly placed: if it is out of the water, in the sun on the rim, its water warms up more than the pool's and evaporates faster. It must be partly submerged.
- Losses at the plant: a pump or filter gasket that drips, an open drain cock or a multiport valve that lets water through to the drain make the pool drop without any buried break. Look under the equipment and at the drain in the plant room before you start.
What the bucket test does not tell you
It confirms that there is a loss and gives a clue as to whether it depends on the pump, but on its own it does not locate the point or diagnose anything. A pool can lose water at the same time through a skimmer gasket and through a delivery pipe, and intermittent losses (a fissure on the waterline that stops losing when the level drops below it) can give a misleading result depending on the day. If the difference is clear and repeats, the next step is diagnosis, not carrying on topping up.
What to do if the pool loses more than the bucket
Before calling anyone, gather these details. They allow the visit to be targeted from the very first moment:
- How much it drops per day in millimetres, with the pump running and with the pump stopped.
- Whether the level stabilises at some height and, if so, which one.
- Type of basin: mosaic tile, liner (reinforced membrane), polyester, fibreglass or concrete.
- Since when you have noticed it and whether it coincides with any works, frost or change to the installation.
- Whether damp areas or subsidence appear around the basin or the plant room, or whether you use more chlorine and the pH goes out of balance for no apparent reason.
When you call us about a pool, the first thing we ask is precisely the type of basin and how much the level drops. On the visit we work with the pool full, because the water pressure in the basin helps locate the exit point: we check the watertightness of the skimmer, delivery and cleaner pipes separately under controlled pressure to isolate the section with the break, and when the problem is in the basin a diver inspects lights, main drain, jets, joints and walls with tracers and a camera, without draining the pool. Do not empty the pool on your own to look for the leak: emptying certain types of pool can put the basin at risk and, in addition, removes the normal conditions in which the loss shows up.
If you want us to take a look, you can tell us about your case in pool leak detection or request a quote through the form. And if the clue points to the circuit and you want to know how leaks in circuit pipes are searched for, we explain it in how a geophone works to detect water leaks: complete technical guide.
