Pool losing water? How to tell evaporation from a leak
Every outdoor pool loses water to evaporation, and on hot, dry, windy days it can be a lot. The bucket test tells you, with no special tools, whether your pool is losing more than evaporation explains. If it is, a few more checks narrow down where the leak is.
The bucket test
A bucket of water sitting in the pool sees the same sun, wind, heat and humidity as the pool. So it evaporates at about the same rate. If the pool's level falls clearly more than the bucket's, the extra loss is a leak (or splash-out).
- Bring the pool to its normal level. Turn off any automatic fill valve.
- Fill a bucket with pool water to about an inch from the top, and set it on the first or second step, so the water inside and outside the bucket are at roughly the same height. On an above-ground pool, put the bucket on the ladder step or a sturdy stand in the pool.
- Mark the water level inside the bucket. Mark the pool's water level on the outside of the bucket too, or on the skimmer face.
- Run the pump as normal, and don't swim, splash or add water.
- After 24 hours, compare. If the pool dropped more than the bucket, most guides treat a difference of more than about a quarter inch as a sign of a leak.
- Repeat once with the pump off for 24 hours.
What the pump-on and pump-off results tell you
| Result | Likely area |
|---|---|
| Loses more with the pump running | The pressure side: return lines, the filter, valves or fittings after the pump. Check the equipment pad for drips and the backwash or waste line for flow. |
| Loses more with the pump off | The suction side (skimmer and main drain lines), where the pump pulls water in and can draw air rather than leak water while running. |
| Loses the same either way | The pool itself: a liner tear, a crack in the shell, a light niche, the skimmer throat or a return fitting. |
| Stops dropping at a certain level | The leak is probably at that height, such as the skimmer, a return, a light or a tear in the liner at that line. |
Your test kit can help too
Evaporation removes only water. Stabilizer (CYA), calcium hardness and salt stay behind, so in a pool that's only evaporating and being topped up, those readings slowly creep up. A leak takes them out with the water. If salt or CYA keeps dropping and you haven't been draining, backwashing or getting heavy rain overflow, water is leaving the pool somewhere. This works best with salt, because salt readings are fairly precise. The drain and refill calculator shows the same dilution math in reverse: a 10% drop in salt means roughly 10% of the water was replaced.
Turning inches into gallons
For a 16 × 32 ft pool (512 sq ft) that loses half an inch a day: 512 × 0.5 ÷ 12 × 7.48 ≈ 160 gallons a day. Over a month, that's about 4,800 gallons, a large share of a typical pool, and all of it needs rebalancing. The pool volume calculator gives your surface area.
Where leaks usually hide
- Equipment pad: pump seals, filter tank O-rings, valve stems, unions and the backwash line. Easy to check first, and often the culprit.
- Skimmer: the joint between the skimmer and the pool shell or liner is a common spot, especially after freezing winters.
- Return fittings and lights: gaskets and the conduit behind light niches.
- Vinyl liners: small tears, often near steps, ladders and corners.
- Plaster and gunite pools: cracks in the shell or tile line.
A dye test helps confirm a suspected spot. With the pump off and the water still, squeeze a little pool test dye (or food coloring) near the fitting or crack. If the dye is drawn into it, water is escaping there. Underground pipe leaks usually need a leak detection company with pressure-testing and listening equipment.
What to do while you look for it
- Keep the water above the bottom of the skimmer opening, so the pump doesn't run dry and lose prime.
- Retest and rebalance after large top-ups: fresh water dilutes chlorine, alkalinity and stabilizer.
- If the water level is dropping fast or you see wet ground, sinking soil or water near the equipment, turn the pump off and call a professional. Large leaks can wash out soil under a pool or deck.
Frequently asked questions
How much evaporation is normal?
It depends heavily on weather: sun, heat, wind and dry air all speed it up, and a heated pool on a cool night loses much more. That's why the bucket test compares against the actual conditions instead of a fixed number. A pool cover cuts evaporation a lot.
Does running the waterfall or fountain increase water loss?
Yes. Spraying and splashing water increases evaporation and can throw water out of the pool. Run the bucket test with water features in their usual setting, then again with them off if you suspect them.
Will a leak affect my chemicals?
Yes. Each top-up brings in fresh water that dilutes stabilizer, salt and alkalinity, so a leaking pool needs more frequent rebalancing. Fixing the leak usually makes the chemistry easier to keep steady.