Most "my pool is leaking" calls aren't leaks.
They're evaporation, and the owner has never actually measured it. In Phoenix in July a pool loses a quarter to half an inch a day to evaporation alone. Add a heater and a bit of wind and it's more. That's two to four inches a week, disappearing, entirely normally.
So before anybody quotes you for leak detection, spend a day proving there's something to find.
Key takeaways
- Evaporation of a quarter to half an inch a day is normal in hot dry weather — call it 1.75 to 3.5 inches a week. Past that, suspect a leak.
- The bucket test costs nothing and settles the question in 24 hours. Do it before you call anyone.
- Losing more water with the pump running points at pressure-side plumbing. Losing more with it off points at the shell or suction side.
- Dye testing only works with the pump off and the water dead still. Any current and you'll chase your own dye around.
- Leak detection services run $250-$600. Cheap compared to digging in the wrong place.
Start with the bucket test
Before anything else, and before you pay anybody: prove there is something to find.
Weight a bucket, stand it on a pool step so it sits about two-thirds submerged, fill it until the water inside matches the pool outside, mark both, and wait a day. Both drop from evaporation. Only the pool drops from a leak.
Then run it a second time with the pump off. That comparison is what separates plumbing from shell, and it decides who you call.
The full method is here — the five things that quietly invalidate the test, how to convert a drop in inches into gallons, and how to read the pump-on run against the pump-off one. It is worth doing properly, because every leak detection company will ask whether you have, and a result you can hand them shortens the job.
One finding from it is worth repeating up front, because it saves the most time: a pool that drops a few inches and then holds has a leak at exactly that level. The water has fallen below the hole. Nine times out of ten that band contains the skimmer throat, a return fitting or the tile line — and you have gone from searching a whole pool to searching a stripe.
Look at the easy things first
Before dye testing anything:
The equipment pad. Drips at unions, a weeping pump seal, a filter clamp band that's wet. Run the pump and look properly — get down at ground level.
The backwash line. A multiport valve with a worn spider gasket dribbles water out the waste line constantly. Stick your hand in the end of the backwash pipe. If it's wet with the valve on FILTER, there's your leak, and a spider gasket is about $15.
The ground. Walk the plumbing runs. Soft spots, sunken pavers, a patch of grass that's noticeably greener and growing faster than the rest. Underground leaks announce themselves through the lawn long before anything else.
Around the pool shell. Erosion at the edge of the deck, settling coping, a gap opening at the expansion joint.
Dye testing the shell
Only works in dead still water. Pump off, cleaner out, and give it a couple of hours to settle. Any current at all and you'll just watch dye drift around and conclude nothing.
Get in with goggles and a bottle of leak dye — or dark food coloring, which works fine — and squeeze a small amount right next to each suspect area:
- The skimmer throat, where the plastic meets the shell. The single most common shell leak on any pool.
- The light niche, particularly around the conduit entry.
- Return fittings, at the joint between fitting and wall.
- Main drain, around the cover and the sump.
- Steps and benches, especially where a step meets the wall.
- Any visible crack in plaster.
- The tile line, where grout has fallen out.
If there's a leak, the dye gets drawn into it in a thin stream. It's unmistakable when you see it.
On a vinyl liner, also check every seam, the corners, and around the fittings. Liner leaks are frequently tiny — a pinhole from a dropped earring or a dog's claw. Run your hand slowly over the liner in the general area; you can often feel the draw before you see it.
Isolating the plumbing
If the bucket test points at plumbing, you can narrow it down without any special gear.
Plug the returns with rubber expansion plugs — a set is about $25. Plug the skimmer too. Then top the pool up and watch it for 24 hours with the pump off.
- Loss stops: the leak is in the plumbing you just isolated.
- Loss continues: it's the shell.
Then unplug lines one at a time to work out which one. Tedious, and it costs nothing but time.
Pressure testing is the professional version — capping a line, pressurizing it with air or water, and watching a gauge for a drop. It's how a tech pinpoints an underground leak, along with acoustic listening equipment that hears water escaping through soil.
What it costs
Leak detection service: $250–$600 typically. Worth every penny compared to a wrong guess with a jackhammer.
Repairs, roughly:
| Repair | Typical cost |
|---|---|
| Spider gasket in a multiport | $15 part, DIY |
| Return fitting reseal | $30–$150 |
| Skimmer throat repair (epoxy) | $150–$500 |
| Skimmer replacement | $600–$1,500 |
| Vinyl liner patch | $15 DIY, $150–$300 with a tech |
| Light niche conduit seal | $200–$600 |
| Underground pipe repair | $500–$3,000+ depending on access |
| Full re-plumb of a line | $1,500–$5,000 |
The spread on that underground row is entirely about what's on top of the pipe. Under lawn, cheap. Under a stamped concrete deck, expensive.
The one that catches people out
A leak under a fiberglass or concrete shell in a high water table can float the pool if you drain it to investigate.
Don't drain a pool to look for a leak without understanding your groundwater situation. Fiberglass shells especially — I've seen one lift three feet out of the ground overnight after a well-meaning owner emptied it. That's a five-figure mistake and it isn't recoverable.
Hydrostatic relief valves exist for this. If you don't know whether yours works, that's a conversation with a pro before any water comes out.
Go deeperLevel stopping at the skimmer?That is the classic cracked-skimmer signature. Here is how to confirm it.Frequently asked questions
How much water loss is normal for a pool?
A quarter to half an inch a day from evaporation in hot dry conditions, which is 1.75 to 3.5 inches a week — and 3.5 is the top of normal, not the middle of it. Less in humid or cool weather, more with a heater running, a lot of wind or no cover. Losing an inch a day or more is a leak.
How do I do a bucket test on a pool?
Put a weighted 5-gallon bucket on a step so it's mostly submerged. Fill it so the water inside matches the pool level, then mark both. After 24 hours, compare. Evaporation affects both equally, so if the pool dropped more than the bucket, the difference is your leak.
Does a pool leak more with the pump on or off?
Depends where it is. A pressure-side plumbing leak gets worse with the pump running because the pipes are pressurized. A suction-side leak often gets worse with the pump off, because when running it draws air rather than losing water. A shell leak loses at roughly the same rate either way.
How much does pool leak detection cost?
Typically $250-$600 for a residential pool depending on region and how much of the plumbing needs pressure testing. Worth it — a good technician with listening equipment and pressure testing gear will locate it in a few hours, versus digging up your deck on a guess.
Can a pool leak fix itself?
Very occasionally a small liner or plaster leak seals with debris, and that's temporary. A leak that seems to have stopped usually means the water level dropped below the leak. That's a clue about where it is, not a repair.
