How do I find a leak in my pool's underground plumbing?

Before you spend a dime, find out whether the water is leaking or just evaporating. The bucket test gives you a side-by-side comparison, because the bucket loses water to the air at the same rate your pool surface does. In a Phoenix summer, a pool can lose a quarter inch or more a day to evaporation alone, so this step saves a lot of guessing.

Run it like this:

1. 1Fill a clean five-gallon bucket with pool water and set it on a pool step so the water inside and the pool outside start at the same level.
2. 2Mark both levels with tape or a grease pencil, one line for the water inside the bucket and one for the pool surface.
3. 3Turn the pump and any autofill off, and keep swimmers out for the test window.
4. 4Wait 24 hours, then compare how far each line dropped.

If the bucket and the pool dropped about the same amount, you are looking at normal evaporation, not a leak. If the pool dropped more than the bucket, the difference is your leak. A common rule of thumb is that a gap of more than about a quarter inch points to a real leak rather than evaporation. Run the test over two or three days for a clearer reading, since wind and temperature swings can skew a single day.

Evaporation is a real physical process, not a fudge factor. As the USGS puts it, "Evaporation is the process by which water changes from a liquid to a gas or vapor." The hotter and drier the air, the faster that change happens, which is why a Phoenix pool in July behaves nothing like the same pool in December. The bucket cancels that variable out so you can trust the result.

A few habits make the bucket test more accurate. Run it during a calm stretch of weather, since a windy day strips extra water off both the pool and the bucket and can exaggerate the loss. Keep the bucket in the shallow end on a step so it stays in the same sun and shade pattern as the pool. Top off neither one during the test, and switch off any autofill valve, because a hidden float that refills the pool will mask the very loss you are trying to measure. If your numbers come out inconsistent day to day, repeat the test until you see a steady pattern rather than acting on a single odd reading.

A bucket test confirms loss, but other clues tell you a leak is already at work and where it might be. Watch for these signs around the pool and equipment pad:

• Water loss beyond normal evaporation, especially more than about a quarter inch a day once the bucket test rules out the weather.
• Soggy or wet spots in the yard, settling soil, or persistent puddles near the pool, the equipment pad, or along the run of buried pipe.
• Air bubbles at the return jets, which can mean a suction-side line is drawing air through a crack instead of pulling only water.
• A rising water bill with no change in how you use water, since an autofill quietly tops off a leaking pool and hides the loss.
• Needing more chemicals than usual, because fresh fill water dilutes your chlorine and throws off the balance.
• Cracks, settling, or shifting in the deck, the bond beam, or the pool shell itself.

These signs matter because pool leaks waste a surprising amount of water. EPA WaterSense reports that household leaks nationwide waste nearly one trillion gallons of water annually, and that the average home loses more than 9,400 gallons a year to leaks. A leaking pool can dwarf those household figures on its own, so catching one early protects both your water bill and the structure around the pool.

Once you know water is escaping, a dye test helps you find a leak in the pool shell, around fittings, or near a visible crack. It works because still water near a leak is slowly being drawn into the opening, and colored dye makes that hidden movement visible. You can buy leak-finding dye or use a few drops of dark food coloring.

Do it this way:

• Turn the pump off and let the water go completely still. Moving water scatters the dye and hides the pull.
• Get in the pool or reach down to a suspected spot, such as a crack in the plaster, a light niche, a skimmer throat, or a return fitting.
• Squirt a small amount of dye right next to the suspected area without stirring the water.
• Watch the dye. If it drifts toward the spot and gets pulled in, water is escaping there. If it just hangs in a cloud, that spot is sealed.

The dye test is cheap, repeatable, and good for confirming a crack you can see or reach. Common spots to check are the skimmer, where the throat meets the pool wall and where the skimmer body joins its pipe, the return fittings and their eyeball jets, the main drain at the deep end, light niches, and any visible crack in the plaster or tile line. Work methodically around the pool so you do not miss a spot, and retest anywhere the result looks borderline.

It does have a hard limit. The dye test cannot find buried-line leaks, because the leak is in a pipe under the deck or yard, not in the pool wall where you can place the dye. If the shell and fittings test clean but the pool still loses water, the leak is almost certainly in the underground plumbing. A useful clue: if the pool keeps dropping even with the pump and lines turned off and isolated, the loss is in the shell; if it drops faster while the pump runs and the lines are pressurized, suspect a pressure-side pipe leak.

When the bucket test confirms a leak and the dye test clears the shell, the problem is in the underground plumbing that connects the pool to the pump. These lines, the main drain, the skimmer, and the returns, run buried under concrete and soil, so you cannot see, dye, or reach the break. This is where guesswork gets expensive, because digging in the wrong place wrecks decking for nothing.

A professional finds the break with pressure testing. The technician isolates each line in turn, plugs it, and pressurizes it with air or water while watching a gauge. A line that holds pressure is sound. A line that loses pressure has the leak, and the rate of loss and listening equipment help localize the break to a short section before any digging starts. Testing each line separately, rather than the system as a whole, is what turns "somewhere underground" into "right here." As Pentair notes in its troubleshooting guidance, leak diagnosis starts by checking the equipment and fittings, then moves to the plumbing lines themselves when the visible parts test sound.

Pressure testing also protects your equipment. Both Pentair and Hayward design their pumps and filters to run with full, primed lines, and a suction-side leak that pulls air can cause the pump to lose prime, run dry, and overheat. Finding and sealing that leak keeps the whole system running the way the manufacturer intended. If you suspect the pump itself rather than the buried lines, see our guide on why is my pool pump leaking.

Some leaks are worth a pro from the start. Call a leak-detection specialist when the bucket test shows steady loss but the dye test finds nothing, when you see soggy ground or settling along the pipe run, when air keeps coming out of the returns, or when the pump struggles to hold prime. These all point to buried plumbing, which is past the reach of a do-it-yourself test.

A specialist brings tools you do not have at home: pressure-testing rigs, acoustic listening gear, and sometimes tracer gas, all aimed at pinpointing the break before anyone breaks concrete. The same non-invasive approach plumbers use to locate hidden leaks inside a house applies under a pool deck, and you can read more in our guide on how plumbers find hidden leaks. Paying to find the exact spot is almost always cheaper than digging up a deck on a hunch.

In the Phoenix area, where slab construction, hard water, and long pool seasons all add wear, getting the diagnosis right the first time matters. Start with the bucket test to confirm you actually have a leak, use the dye test to clear the shell, and bring in a pro to pressure-test the lines when the water keeps dropping with no visible cause. That order saves water, money, and your decking.