Why Is My Pool Losing Water? Evaporation, Leaks and Fixes

Why is my pool losing water? If it's about a quarter inch a day in summer, it's almost certainly evaporation, and a hot, dry, windy week can push that toward half an inch. A pool that loses an inch a day, keeps dropping under a solid cover, or loses a different amount with the pump on than with it off is probably leaking. The tests below tell the two apart within a day or two, and the last section covers putting the water back.

How much water loss is normal

Pool professionals commonly treat about a quarter inch a day in warm weather as normal, within roughly an eighth to half an inch depending on conditions. The EPA's WaterSense program puts its leak line at the weekly total: a pool losing more than 2 inches a week, or more than 3 inches in hot, dry areas, probably has a leak. Evaporation rises with air temperature, wind, low humidity and warm water, and falls with a cover, shade and still air. A heated pool in a dry, windy spot can lose more than half an inch a day, while a covered pool in humid weather can lose almost nothing.

What a quarter inch means in gallons depends on the surface area. One square foot of surface, one inch deep, holds about 0.62 gallons.

A 20 x 40 pool evaporating normally loses roughly 870 gallons a week in summer. Topping that off from a hose takes about 1.5 to 3 hours at 5 to 10 gallons a minute, which is why evaporation alone rarely justifies a truck. The pool evaporation rate guide goes into how climate and season change the number.

The decision tree: evaporation, splash, leak

Work through these in order and stop at the first one that explains your loss.

1. Is the loss more than about 2 inches a week (3 inches in hot, dry weather), or steady through cool, humid, still days? If not, it's most likely evaporation. Measure it anyway with the bucket test so you have a baseline.
2. Did it start after a change in how the pool gets used, such as kids, a dog, a new slide or a lot of backwashing? On a busy week, splash-out and backwash together can match evaporation.
3. Is the loss the same with the cover on? The EPA says a solid cover cuts evaporation by more than 90 percent, but a cover does nothing for a leak. A pool that drops a quarter inch overnight under a solid cover is leaking.
4. Does the bucket test show the pool dropping faster than the bucket? Then it's a leak, and the next question is where.
5. Does the loss change with the pump on versus off? That points you to the plumbing or to the shell.
6. Where does the level stop dropping? A pool that leaks down to a certain level and stops is usually leaking at that level, at a skimmer, a light niche, a return fitting or a crack along the tile line.

That first question explains most "pool losing water but no leak" cases. It's July, the air is dry, and a quarter inch a day over a week adds up to nearly two inches, which looks alarming on the tile line and is still normal.

Step 1: bucket test

The bucket test separates evaporation from everything else, because the water in the bucket evaporates at about the same rate as the pool but can't leak.

1. Fill a 2.5 to 5-gallon bucket with pool water to about an inch from the top and set it on a step or bench so at least two-thirds of it sits underwater, with the water inside a little higher than the pool. Weight it with a brick if it floats.
2. Mark the water level inside the bucket with tape or a marker, and mark the pool level on the outside of the bucket at the same time.
3. Turn off the auto-fill and leave the pool alone for 24 hours, with no swimming or backwashing. The EPA's version has the pump off too, and Step 2 explains why you'd also run it with the pump on.
4. Compare the two drops. If the pool fell the same amount as the bucket, that's evaporation. If it fell more, the difference is a leak, although an eighth of an inch can be measuring error.

Give it the full 24 hours so a small leak has time to show. The bucket test guide covers the setup and the usual mistakes, such as leaving the auto-fill on.

Step 2: pump on vs pump off

Repeat the bucket test twice, once with the pump running for 24 hours and once with it off for 24 hours, and compare the two leak figures (pool loss minus bucket loss).

If it loses more with the pump on, look at the pressure side: the return lines, heater, filter, backwash valve or a return fitting. When the system is pressurized, water gets pushed out through any crack, so check for wet ground near the equipment pad or along the return line route.

If it loses more with the pump off, look at the suction side or the shell: the skimmer, the main drain line, the light niche or a crack. While the pump runs, the suction lines are under vacuum, so a crack pulls air in instead of letting water out. Once the pump stops, the same crack leaks water.

If it loses the same either way, the problem is likely in the structure, such as a crack in the shell, a liner tear, a failed seal between the skimmer and the wall, or a fitting where the plumbing passes through the shell.

That one comparison rules out a large part of the pool before you start looking. Pump on vs off has the full diagnostic table.

Step 3: dye test and the usual leak spots

With the pump off and the water perfectly still, squeeze a little leak-detection dye (or food coloring from a syringe) about half an inch from a suspected spot. If the dye gets pulled in, you've found it. Check these spots in order of likelihood:

1. The skimmer. The seal between the plastic skimmer body and the concrete or liner wall is one of the most common leak points on both pool types. Dye the inside of the skimmer mouth and the bottom of the throat.
2. Return fittings and the light niche. The conduit behind a pool light is a common leak path. Dye the face of each fitting.
3. The tile line and any visible crack. Plaster cracks that go through the shell can leak, while surface crazing usually doesn't.
4. The main drain. It's hard to dye test without goggles and a calm day, and if everything else checks out clean, this is where a leak detection company would pressure test.
5. On a vinyl liner, check every seam, every fitting and anywhere the liner has pulled away from the wall. Even a small hole can lose a surprising amount of water over a week.

If the pool stopped dropping at a specific level, dye test at that level first. Underground plumbing leaks, often given away by a wet patch in the yard that never dries, need a pressure test on each line by a leak detection company.

Backwash, splash-out and auto-fill errors

A few things get mistaken for leaks every summer.

Each sand or DE filter backwash sends anywhere from about a hundred to a few hundred gallons to waste, depending on the filter and how long you run it. On a 16 x 32 pool, where an inch is about 320 gallons, three backwashes in a week can take an inch or more that was never evaporation or a leak. Note the level before and after.

A pool party or an afternoon of cannonballs can splash out an inch on its own. If the level drops on weekends and holds on weekdays, that's your answer.

Then there's the auto-fill. An auto-fill valve hides a leak by replacing the water as fast as it leaves, so the signs are a water bill that has climbed or a valve that runs all the time. A stuck valve can also overfill the pool and push water out the overflow, which shows on the bill but not on the tile. Turn it off for every bucket test. The pool auto fill guide covers how to check one.

As for the level itself, the water should sit at about the middle of the skimmer opening. Below that the skimmer gulps air and the pump can lose prime, which is a far more expensive problem than the water. Pool water level explains the mark.

Topping off: hose, auto-fill or truck

Once you know the cause, put the water back the cheapest way that works.

For evaporation and splash-out on city water, use the hose. A 24 ft round pool 3 inches down needs about 840 gallons, roughly 1.5 to 3 hours of hose. An auto-fill does the same job automatically and earns its keep in hot, dry climates, as long as you check the level by hand now and then so it isn't hiding a leak.

A leak repair that needed a partial drain is where the gallons get big. A 16 x 32 pool drained 2 feet below the skimmer for a return-line repair needs about 7,700 gallons. That's 13 to 26 hours of hose, or one load on an 8,000-gallon truck (two on a 6,000-gallon truck). On a well it's the same 13 to 26 hours of pump run time, and the truck is the obvious call.

Big pools well down, or a local restriction on filling from the tap, change the math. A 20 x 40 pool a foot down needs about 6,000 gallons, a full load on many trucks. Haulers price per trip, so a partial load costs about the same as a full one, and the truck makes sense when the gallons approach a load or the hose isn't an option. Our pool top-off page covers when a delivered top-off is worth it. After any top-off of more than a few inches, test and rebalance, because fresh water dilutes the stabilizer and sanitizer.

Frequently asked questions

How much water should a pool lose a day?

About a quarter inch a day in warm weather is normal evaporation, within a range of roughly an eighth to half an inch depending on heat, wind, humidity and whether the pool is covered. The EPA treats more than 2 inches a week (3 inches in hot, dry areas) as a likely leak, and any loss under a solid cover deserves a bucket test.

Why is my pool losing an inch a day?

An inch a day is far beyond normal evaporation, so it's almost always a leak, or a leak combined with heavy backwashing or splash-out. Run the bucket test with the pump off for 24 hours, then with it on, and dye test the skimmer first.

Why is my pool losing water but there is no leak?

Usually it's a hot, dry, windy week, and a quarter inch a day adds up to nearly two inches, which looks like a lot on the tile line. Backwashing and splash-out add to it. The bucket test settles it, because if the bucket dropped the same amount as the pool, nothing is leaking.

Does a leaking pool need to be drained to fix?

Often not. Skimmer seals, return fittings, light niches and small liner punctures can usually be patched underwater or with the level dropped a few inches. Main drain and underground line repairs may need a partial drain, and refilling after one can run to a tanker load.

Leak fixed and still low? A top-off may still be one minimum load, and if the hose is the smarter choice for your pool, we'll say so in the reply. Get your delivered price.