Why Is My Pool Losing Water? Evaporation vs a Leak

Your pool is down an inch and you did not drain it. Before calling a leak company, spend two minutes on the math and 24 hours on a bucket, most "leaks" are just physics doing its thing.

First: is the loss even abnormal?

Evaporation from a pool is driven by water temperature, air temperature, humidity, and wind, and it is bigger than most owners expect. A typical 512 sq ft pool loses 30 to 120 gallons per day in summer conditions, up to a quarter inch of level, and hot dry wind can push that toward half an inch. Run your actual conditions through the pool evaporation calculator: if your observed loss lands inside the estimate, you very likely do not have a leak, you have weather.

Two quick sanity checks before anything else: a rough rule is that losing more than half an inch per day, day after day, regardless of weather, points away from evaporation. And evaporation varies with conditions, while leaks are stubbornly constant, if your loss is identical on a humid still day and a dry windy one, suspect the leak.

The bucket test, done properly

The bucket test works because both the bucket and the pool experience identical weather, so any extra drop in the pool is water going somewhere it should not. The details matter:

1. Fill a bucket two-thirds with pool water and set it on the second step of the pool, weighed down with a rock. Sharing the pool's water temperature is what makes the comparison fair.
2. Mark the water line inside the bucket and the pool's level outside it (tape or a grease pencil on the tile).
3. Turn the pump OFF and leave it off. Wait 24 hours with no swimming, no rain, no auto-fill.
4. Compare the two drops. Equal drops = evaporation, you are done. Pool dropped noticeably more = leak.
5. Now repeat the same 24 hours with the pump ON. Bigger loss with the pump running than off points to a pressure-side plumbing leak; loss only with the pump off points to suction-side or shell; equal loss both ways points to a structural leak below the water line.

Reading the clues

• Level stops dropping at a specific height: the leak is at that height. Stops at the skimmer mouth? Skimmer joint. Stops at the light? Light niche or its conduit.
• Wet spots or unusually green grass near the pool or between pool and equipment pad: underground plumbing.
• Air in the pump basket or bubbles from the returns: suction-side plumbing pulling air, which usually also leaks water when the pump is off.
• Constantly rising water bill plus a running auto-fill can mask a large leak completely; shut the auto-fill off during any test.

What a real leak costs you (beyond water)

A quarter-inch-per-day leak on a 16x32 pool is about 2,400 gallons a month. The water itself might be $15 to $30, but the leak also drains everything dissolved in it: salt (recalculate with the salt calculator after any big refill), stabilizer, and heat. Replacing 2,400 gallons of 85°F water with 60°F fill water quietly adds real money to a heating bill, priced out in the heating cost calculator. Fixing a skimmer leak is usually a few hundred dollars; ignoring it for a season can cost that much in water, chemicals, and heat alone.

When to call a professional

Call a leak detection company (not a general pool cleaner) when the bucket test confirms a leak and the level-stop clue does not localize it, when you suspect underground plumbing, or when the shell itself may be cracked. Professional pressure testing and dye work typically runs $250 to $500 and beats excavating on a guess.