Why Is My Pool Losing Water? Evaporation vs. a Leak

Every pool loses some water. Between evaporation, splash-out from swimming, and routine maintenance like backwashing the filter, a certain amount of water leaving the pool each week is just part of owning one, not a sign anything is wrong. Most owners first notice it as the waterline creeping below the tile line, or as topping off the pool becoming a weekly chore instead of an occasional one. The question that actually matters is whether your water level is dropping at a normal rate or a rate that points to something else, most commonly a leak somewhere in the shell, the plumbing, or a fitting. Figuring that out doesn't take a plumber or a leak-detection crew. It takes a bucket, a permanent marker, and about a day of patience, in the form of a simple comparison test almost every pool professional recommends running first.

How much water loss is actually normal

Evaporation rates vary quite a bit depending on your climate and how the pool is used. Heat, direct sun, low humidity, and wind all speed evaporation up, which is why a pool in a hot, dry, breezy backyard can lose noticeably more water over a week than an identical pool somewhere shaded and humid. Running a water feature, a fountain, or a spa spillover adds surface agitation that speeds evaporation further, and heating the pool above the surrounding air temperature increases it too, since warm water evaporates faster than cool water. Screened or covered pools, and pools with a lot of shade, tend to lose noticeably less. As a rough rule of thumb that many pool professionals use, losing up to around a quarter inch of water a day during hot, dry, or windy stretches is typically normal evaporation, and it should slow down noticeably once the weather cools, calms, or gets more humid. Losing water at that rate, or faster, even in mild weather, or losing it at a steady pace no matter what the forecast looks like, is the pattern worth investigating.

The bucket test

• Fill a clean bucket with pool water and set it on the first or second step in the shallow end, or weight it so it floats, so the water level inside the bucket matches the pool's water level.
• Mark the water level on the outside of the bucket with tape or a permanent marker, and mark the pool's water level at the same spot, usually right at the skimmer opening or tile line.
• Run the pool exactly as you normally would for the next 24 hours: pump, filter, and any water features on their usual schedule, and swim in it if you'd normally swim.
• After 24 hours, compare the two marks. Both the bucket and the pool were exposed to the same sun, wind, and heat, so if the pool's level dropped noticeably more than the bucket's, evaporation alone probably isn't the explanation.

Where pools typically leak, and when to call a leak-detection service

If the bucket test points to something beyond evaporation, that's a solvable problem, not an emergency. Most leaks trace back to a fairly short list of usual suspects: a skimmer throat that's cracked where it meets the deck, a fitting around a return or light niche that's worked loose or degraded, a small tear in a vinyl liner, a crack along a plaster or pebble surface, or a plumbing line under the deck that's separated at a joint. Surface-level leaks like a cracked skimmer or a liner tear are often visible on close inspection, or at least locatable with a little patience and a mask if you're checking underwater. One thing worth ruling out first: a pool that was recently replastered can lose a bit more water than usual for the first few weeks as the new surface cures, which isn't a leak and typically settles down on its own. It's worth calling a professional leak-detection service once you've confirmed the loss is real and you can't find an obvious source yourself, once you're topping off the pool often enough that it's showing up on your water bill, or once you notice a damp or unusually green patch of lawn near the equipment pad or pool shell that never quite dries out. Underground plumbing leaks in particular are hard to find without specialized equipment: dye testing, electronic listening devices, and pressure-testing the lines are all standard tools a leak-detection technician will use, so there's no need to go digging on your own before calling one. Logging your water tests and top-offs somewhere consistent, which is exactly what Skimmer's maintenance log is built for, makes it much easier to spot a slow leak early instead of discovering it months later as an unexplained jump in your water bill.