Pool Losing Water: Evaporation or a Leak? Here's How to Tell

A pool losing 1 to 1.5 inches of water per week is normal evaporation - no action needed. Losing more than 2 inches per week, or noticing water loss only when the pump runs, is a strong signal you have a leak. The bucket test (explained below) is the most reliable way to separate evaporation from an actual problem, and it costs nothing. Most pool leaks start at the skimmer, return fittings, or equipment pad - not in some mysterious crack in the shell.

Why do pools lose water even without a leak?

Water evaporates constantly from a pool surface, and the rate depends on four things: temperature, humidity, wind, and surface area. A hot, dry, breezy day can pull 1 inch of water off a pool in 24 hours without any leak involved. Running water features like waterfalls or fountains accelerates evaporation further because it exposes more water surface to moving air. Splash-out from swimmers also adds up faster than most people expect, especially in pools with heavy use.

If you live in a warm, low-humidity climate like the Southwest or South Florida, 1.5 to 2 inches of weekly water loss during summer is not unusual even in a perfectly sealed pool. Using a pool cover when the pool is not in use can cut evaporation by 30 to 50 percent, which is also the easiest way to confirm evaporation is the culprit - if covering the pool stops the loss, you're not dealing with a leak.

How do you do the bucket test for a pool leak?

The bucket test is the standard first step for any suspected pool leak, and it separates evaporation from a real problem with a 24 to 48-hour test you set up yourself.

1. Fill a 5-gallon bucket with pool water to about 1 inch from the top.
2. Set the bucket on a pool step or bench so it sits partially submerged in the pool water. This equalizes the water temperature so both the bucket and pool evaporate at the same rate.
3. Use a marker or piece of tape to mark the waterline inside the bucket and the pool waterline on the wall or skimmer.
4. Turn off the pump and auto-fill valve if you have one.
5. Wait 24 to 48 hours. Do not use the pool during the test.
6. Check both water levels. If the pool dropped significantly more than the bucket, you have a leak. If both dropped the same amount, it's evaporation.

Run the test a second time with the pump running to narrow down the leak location. If the pool loses more water with the pump on than off, the leak is in the pressurized plumbing - return lines, fittings, or the equipment pad. If water loss is the same whether the pump runs or not, the issue is structural: the shell, liner, or fittings at the waterline.

Where do pools actually leak most often?

The vast majority of pool leaks are not dramatic cracks in the shell. They're small, unglamorous failures at connection points and fittings that are easy to miss unless you know where to look.

• Skimmer throat and faceplate: The joint where the skimmer body meets the pool wall is one of the most common leak spots, especially in concrete pools as they age and shift slightly. Run your hand or dye test around the skimmer edge when the pump is off.
• Return jet fittings: The threaded fittings that screw into return ports can develop leaks as the gaskets degrade. These are easy to test with dye and often just need the fitting re-seated or the gasket replaced.
• Main drain: Leaks at the main drain housing are less common but harder to fix. A dye test near the drain at the deep end can confirm a leak there.
• Equipment pad plumbing: The joints, unions, and valves at the pump and filter are under constant pressure and vibration. Look for water stains, mineral deposits, or actual wet spots around any pipe connections.
• Vinyl liner tears: In liner pools, small tears form near step seams, around skimmer and return cutouts, and in corners. These are often pinhole-sized and hard to see, but dye will find them.
• Light fixtures: The conduit behind a pool light is a direct path for water to travel out of the pool if the gasket or conduit seal fails. This is especially worth checking if the light was recently serviced.

How to use dye to find a small pool leak

Pool dye (or even a few drops of regular food coloring) is a low-cost, high-value diagnostic tool. Squeeze the dye close to a suspected area - near a skimmer edge, a return fitting, or a liner seam - while the pump is off and the water is calm. If the dye drifts toward a spot and gets pulled into it rather than dispersing evenly, that's your leak. Move slowly and work in sections. Starting at the waterline makes sense because leaks at or just above the waterline are the ones causing you to lose water; leaks below the waterline would still show up even if the water level dropped and the leak point ended up exposed.

Some pool owners find it easier to do this test early in the morning before wind picks up, since moving water makes the dye hard to read. The folks at River Pools and Spas have written solid guides on diagnosing fiberglass pool issues including leak detection if you want a deeper look at how pros approach this systematically.

When should you call a professional leak detection service?

If the bucket test confirms a leak and you've checked all the obvious spots without finding the source, it's time to call a leak detection service. Pros use pressure testing on plumbing lines, electronic listening equipment, and sometimes scuba to inspect underwater surfaces directly. This isn't cheap - expect $200 to $500 for a professional leak detection visit - but it's far less expensive than the water bill, chemical costs, and potential deck or soil damage that come from an ongoing leak. A loss of 3 or more inches of water per week in a 20,000-gallon pool can waste thousands of gallons a month.

One thing worth knowing: if your pool has an auto-fill valve, it can mask a leak entirely. The valve keeps refilling the pool to the correct level, so you never see the waterline drop - you just get a high water bill. Disconnecting the auto-fill before running the bucket test is essential if you want an accurate read. This is the same kind of "check the obvious stuff first" mindset that applies to diagnosing why your pool loses chlorine overnight - the answer is usually simpler than you expect once you isolate the variables.

Does winterizing change the leak picture?

If you're heading into the off-season and suspect a leak, it's worth finding it before you close the pool. Water in a leaking plumbing line that isn't fully drained can freeze, expand, and crack PVC - turning a cheap repair into a major one. Knowing how to close your pool for the winter properly includes making sure the equipment pad plumbing is blown out and any known leak points are addressed before the first hard freeze.

Frequently Asked Questions

How much water does a pool lose to evaporation per week?

A typical pool loses 1 to 1.5 inches of water per week to evaporation in warm, dry, or windy conditions. In hot climates during summer, 2 inches per week is not unusual. Anything consistently over 2 inches per week warrants a bucket test to rule out a leak.

How do I do the bucket test for a pool leak?

Fill a bucket with pool water to about 1 inch from the top and set it on a pool step so it sits in the water. Mark the waterline inside the bucket and on the pool wall, then wait 24 to 48 hours with the pump off. If the pool loses significantly more water than the bucket, you have a leak.

Where do pools most commonly leak?

The most common leak locations are the skimmer throat, return jet fittings, main drain housing, equipment pad pipe connections, and - in vinyl liner pools - tears near steps and fittings. Light fixture conduit seals are also worth checking if the light was recently worked on.

Does a pool lose more water with the pump running or off?

If your pool loses more water with the pump running than with it off, the leak is likely in the pressurized plumbing - return lines, fittings, or equipment connections. If water loss is the same either way, the leak is structural, in the shell, liner, or waterline fittings.

Can I use dye to find a pool leak myself?

Yes. Pool leak dye or a few drops of food coloring squeezed near a suspected area will get pulled toward any active leak while the water is calm and the pump is off. Work slowly in sections starting at the waterline, and do it early in the morning before wind disturbs the water surface. AquaDoc's pool dye is one option pool owners use for exactly this kind of targeted leak hunt before calling in a professional.