Pool Losing Water: Is It Evaporation or a Leak?

A pool that loses up to 1/4 inch of water per day is almost certainly just evaporating – that is completely normal. If you are losing more than 2 inches per week, or if the water level keeps dropping even on cool, calm days, you are probably dealing with a real leak. The bucket test is the single most reliable way to tell the difference, and you can do it yourself in about 48 hours with a five-gallon bucket and a marker.

How Much Pool Water Loss Is Normal?

In hot, dry, or windy weather, evaporation alone can pull 1/4 inch of water out of your pool every single day. Over a week, that is 1 to 2 inches. During a heat wave with consistent wind, some pools lose close to 2 inches in a week and there is nothing wrong with them. Humidity plays a big role too – low humidity in desert climates means faster evaporation, while humid climates slow it down significantly.

The numbers that should get your attention: more than 2 inches of loss in a week with no significant splash-out or heavy use, any sudden change in your normal water loss rate, or a pool that needs to be topped off more than once a week. Those patterns suggest a problem beyond evaporation.

How Do You Run the Bucket Test?

The bucket test is the standard DIY method for separating evaporation from a leak, and it works because it exposes both the pool water and a reference bucket to the exact same weather conditions at the same time. Here is how to do it:

1. Fill a 5-gallon bucket with pool water to about 1 inch from the top.
2. Set it on the first or second step of the pool so it sits in the water, keeping the temperature the same.
3. Use a marker or tape to mark the water level inside the bucket and the pool water level on the outside of the bucket (or on the pool wall).
4. Leave everything alone for 24 to 48 hours. Do not top off the pool. Try to keep normal swim activity to a minimum.
5. Check both marks. If the pool dropped the same amount as the bucket, it is evaporation. If the pool dropped more than the bucket, you have a leak.

Run the test twice if you want better data: once with the pump running normally, and once with the pump off for 24 hours. A leak that only shows up when the pump is running points to the pressure side of your plumbing. A leak that shows up when the pump is off suggests the suction side, the shell, or a fitting. This distinction can save a leak detection specialist a lot of time.

Where Do Pool Leaks Usually Come From?

Most pool leaks are not in the ground. The majority happen above the waterline or at equipment connections, which is good news because they are findable and fixable without major excavation. Here are the most common culprits:

• Skimmer faceplate and gasket: The seal between the skimmer and the pool wall cracks over time, especially in fiberglass and concrete pools. Run your hand around the faceplate underwater – bubbles coming in while the pump runs are a giveaway.
• Return fittings: The eyeball fittings and their gaskets degrade. Check them for cracks and loose connections.
• Light niche: Pool lights sit in a housing that can crack. A failing conduit seal is one of the more common hidden leak sources in in-ground pools.
• Equipment pad: Pumps, filter valves, and multi-port valves all have o-rings and seals that wear out. Wet ground or puddles near equipment after the pump runs is a clear sign.
• Liner tears (vinyl pools): Small tears near steps, corners, or under the waterline can be easy to miss. A dye test – using a few drops of food coloring or a leak dye near suspect areas – shows you if water is being pulled through.

Underground plumbing leaks do happen but they are the least common cause. Before assuming the worst, check every fitting, seal, and visible surface first. The pool professionals at Poolwerx note that a large percentage of leak calls they receive turn out to be equipment pad issues or failed gaskets – not buried pipe problems.

What If the Water Stops Dropping at a Certain Level?

This is actually a helpful clue. If your pool consistently drops to a specific level and then stops, the leak is almost certainly at or near that water line. A pool that stabilizes right at the skimmer mouth probably has a cracked skimmer. One that drops to the level of a return fitting has a strong suspect. Mark where the water stopped and check every fitting and penetration at that exact height.

How to Rule Out Other Causes First

Before calling a leak detection company, go through this quick checklist. It is easy to chase a “leak” that turns out to be something much simpler:

• Splash-out: Heavy pool use – especially kids doing cannonballs – can displace more water than you think. A busy weekend can account for an inch of loss on its own.
• Backwashing: If you backwashed your sand or DE filter recently, that accounts for water loss. A typical backwash uses 200 to 300 gallons.
• Automatic water leveler: If yours is stuck open, it will mask a leak by constantly refilling. Shut it off before running the bucket test.
• Pool cover condensation: Water that collects on a solid cover and drains off the edge is lost water, but it is not a leak in the pool structure.

Keeping a simple log of your water level – just a weekly mark on your skimmer wall and a note of the weather – makes it much easier to see patterns over time. A lot of pool owners also keep an eye on their chemistry as a secondary signal: if you are constantly adding chemicals just to maintain levels you just set, unusual dilution from top-off water is a sign something is off. Products like AquaDoc’s pool chemical line are designed to hold up between adjustments, so if your numbers keep sliding even with proper dosing, water loss from a slow leak is worth investigating.

When to Call a Leak Detection Professional

If the bucket test confirms a leak and you cannot find the source yourself, a professional leak detection service is worth the cost. They use pressure testing on plumbing lines and electronic listening equipment to find underground leaks without guesswork. Expect to pay $150 to $400 for a leak detection visit depending on your area. That is a fraction of what a neglected leak costs in water bills, chemical waste, and potential structural damage to your pool deck or yard.

For more detail on what the detection process looks like from the inside, the team at River Pools and Spas has covered the professional side of leak diagnostics in depth – worth reading if you are heading into a service call and want to know what to expect.

Frequently Asked Questions

How much water should a pool lose per week?

Losing up to 1/4 inch of water per day is considered normal evaporation in most climates. That works out to roughly 1 to 2 inches per week depending on heat, wind, and humidity. Anything more than 2 inches a week is worth investigating with the bucket test.

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

Fill a 5-gallon bucket with pool water to about 1 inch from the top, set it on a pool step, and mark the water level inside the bucket and on the pool wall. Leave it 24 to 48 hours. If the pool dropped more than the bucket, you have a leak – not just evaporation.

Where do pool leaks most commonly occur?

The most common leak locations are around the skimmer faceplate, return fittings, light niches, and the pump or filter equipment pad. Underground plumbing leaks are less common but harder to find and usually require a pressure test by a professional.

Can I still swim in my pool if it has a small leak?

Yes, a slow leak is not a safety hazard for swimmers. But ignoring it is a mistake – constant top-offs dilute your sanitizer and chemicals, and a small crack in a liner or fitting can grow into a much larger repair quickly.

Does running the pool pump affect whether the pool leaks?

Yes. Some leaks only appear when the pump is running (pressure-side plumbing) and some only when it is off (suction-side plumbing). Running the bucket test once with the pump on and once with it off helps you narrow down which side of the system is involved.

A leaking pool is not just an annoyance – it is a slow drain on your water bill, your chemical budget, and eventually your liner or structure. The bucket test costs you nothing and takes two days. Do it before you convince yourself it is just evaporation.