Pool Losing Water: Evaporation or a Leak? Here’s How to Know

A pool losing around a quarter to half an inch of water per day is almost always evaporation – no leak, no problem. Anything beyond 1 inch of water loss per week, especially when the weather is not unusually hot or windy, is worth investigating. The fastest way to tell the difference is the bucket test: fill a bucket with pool water, set it on a step, mark both levels, wait 24 to 48 hours, and compare. If the pool dropped more than the bucket, something is leaking. This article walks through exactly what to look for, where leaks actually hide, and when to call a pro.

How Much Water Loss Is Normal From Evaporation?

Evaporation is the leading cause of water loss in residential pools, and most owners underestimate how much it accounts for. In a warm, sunny, dry climate, a pool can lose half an inch of water or more per day entirely through evaporation – that adds up to 3 to 4 inches per week during a hot summer stretch. In cooler or more humid regions, evaporation typically runs closer to 1 to 2 inches per week in summer. Wind is the biggest variable: a breezy afternoon pulls water off the surface faster than still, humid air does.

Other factors that increase normal evaporation include high bather load (swimmers splash and carry water out), running water features like waterfalls or fountains, and heated pools with large temperature differentials between the water and the air. If your pool is heated in cool weather, evaporation goes up noticeably because warm water evaporates faster into cold dry air.

How to Do the Bucket Test Correctly

The bucket test is the standard DIY method for confirming whether water loss is evaporation or a leak, and it works because both the bucket and the pool are exposed to the same weather conditions. Done correctly, it takes about 5 minutes of setup and 24 to 48 hours of waiting.

1. Fill a 5-gallon bucket with pool water until it is about 3/4 full.
2. Place the bucket on the first or second step of the pool so it is partially submerged and exposed to similar sun and air as the pool surface.
3. Mark the water level inside the bucket with tape or a marker.
4. Mark the pool’s water level on the wall or skimmer with tape.
5. Turn off the pool pump and auto-fill valve (if you have one) for the full test period.
6. Wait 24 to 48 hours without running the equipment or adding water.
7. Compare the drop inside the bucket to the drop in the pool.

If both dropped the same amount, you are dealing with evaporation. If the pool dropped noticeably more than the bucket – say the bucket lost half an inch and the pool lost an inch and a half – you have a leak. A good rule of thumb: any difference greater than a quarter inch between the two readings warrants further investigation.

Where Do Pool Leaks Actually Hide?

Most people assume a leaking pool means a crack in the shell. That does happen, but the majority of residential pool leaks occur at fittings, transitions, and equipment connections – not in the middle of the floor or wall.

Common leak locations include:

• Skimmer throat: The seam where the plastic skimmer body meets the concrete or fiberglass shell is a classic weak point, especially in older pools. Even a hairline separation here can lose significant water.
• Return fittings and main drain: The gaskets and fittings where jets and drains penetrate the pool wall can deteriorate over time.
• Underground plumbing: Pipes under the deck can crack from ground movement, root intrusion, or age. You will not see this one from the surface.
• Equipment pad: Leaks at the pump housing, filter tank, or pressure-side plumbing fittings can bleed water steadily but only when the system is pressurized and running.
• Shell cracks: Vinyl liner pools can develop tears or separations at fittings. Gunite and fiberglass pools can crack at stress points, though this is less common.

One useful diagnostic trick: run the bucket test twice – once with the pump on and once with the pump off. If the pool loses more water with the pump running, the leak is almost certainly on the pressure side of the plumbing or at the equipment pad. If water loss is similar either way, the leak is in the shell, fittings, or on the suction side.

What to Check Before Calling a Leak Detection Pro

Before spending money on professional leak detection equipment, walk the pool and check the obvious spots yourself. Look at the skimmer face plate and throat for visible gaps or deteriorated putty. Check around every return jet fitting – press lightly and feel for movement or soft areas in a plaster pool. Inspect the equipment pad for wet soil or puddles under the pump and filter. Look at your pool deck for areas where concrete has sunk or settled, which can indicate water washing out soil underneath.

For vinyl liner pools, check along the floor seams and especially around the main drain and any steps or benches. A liner that has pulled away from a fitting is often visible on close inspection. For fiberglass pools, look for a waterline ring that seems lower than it should be, and check the shell around steps and benches.

If you have an auto-fill valve (the float-controlled top-up device most pools have in the skimmer), turn it off during your bucket test. Auto-fill valves can mask significant water loss by constantly topping up the pool, making owners think everything is fine for months while a slow leak steadily washes out the ground beneath the deck. AquaDoc pool owners using dye test kits have found that pulling a little dye near a suspected fitting and watching where the cloud moves is a surprisingly effective low-tech way to confirm a fitting leak before calling a professional.

When Evaporation Rates Suddenly Change

If your pool has been stable for years and suddenly seems to be losing more water than usual, do not immediately assume a new leak. Check whether anything changed: a new water feature was added, the pump run time increased, a solar cover was removed, or a stretch of unusually dry and windy weather arrived. These factors can easily double your evaporation rate without any plumbing failure at all.

A solar or thermal pool cover reduces evaporation by 50 to 70 percent. If you used a cover last summer and are not using one this summer, the increase in water loss you are seeing may be entirely normal. If you want to understand the chemistry side of what changing water levels does to your pool’s balance, the post on cyanuric acid in pools covers why topping up frequently with fresh water gradually dilutes stabilizer and throws off your CYA levels over a season.

When to Call a Professional Leak Detection Service

Call a leak detection professional when you have confirmed through the bucket test that a leak exists but cannot locate it yourself, when water loss exceeds 1.5 inches per week consistently, or when you have already patched a suspected spot and the loss continues. Professional leak detection uses pressure testing on individual plumbing lines and acoustic listening equipment to pinpoint underground pipe leaks without digging. Most leak detection calls run between $150 and $400 depending on pool size and complexity, and that cost is almost always worth it compared to the structural damage a long-running underground leak can cause.

Pool service companies that work with plaster and fiberglass pools, like those affiliated with Poolwerx, offer leak detection as a standard service and can pressure-test lines the same day. Getting a professional diagnosis before attempting any repair saves a lot of time, money, and unnecessary demolition.

Frequently Asked Questions

How much water should a pool lose per week?

Losing up to 1 inch of water per week is normal evaporation in most climates. More than 1 inch per week, especially without extreme heat or wind, suggests a possible leak worth investigating.

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

Fill a bucket with pool water, mark the water level inside the bucket and on the pool wall, place the bucket on a submerged step, and let both sit undisturbed for 24 to 48 hours with the pump off. If the pool drops more than the bucket, you likely have a leak.

Can a pool lose water without a visible crack?

Yes. Many pool leaks occur at fittings, return jets, skimmer throats, or underground plumbing – places where there is no visible crack in the shell. A leak in the plumbing or at the equipment pad can drain significant water with no obvious sign at the waterline.

Does running the pool pump cause more water loss?

It can. Leaks at the pump, filter, or pressure-side plumbing only lose water when the system is running under pressure. If your pool drops more with the pump on than off, the problem is likely in the plumbing or equipment connections rather than the pool shell.

When should I call a professional leak detection service?

Call a professional if your pool consistently loses more than 1.5 inches per week, if the bucket test confirms a leak but you cannot find the source, or if water loss worsens after heavy rain or ground movement. Pressure testing and acoustic equipment can locate underground leaks without any digging.

The bottom line: most pool water loss is boring, normal evaporation. But if the bucket test shows the pool dropping faster than the weather can explain, stop adding water and start looking – because a leak that seems minor today can silently erode your deck foundation for months before anyone notices the real damage.