How to Detect a Leak in Your Vinyl Liner Swimming Pool

Inground pools regularly lose water each day to evaporation, but an excessive loss of water (over ½ to 1 inch per day) is indicative of a leak. Leaks must be repaired, because they not only empty a pool of its water, but also increase water bills and can weaken subsoil. Before deciding how best to repair a leak in an inground pool’s vinyl liner, however, the leak’s location must be identified and severity assessed. If you suspect that your pool has a leak, here’s how to tell whether it does and figure out where the leak is.

How Much Water Loss in a Pool is Normal?

It’s normal for your pool to lose a small amount of water to evaporation each day, especially if you live in a hot climate or your pool gets lots of direct sun exposure. However, if your pool is losing more than half an inch to an inch of water per day, it’s almost guaranteed you’ve got a leak on your hands. Unlike evaporation, which is a harmless natural process, a leak creates safety hazards by pumping water directly into the ground, causing weakness and instability that can ruin your pumps, damage the pool, or even lead to a collapse.

How Much Water Does a Pool Lose Per Week?

On average, a typical swimming pool loses anywhere from approximately one quarter of an inch to two inches of water per week. That adds up to a whopping 10,000 to 20,000 gallons per year — enough to fill a second pool! Factors that can cause or accelerate pool water loss include splashing, hot weather, and filter backwashing, which occurs when pool waste water drains out into the sewer system.

While you can’t prevent water loss completely, you can take steps to reduce evaporation and waste, such as keeping your pool covered, avoiding decorative water features, and shutting off heating systems when the pool is not in use, since warm water evaporates more quickly than cool water.

Why Does My Pool Lose Water Overnight?

You might think that, without the sun beating down or your pets and kids splashing, overnight water loss would be impossible. However, to many homeowners’ surprise, it is actually common for swimming pools to lose water at night. Even without sunlight, substantial amounts of evaporation can still occur, especially if the pool temperature is higher than the air temperature. Fortunately, you can limit overnight evaporation and water loss by using a swimming pool cover, or by turning off the heating system once you are done with the pool for the evening.

What Are the Signs of a Pool Leak?

Watch out for these tell-tale warning signs that your fiberglass, gunite, or vinyl pool is leaking, especially if your pool is old or was installed recently:

1. There are puddles, damp spots, or pools of standing water near the swimming pool — even though it hasn’t rained recently.
2. Your pool is growing algae (which looks like green scum), even though you’ve been treating and shocking the water diligently.
3. You notice cracks, depressions, soft spots, or stains on the pool deck, which indicate that water is not being contained.

What is the Best Way to Identify a Leak in a Pool?

There are several easy ways to check for pool leaks if your swimming pool’s water loss seems excessive. For example, you can touch or walk on the floor of your pool to feel for squishy areas, which are indicative of leaks; spread non-toxic food coloring to reveal where eddies, ripples, or currents appear; or simply use the five-gallon bucket test, which is described step by step in the next section. 

Pool Leak or Evaporation? Identify a Leak With a 5-Gallon Bucket

Before searching your pool for a leak, you should confirm that there is a leak. Doing so only takes one day, and all you’ll need is a 5-gallon bucket, painter’s tape and water. Simply:

1. Fill a 5-gallon bucket up with water and set it next to your pool.
2. Mark the water level in the bucket with painter’s tape.
3. Mark the water level in the pool with painter’s tape (which won’t hurt your vinyl liner).
4. After one day, measure the difference between the water level and each piece of painter’s tape.
5. If the difference in the pool is greater than the difference in the bucket, then your pool has a leak; if there isn’t a significant disparity, the loss of pool water is from evaporation.

Once you determine that there is a leak, the next step is to identify where it is. (If there isn't a leak, just go for a swim!)

Check for a leak in the electrical and filtration systems

It’s easy to check the electrical and plumbing systems for leaks. Look at the electrical conduit lines that lead from the pool’s lights for moisture or leaks. There shouldn’t be any water on the wires. If there is, call a professional to determine the precise location of the leak and repair it -- you don’t want to get shocked.

To see whether your pool’s water system is leaking, turn off the filtration. Look around all pipes, fittings and the pump for moisture. If you see any, this is likely the location of your leak. Depending on the exact location of the leak, you may be able to replace a pipe yourself, or, for more complex issues like a failing pump, you might need to have a professional service your pool’s filtration system.

Look for a hole in a vinyl liner with food coloring

If your pool has a leak, but it’s not in the electrical conduits or filtration system, you’ll need to search for a leak in the vinyl liner. This is also easy to do. You will only need food coloring. Then:

1. Ensure the pool is full (so the leak’s not above the water level).
2. Feel the pool’s floor to see if it’s squishy, which suggests a leak in the bottom of your liner.
3. Wherever you suspect a leak, place a few drops of food coloring.
4. Continue placing food coloring around your pool until you find the leak.

The food coloring will let you see the flow of water. Once you place the food coloring near the leak, you’ll see it flowing out the exact spot where the leak is.

How Can I Tell Where my Pool is Leaking?

Once a pool leak test has confirmed that a leak exists, the next step is pinpointing the leak’s location so that it can be quickly repaired before the damage worsens. To find a leak in a vinyl pool, fiberglass pool, or gunite pool, use the simple food coloring method described above, or hire a trusted professional to thoroughly inspect the pool for you. 

How to Fix a Leak in a Pool: Patches and New Liners

Once you identify where your pool’s leak is, you’ll see whether a patch will suffice. If the leak is too big for a patch, you’ll need a vinyl replacement liner. Order one as quickly as possible, so you can prevent the leak from weakening the subsoil and so you can enjoy your pool again soon.