Pool Evaporation: How Much Water Do You Actually Lose?

Evaporation is when liquid turns into a gas or vapor.

This process happens to almost any body of water under the right conditions, including swimming pools.

This means, as a pool owner, you lose a measurable amount of pool water every day through evaporation. Several thousands of gallons of water per year for an average residential pool, in fact.

Let’s talk about why this happens, how to calculate the amount of water you lose through evaporation, and how to prevent it.

Quick answer: You can expect to lose between 2 mm to 2 inches of water in your pool per week due to evaporation, depending on the various factors we’ll get into in this article.

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Why Does Pool Water Evaporate?

There are multiple conditions that govern the rate of evaporation for any body of water, including swimming pools.

These conditions are:

Temperature. Heat increases the rate of evaporation (yes, even at night), so you should expect your water to evaporate faster if you live in a warm climate, and slower if you live in a cold climate.
Humidity. Air can only hold a certain amount of liquid vapour, so, unlike temperature, the more humid the air is around your pool, the slower the water will evaporate.
Wind speed. The stronger the wind, the more air it moves. This counteracts humidity by clearing the “heavy” air and relieving pressure on the water, allowing evaporation to ramp up.
Surface exposure. If you have a large pool you probably also have a large surface area of water. The more spread out a body of water, the faster it evaporates. You can reduce this using a pool enclosure, however.

How Quickly Does It Evaporate?

For evaporation, anywhere between 2 millimeters to 2 inches per week is about what you should expect in terms of pool water loss.

This largely depends on the conditions we mentioned above which are mostly governed by the climate you live in, but it also has a lot to do with the unique conditions of your swimming pool.

For example, if you live in Florida or Texas, run numerous water features all day and hardly ever cover your pool, you’ll almost certainly be losing closer to 2 inches per week, if not more.

On the flip side, if you live in Wyoming or Montana, have no water features and use a high grade solar blanket whenever the pool is not in use, it’s very likely you’ll be closer to losing 2mm per week, if not less.

How To Calculate Your Water Evaporation Rate

There are pool evaporation calculators you can use, and they work pretty well if you can feed them with accurate information.

Of course, that’s easier said than done, especially because you have to consider other factors like rainfall, heavy pool traffic, water splash-out, and even potential leaks (more on that soon).

The truth is, you won’t know for sure how much water you’re losing to evaporation until you actually test it.

For this, we recommend the Bucket Test:

Fill a bucket with the water from your pool.
Using the pool steps or possibly the shallow end of the pool, place the bucket in the water so the rim is just above the waterline.
On both the inside and outside of the bucket, mark the water level with a black marker or some tape.
Wait a full 24 hours. Maybe it’s time to finally watch that Netflix show you’ve been holding off on.
Check the results and measure how much water was lost in the bucket. This is your daily evaporation rate.

You could simply mark the side of your pool to the same effect, but using a bucket allows you to remove many of the outside factors mentioned above, and get the most accurate reading on evaporation alone.

Is It Evaporation Or A Leak?

It’s not uncommon to attribute water loss to evaporation when, in fact, your pool water is seeping out from your pool shell or plumbing due to a leak.

If you’re losing more pool water than expected on a daily or weekly basis, or if you notice a considerable drop in cyanuric acid (which never drops), it’s worth investigating to see if you have a leak.

This is another reason we recommend The Bucket Method.

If, after performing the test, the water level on the inside of the bucket is noticeably higher than on the outside, you probably do have a leak.

How To Find The Leak

If you’re confident you have a leaky pool, the next step is to figure out exactly which part of the pool has been breached.

First, you’ll want to repeat The Bucket Test with your pool pump off.

If you lose more water when the pump is off, the leak will likely be found in the return lines.
If you lose less water when the pump is off, the leak will likely be found in the skimmer and main drain.
If you lose the same amount of water when the pump is off, the leak will likely be found in the shell, pool liner or fittings.

Now that you have a general sense of where the leak is, the next step is to pinpoint exactly where it is in your pool.

The best way to do that is using colored dye.

Submerge yourself in the pool with a bottle of dye
Slowly release a small amount of liquid near the suspect areas of your pool
Keep going until the dye gets sucked into a particular point
When it does, you’ve found your leak!

Finally, you’ll need to patch the leak, though the type of patch will depend on the type of pool you have, as well as where the leak is located.

How Can You Slow It Down? (Preserve Water)

You’ll never completely stop Mother Nature from sipping on your pool water, but you can slow her down.

Here are some “evaporation prevention” measures you can take:

Use a regular pool cover. A pool cover or even a safety pool cover will do a great job blocking water evaporation since water particles can’t escape through solid material
Use a solar pool cover (or blanket). They’re like regular pool covers, but they also absorb and transfer heat into your pool water. In other words, not only do they block evaporation, but they provide additional heat to your water. They also come in the form of smaller rings and even bottles of liquid.
Keep your pool shaded. You can use a pool shade or shaded enclosure to keep the sun off your pool and reduce temperatures. Tall trees work well for this too, of course.
Avoid aeration. Aerating your pool water (for example, using waterfalls, misters, fountains or other water features) has several benefits, but one of the side effects is an increase in pool water evaporation as more of it is exposed to the air.
Avoid heating at night. The greater the temperature difference between the water in your pool and surrounding air, the faster it evaporates. Not only is warming your pool on cool nights a waste of energy, it’s also a waste of water.

Don’t Sweat It!

All pool owners deal with evaporation to some degree, so don’t panic if you find your water level slowly depleting.

Just get a handle on what your evaporation rate should be, and be prepared to take swift action if you suddenly start losing more water than expected.