How Much Water Do You Actually Lose in Your Pool From Evaporation?

Since a pool is a water source that generally sits outside, it’s no secret that evaporation is an issue all pool owners face at some point. While some degree of evaporation is normal, understanding what factors affect it is important. It’s also vital to keep an eye on other possible causes of your declining pool water so you know you’re not dealing with a bigger problem, like a leak.

In this article, I will explain how much water you should expect to lose to evaporation, the factors that cause evaporation, the difference between evaporation and leaks, and even share tips on how to slow down evaporation.

Main Takeaways

• You can lose between two millimeters to two inches of water each week from evaporation.
• Climate factors that affect your evaporation rate include: rainfall, wind velocity, solar radiation, temperature, and difference in vapor pressure between the air and water.
• Human factors that affect your evaporation rate include: pool location, traffic, amount of splashing, and use of water fountains and other features.
• To calculate your evaporation rate, fill a bucket with pool water and place it on your steps. Then, mark where the water is and, at 24 hours, mark the new water line.

How Much Pool Water Do You Lose from Evaporation?

Your pool may lose anywhere from two millimeters to two inches of water every week due to evaporation.

Many factors cause evaporation, and I’ll cover them shortly. But for now, know that while evaporation is a natural process, there are strategies you can use to prevent Mother Nature from taking away your pool water.

Why Does Pool Water Evaporate?

From a scientific standpoint, evaporation happens when water molecules begin moving around so quickly that they bump into each other, releasing water vapor molecules into the atmosphere.

What Is Evaporation?

If that’s too technical for you to follow before your first cup of coffee, here’s the layman’s version: Evaporation is when a liquid turns into gas. If you’re a visual learner, imagine a boiling pot of water; the steam from it is evaporation at its finest.

Luckily, you don’t have to worry about your swimming pool’s water evaporating as quickly as boiling water. Nevertheless, evaporation is a natural part of the water cycle powered by the sun, and where you live significantly impacts how quickly or slowly your pool water will evaporate.

Air Pressure Affects Evaporation

I’ll cover more environmental factors that determine how quickly pool water evaporates shortly. However, one other important factor worth mentioning here is air pressure.

The higher the air pressure on the surface of your pool, the less likely your pool water will evaporate quickly. As its name suggests, the air pressure literally presses down on the water’s surface, making it difficult for water molecules to escape.

Thunderstorms are typically a high-pressure system, so if you live in an area that rains a lot, you receive the double benefit of less evaporation and rainwater, refilling the small amount of water your pool loses. That said, when it rains, make sure you clean your pool and test your water.

How Quickly Does Pool Water Evaporate?

There’s an extensive range regarding how quickly pool water evaporates—it can be as little as two millimeters to as much as two inches per week. That’s up to a 1/4″ of water per day!

Climate is the biggest factor that determines how quickly you lose pool water to evaporation. However, there are other factors that you have some control over.

For example, if you’re building a pool, consider the location of where you place it. A pool that sits in the sun all day will evaporate more quickly than a pool that spends a portion of the day in full or partial shade.

Of course, few people want to swim in a shaded pool, so enjoyment is also a factor when determining where to put your pool.

On the other hand, if your pool sits in a hot, sunny climate all day but you keep it covered when it’s not in use, you’ll significantly reduce the water evaporation rate.

Similarly, running a water fountain or other water features speeds up evaporation since you’re exposing a larger amount of water surface area to the air.

Therefore, if you have water features and are concerned about how quickly your pool loses water, consider only turning them on at night or when you are using the pool.

Long story short, even if you live in a hot climate with high evaporation rates, you can keep your pool evaporation levels closer to the two millimeters per week mark by using a solar pool cover and being mindful of how you use water features.

Factors That Go Into Water Evaporation Rate

As mentioned earlier, numerous factors determine a pool’s evaporation rate, and climate is the most important. Furthermore, depending on the environment, your pool will likely change its evaporation rate throughout the year.

Examples of climatic situations that impact water evaporation in pools include:

• Rainfall
• Wind velocity
• Solar radiation
• Temperature
• Difference in vapor pressure between the water and air

From an environmental standpoint, the highest water evaporation in pools often happens in areas where water is already scarce. For example, swimming pools in hot, dry climates like Arizona typically have higher evaporation rates than those in cooler climates with higher humidity levels, like New York.

As I’ve previously mentioned, human factors also influence pool evaporation rates. Below are some of the most common scenarios that impact them:

• Pool location
• Traffic (the more people use the pool, the faster evaporation occurs)
• Amount of splashing
• Use of water fountains and other features

Furthermore, sometimes, the pool itself is to blame for water loss. Toward the end of this article, I’ll discuss how to tell whether your pool is losing water because of evaporation or a leak.

How to Calculate Your Pool Water Evaporation Rate

Do you want to check exactly how much water your pool is losing every day or week? There are a couple of different methods for doing so.

But before we get into it, here’s a top tip: Pick a time to do this when rain isn’t in the forecast. Otherwise, the rain will undo all your hard work!

Simply Measure It

Measure the distance between the pool deck and the surface of the water of the pool. Note down the date you measure this. Then, to get your weekly water loss exactly one week later, use the tape measure once again to measure the distance between the deck and the water. Make sure to measure the distance in the exact location.

Now, subtract the first measurement from the second to calculate the change in water level. This indicates how fast your pool is losing water.

To get the evaporation rate, calculate the change in water level in feet, not inches. Multiply this number by 7.48 and divide it by the surface area of your pool. This new number is the gallons of water lost.

Finally, divide the number of gallons lost by the number of days, which is 7. This is the pool’s evaporation rate per day.

Bucket Method

Fill a bucket with pool water and place it on your pool steps. Mark where the water level comes to with a permanent marker or duct tape.

The next step can get a little tricky, but you’ll want to situate the bucket on the step so that the rim is slightly higher than the water level. That way, it prevents the sides of the bucket from heating up, leading to higher evaporation rates than your pool.

After 24 hours, mark the new water line in your bucket and measure its distance from the mark you made the day prior. That’ll give you your evaporation rate per day.

Water Loss Calculators

If you’ve already spent some time researching pool evaporation rates, you’ve likely stumbled upon water loss calculators.

Such calculators are great if you have the data to feed them, but they require you to measure the water loss per day, which can get tricky if your pool only loses a couple of millimeters of water per week.

Furthermore, as mentioned, rainfall and pool use can make a pool’s water level fluctuate throughout the day.

Pool Water Evaporation Chart

Of course, the rate of evaporation in a pool is affected by various factors, but it’s still helpful to know whether your pool is within the average range.

Here’s a quick chart to see roughly how much water you should be losing every month. Keep in mind that these numbers are for a 400-square-foot pool with minimal activity and no rainfall in a dry climate. So, don’t be alarmed if your pool’s evaporation rate does not follow the chart below, as numbers can vary widely depending on the various factors mentioned above.

For more specific information regarding average evaporation rates in your area, contact your local authorities for the proper data.

How to Tell If the Problem Is Actually a Leak vs. Evaporation

If you notice that your pool is suddenly losing water quicker than usual and the weather isn’t in an abnormally long, hot, and sunny stretch, there’s a good chance you have a leak.

Using the bucket method I described above is a great way to help you determine a leak, provided that you couple it with marking the side of your pool; if the bucket loses less water than your pool, a leak is likely the culprit.

Finding the Leak

Now that you know you have a leak in your pool, the next part is determining where the leak is. To do so, repeat the bucket test with the pump off, and determine which of the following scenarios is the case for your pool:

1. You lose more water. In that case, the leak is in the return lines.
2. You lose less water. In that case, the leak is in the skimmer or drain.
3. You lose the same amount of water. In that case, the leak is structural—in the shell, pool liner, or fittings.

Knowing the general area of your leak will make the next step easier, for you’ll need to apply a colored dye in that region; the dye will point you to the exact location of your leak.

Once you identify the leak, you’ll need to visit your local pool store to purchase material to patch it up. For more details on how to find the leak and fix everything, read my complete guide on pool leak detection.

How to Slow Down Pool Water Evaporation

Let me start by saying that it’s impossible to prevent your pool water from evaporating entirely. However, there are numerous strategies you can use to reduce the amount of water leaving your pool.

1. Keep your pool covered. Any solid pool cover will significantly reduce evaporation since it lowers the surface area where water particles can escape. Even better, use a solar cover if you can. That way, your water will heat up so that it’s closer to the air temperature, which will reduce evaporation rates. You can read my article on do solar covers work for studies on their effectiveness, and my research on the top solar pool covers and best pool covers for inground pools for model recommendations.
2. Build some shade. If your pool isn’t already in a shady spot, consider planting a few tall trees near it or installing a shaded enclosure.
3. Shut off water features with aeration. Fountains and misters are fun to look at, but they significantly increase evaporation rates.

By implementing these strategies, you’ll spend less time filling your pool with more water, you’ll save money on your water bill, and you can feel good knowing you’re implementing these environmentally friendly strategies.

Frequently Asked Questions

How Often Should You Add Water to Your Pool?

This really depends on a variety of factors, including climate, how much use your pool gets, and your pool’s location. But generally, most pool owners will have to top up their pool every two weeks or so.

Does Pool Water Evaporate at Night?

Yes, definitely! Water evaporates even when the sun is down. You might be able to spot this process if you see a slight mist above your pool.

How Much Water Should My Pool Lose Each Week?

On average, pools lose around 1/4 inch of water per day, which comes to around 2 inches a week.

Keep Track of Evaporation Levels

Lessen your water bill and understand how much water your pool needs by keeping track of your pool’s evaporation rate. By keeping a close eye on it and by implementing strategies like the bucket method, you can easily detect a leak in your pool before you waste any more water than needed.

Questions about evaporation or other issues with your pool water levels? Drop me a line.