Pool Evaporation Rate: How Much Water Is Your Pool Losing?

Understanding pool evaporation rate is key to making sure the water in your swimming pool is at the proper level. Pools are a perfect way to cool off on hot summer days, but maintaining a pool requires work. On top of making sure it is chemically balanced, you also need to regulate the level of water in your pool. The water level affects the operation of the filtration system, water circulation, and the chemical balance of the pool.

Knowing whether your pool is losing water through evaporation or a leak is essential for overall pool performance as well as the safety of swimmers. This article will walk you through these differences and offer ways to help slow down the evaporation process.

What Is a Normal Pool Evaporation Rate?

You should take notice when you notice that your pool is losing water. Pool evaporation is a normal process for any body of water, and your pool will naturally lose water over time. The normal pool evaporation rate of a residential pool is usually about a quarter-inch of water per day in the summer.

There are a few ways you can find the evaporation rate for your pool.

• Check an evaporation rate map of your area. Use resources, like the National Weather Service, that offer evaporation maps that indicate the rate of evaporation in your area.
• Use a tape measure or ruler. You can estimate the evaporation rate yourself by measuring it. Measure the distance between the surface of the pool water and the top of the pool deck or wall. Wait for a week, and measure the same distance again. Subtract the first measurement from the second measurement, and this will give you a simple estimate of the evaporation rate. 

Pool Water Evaporation vs. Leak

A good indicator of a pool leak is if the amount of pool water lost is higher than a normal amount of pool water loss. If this is the case, you should check for leaks around your pool. A leak may highlight a bigger problem that needs to be addressed by a professional.

One simple way to test for leaks is the bucket test. Only perform the test if it is not raining and no people or animals are in the pool, as this will make the results inaccurate.

1. Fill your pool to its normal water level.
2. Fill your bucket until about 1 inch from the top remains empty.
3. Place the bucket on the first or second step of your pool so that it's submerged about 5–6 inches into the water.
4. Use a marker or pen to mark where the water stops both inside and outside of the bucket.
5. Wait 24 hours.
6. Check the water levels inside and outside of the bucket.

If the water outside of the bucket dropped more than it did inside the bucket, then you likely have a leak.

If there are no steps in your pool, there are other ways you can check for leaks. These include:

• Looking for wet spots in the yard or near pool equipment
• Checking for cracked concrete or tiles around your pool
• Checking for pool chemical imbalances
• Getting an opinion from a professional

How Pool Water Evaporation Affects Pool Owners

• Replenishing the water levels in your pool after evaporation increases your water bills.
• When water levels fluctuate due to evaporation, it causes the chemicals to become imbalanced, which may damage equipment and harm swimmers.
• Your pool system works harder to accommodate changing water levels from evaporation, putting wear and tear on pool equipment.
• Evaporation naturally decreases the temperature of your pool when the heat of the liquid water is converted to water vapor.
• The decreased pool water temperature as a result of evaporation may cause reduced comfort while swimming.

What Causes Pool Evaporation?

Several factors can influence the rate of evaporation in pools. It can vary depending on the area you live in as well as the specific details of your pool.

Environmental Factors

• Temperature - The rate of evaporation increases the higher the air temperature.
• Humidity - The lower the amount of humidity in the air, the higher the evaporation rate will be. The higher the humidity, the lower the evaporation rate.
• Wind - Winds carry away heat vapor from water evaporation while cooling the pool temperature.
• Sun exposure - Direct sun exposure will heat water quicker, causing more evaporation.

Pool Specific Factors

• Pool surface area, size, and shape - The dimensions and depth of the pool impact water evaporation. The larger and deeper the pool, the longer it takes to heat and cool the water, therefore changing the evaporation rate.
• Water features or fountains in pools - Water features and fountains promote water circulation, cooling the surrounding water and air temperatures as the water evaporates.
• Water temperature - The warmer the temperature of your pool water, the more water you will lose because of the higher evaporation rate.
• Chemical balance - As water evaporates, a higher concentration of chemicals remains in the pool. Make sure the water level is correct to ensure chemical balance.
• Pool usage - The movement of water from swimming in a pool can cause a similar effect that water features have on pool water evaporation rates. The more people swimming at once, the more the water is affected.

How To Prevent Pool Water Evaporation

There are a range of ways to minimize the impact of pool water evaporation.

• Utilize pool covers.
• Create shade with landscaping.
• Manage the water temperature.
• Perform regular pool equipment maintenance checks
• Install wind barriers.

Pool Water Evaporation FAQs

How fast does water evaporate in a pool at 100 degrees?

If you live in an area with very hot days and much cooler night temperatures, this can lead to higher evaporation rates. Pool water usually retains overnight temperatures, meaning there is a large difference in daytime air and water temperatures.

How often should you have to add water to your pool?

How often you should add water to your pool depends on how quickly the water evaporates, but typically you should add 2 to 4 inches of water to the surface level each week.

Why is my pool losing water in winter?

Many people think evaporation only happens in warmer temperatures. However, the colder the water in your pool gets, the greater the pressure difference between the water and the surrounding air. This pressure difference, along with low temperatures and low humidity, causes rapid evaporation.