How To Measure Water Evaporation In A Pool

Kenneth Wilson

When I first had my pool, I was worried when I saw that my water level kept decreasing. I thought it wasn’t normal for it to lose water every day but I learned that this happens due to water evaporation.

Pools can lose a quarter of an inch to half an inch of water every day from water evaporation. Accumulated, this is already two to four inches per week. This means an average-sized pool can lose 25,000 to 50,000 gallons of water a year - that’s a lot of water! This doesn’t even include splash out from constantly using the pool. The water level decreases faster if there are more swimmers who go in and out of the pool, dive in, play, or do cannonballs.

If you want to know how much water you will lose due to evaporation, here’s a guide on how to calculate the water evaporation loss of your pool.

Why does your pool water evaporate?

There are several factors that can cause your pool water to evaporate. The climate, temperature, and amount of shade play a big role in reducing the water levels.

Warm temperatures

When it’s really hot, you can expect your pool water level to go down quickly. During the summer, a hot day combined with a cool evening result in water evaporation. This happens because the temperature of your pool water is different from the temperature of the outside air. When your pool water starts heating up, it will evaporate when it comes into contact with the cool air.

Direct Sunlight and Strong Winds

If your pool doesn’t have a cover or roof, you can expect that the pool water level will decrease due to water evaporation. It doesn’t even have to be fully covered as pools with screen enclosures lose less water than unscreened pools. Structures, fences, and trees can also help reduce the amount of water evaporation levels.

Humidity Levels

An area with low or zero humidity levels will probably lose more water due to water evaporation. If you compare evaporation to a sponge, when the air is drier, more water will be absorbed. This means that areas with higher humidity levels will lose less water.

Is it a leak or just evaporation?

Before you start measuring your water evaporation loss, you have to determine if it’s evaporation or a leak. 

If you’re losing more than a quarter or half an inch of water every day, you probably have a leak. But to be sure, you can use the Bucket Test to see if you have a leak.

This test will help you determine if the leak is not just evaporation. Here’s what you will need

  • A bucket
  • A marker

Step 1. Place the bucket on the step and then partially fill the inside with pool water.

Step 2. Using the marker, draw a line on the inside of the bucket to mark how much water is inside the bucket. Draw another line on the outside to mark the water level of the pool.

Step 3. Turn the pool systems off and wait for 24 to 48 hours. Check the bucket and if the water level drop is the same inside and outside the bucket, it’s just evaporation. If the water level outside the bucket dropped more than the water level inside, you probably have a leak.

Note: Excessive filter backwashing can also result in water loss. 

You can also determine if you have a pool leak if you...

  • Add water twice a week or more
  • Constantly see cracks
  • Find air in the system or your equipment becomes less effective
  • See your pool deck lifting or sinking
  • Find soggy spots around the pool

Calculating for your water evaporation loss

After ruling out the leak factor, it’s time to determine your water evaporation loss. To find out how many gallons of water you are losing from water evaporation, you first need to measure how many inches your water level drops.

Water Evaporation Loss (Inches)

Here’s a simple test to determine how many inches of water you lose:

What you will need:

  • Pencil or marker
  • Ruler

Before you start the test, you first need to turn off the heater system, whether it be solar, electric, or propane. Let your pool cool down for two days before the test. You also need to turn off the water features such as waterfalls to get accurate measurements. The only exception would be spillover from a spa.

Step 1. Fill the pool to the normal level using a hose or other methods. Make sure to not overfill the pool.

Step 2. Turn off the pump for 5 minutes and let the water settle down. 

Step 3. Once the turbulence from the pump has stabled, use the pencil or marker to mark the current water level on your skimmer canister.

Step 4. Turn on your pump and let it run for four days.

Step 5. On the fourth day, turn off your pump again for five minutes and let the water settle down from the turbulence.

Step 6. Once stable, mark the current water level and then get the ruler. Measure the distance from the first mark to the latest mark. This will be your water loss in a span of four days.

Note: For more accurate results, the pool shouldn’t be used during these four days as swimming in it can result in higher water loss.

If you lose an inch or two in a span of four days, then this is reasonable water evaporation loss.

Water Evaporation Loss (Gallons)

After determining how many inches you lose, you can now find out how many gallons of water you lose as well. 

American Leak Detection has a swimming pool water loss calculator to determine how many gallons of water you will lose per minute, hour, and day. It will also show you how many cups of water per minute you may lose.

If you’re going to calculate for this, you will need to know the length and width of your rectangular pool or the diameter of your circular pool. All measurements should be in feet. You will also need to determine how many inches your pool water level drops in a day. Use the first test mentioned in this section but instead of waiting for four days, measure the mark after 24 hours. You can also use the Bucket Test to determine if there’s a leak or not, just measure the distance of the current water level to the mark on the outside of the bucket (check the inside as well and compare results for accurate measurement).

Now that you have all the measurements, simply input the data. Here’s a screenshot of my pool water evaporation loss results:

My 10,000-gallon pool loses around 45 gallons of water per day. That’s around 16,425 gallons per year. This means in a span of one year, I refill my pool almost two times its size.






Fill Counter
Fill Counter
Fill Counter
Fill Counter

What should you do when the water level goes down?

It’s time to get a hose and fill your pool. You would probably need to do this every 7 to 10 days. 

For the proper in-ground pool water level, the water should be halfway up the skimmer line. If you have too much water, the skimmer will not work properly. If it’s too low, the skimmer diverter may experience some issues. Maintaining the proper water level will help the circulation system and make it easier to skim debris off the surface. 

For above-ground pools, the ideal level is a little bit more than half the skimmer. If you have middle screws on your skimmer, that can be your guide. Fill the water up to the top of the screws. If the water is higher than the ideal level, water will start splashing out the skimmer. On the other hand, the pump will start sucking air if it goes down the ideal level. 

In the event that you forget to turn off the hose and overfill your pool, immediately pull out the hose and run the skimmers dry. 

Does water evaporation affect your pool?

Yes, water evaporation can affect your pool water chemistry and systems. If the water is not restored to the ideal level, this may change your pool’s chemical balance and cause calcium scaling. It can also damage your filter and pump systems. 

This will also affect your wallet as you need to pay for the water replacement cost. So it’s best if you can decrease your water evaporation loss.

How to prevent or reduce water evaporation

Refilling a few inches of water once a week may not empty your wallet but when accumulated, it can drain your bank account. That’s why you should try to reduce water evaporation loss and here are some ways you can do it:

Pool Cover

There are many causes of water evaporation that we cannot control such as the sun, temperature, climate, and humidity levels. What we can do is cover our pool to shield it from these harsh elements. This will help decrease water evaporation. Some have said that pool covers can reduce water evaporation loss by 95% - that’s a lot of savings! 

Aside from preventing water evaporation loss, this will also help keep your pool water warm. Evaporation cools your pool water and counters the heat generated by your heating system. Having a cover will reduce the wear-and-tear on your systems and keep debris out of your pool water, which will also reduce the demand on your pool equipment.

There are different types of cover you can get. There’s the usual safety cover, an expensive automatic safety cover, a winter cover, and a solar cover.

You can also opt for a chemical pool cover, which is an invisible barrier that forms on your pool surface to reduce water evaporation loss. You can still swim in it and it will reform once the water settles. You will just need to replace this monthly as the chemical breaks down after some time.

Ideal Water Temperature

You also don’t want your pool water to constantly have a warm temperature as warmer water evaporates faster than cooler water. If you have warm water when the cool air kicks in, your pool water will evaporate quickly. 

Turn Off Your Pool Waterfall

This goes for all your pool water features. These extra features, such as jets, allows more water to be exposed to the sun and air. More exposure to sunlight and wind will increase the water evaporation rate. 

Save money and water by turning them off when not in use. You can just turn them on when you have guests over.

Prevent your pets from swimming

I’m not saying that you should keep your water-loving dog from swimming during a hot summer day. This just means that you should limit their access to the pool when you or your family is not around. 

You might not know that your dog keeps jumping in and out of your pool when you’re at work. It’s also best to stop them from swimming alone to avoid any casualties.


Strong wind is one of the factors that increase the water evaporation rate. One way to combat this is to add windbreaks or barriers around your pool to prevent the fast and strong wind from immediately making contact with the water. This will also avoid creating waves or blowing the water out of your pool which will reduce water evaporation.

You can easily add barriers without ruining your pool area by using landscaping features. You can add thick bushes and shrubs, which will add some charm to your backyard. If you don’t have a fence, a metal or wooden fence will help as well. You may have to add some plants or cover your fence if it is just a chain link or wrought iron post since these won’t be able to block the wind effectively.

Kenneth Wilson
November 22, 2019
Ideas & Inspiration

Kenneth Wilson

Retired contractor. Currently residing in Southwest Florida. Now in semi-retirement, I write and manage this blog focused on helping home owners make savvy decisions when it comes to finding contractors and getting their projects done. I also operate remodeling design service for homeowners.

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