How Much Water Evaporates From a Pool [and how to stop it]

Maintaining a swimming pool can entail a lot of time and money, and constantly having to top up your pool is a large part of that. Sure, topping up your water level is normal due to evaporation, but how much is normal?

It is normal for a pool to lose 2 to 4 inches of water per week from natural evaporation. If your pool is losing more than that per day, it is very likely that your pool is leaking water elsewhere.

Knowing how much water your pool is losing each day is very important. How else would you know if the water loss is normal due to evaporation, or that the loss is more serious, like a leak in the pool shell?

Let’s look at how to measure the weekly water loss in your pool to make sure that it is not something more serious than evaporation. Once we have established that the water loss is just evaporation, let’s look at the factors that cause evaporation, and how we can reduce evaporation rates.

How Do You Calculate Water Loss in Your Swimming Pool?

If you are constantly having to refill your pool, you will first want to find out exactly how much water you are losing per day before deciding what to do next.

Here is a simple way to determine how much water your pool is losing each day.

  1. Fill your pool to the correct level to get an accurate baseline measurement.
  2. Switch off all your water features, pumps, etc. and allow the water the settle.
  3. When the water is calm, using a pencil and ruler, draw a solid line at the current water level and label it “Day 0”
  4. Now switch on your pump and leave everything to run normally for a week.
  5. After EXACTLY one week, and at the same time in the day as the first test, switch off your pump and wait for the water to settle.
  6. After the water has settled, you can now draw another line with a pencil and a ruler.
  7. Measure the difference between the two lines to find your weekly water loss.

Even in a dry and windy climate, losing more than half an inch per day probably means you have a leak. If you suspect that you have a leak in your pool, it’s best to contact your local pool supply store and ask them how to go about it.

What Increases The Rate of Evaporation?

Evaporation is very dependant on where you live and the seasons. Air temperature and humidity, water temperature, exposure to sunlight, and prevailing winds all have a huge impact on evaporation rates.

Here are the main conditions that affect water evaporation in swimming pools.

  1. Water Temperature
  2. Humidity
  3. Air Temperature
  4. Wind Speed
  5. Exposed Surface Area
  6. Pool Usage

1. How water temperature affects pool water evaporation.
Warmer water evaporates at a much faster rate. When your pool is exposed to the hot sun, the pool water can be heated until the water is warmer than the air around it, which will increase the evaporation rate. This is exaggerated at night when the air temperature drops and the pool water is still warm.
[Source: – Evaporation Factors]

2. How humidity affects pool water evaporation.
The humidity of the air has an effect on evaporation. Low humidity means dryer air and a high evaporation rate and conversely, the more humid the air is, meaning it is already quite saturated with water, the lower the evaporation rate. Think of the air around your pool like a sponge, the more water in that sponge the less water it can draw from your pool.
[Source: – Evaporation and Climate]

3. How air movement (wind speed) affects pool water evaporation.
Along with low humidity air that is colder than the pool water, wind moving across the surface of your pool will rapidly increase evaporation as the newly saturation air passes over and new dry air draws more water molecules from the surface of the pool.

4. Does extra exposed water surface area increase evaporation?
The surface area affects the evaporation because more water is exposed to the air. If you have additional exposed water surface areas, such as a rim-flow / infinity edge, or a waterfall feature, you should expect a much higher loss from water evaporation.

6. How does heavy pool usage increase water evaporation rates?
Heavy pool usage will constantly agitate the surface of the pool, essentially aerating the water. This causes more water surface exposure to air and increasing the amount of evaporation. In the same way, the splashing of water by swimmers into the air increases the surface area of exposed water, and that will increase the rate of evaporation of your pool water. In the science world, this would be referred to as “Mechanical Aeration” and is a method used to increase evaporation rates.
[Source: – Effects of mechanical aeration on evaporation rate]

Can pool water really evaporate even in cold weather?
Yes, as a matter of fact, the colder the air is relative to the pool water, evaporation is actually increased! This is even more pronounced when there is low humidity. Water can evaporate even in near-freezing temperatures.

Ok, so now we know what exactly causes evaporation. Let’s use this knowledge and look at ways to reduce evaporation in our pools, saving us time and money!

We have already established that it is not abnormal to lose 2 to 4 inches of water per week. What’s a couple of inches though? That doesn’t really sound like much. Well, let’s see how much water it takes to top up an average-sized pool by an inch.

How Much Water Does An Average Size Pool Lose Each Week?

The typical pool size in the United States is around 15 feet 30 feet. Using an online volume calculator (because I actually suck at math), let’s calculate the amount of water in gallons two inches in that pool would be.

15 feet x 30 feet x 2 inches = 75 cubic feet

Doing some “quick-math”, we find that 75 cubic feet = 561 Gallons.

(I lied, I didn’t use quick-math, I used Google)

The average pool-sized 15ft x 30ft, losing 2 inches of water a week is equal to 561 Gallons of water. That equates to 2,244 Gallons each month, and a whopping 26,928 Gallons every year!

It is quite obvious that the amount of water lost due to evaporation is quite substantial. Even though the cost of that water might not be a problem, it sure as hell is environmentally irresponsible, and if you don’t care about the environment, then we can’t be friends.

So if you want to save water and be my friend, let’s look at some ways we can reduce evaporation in our swimming pools.

How to Reduce Water Evaporation in Swimming Pools

Let’s save ourselves some time and money (and environment) by doing as much as we can to reduce water evaporation. It’s the right thing to do.

1. Solar Pool Covers
You can use solar covers to prevent evaporation. Solar pool covers come in large solid sheets that can be neatly rolled up on a real. You can buy them in smaller round panels that float on your pool like Lilly pads, they come in some really awesome designs and can help complement your pool area decor.

These are also great if you live in a cold area because they help a lot to heat up your pool. These covers also help to protect your chlorine from getting destroyed by the sun’s UV rays.

2. Liquid Solar Pool Covers
Try a liquid solar cover. These are also sometimes called a liquid blanket. They come in a bottle and are non-toxic. Once poured into the pool, the liquid floats on the surface of your pool and creates a barrier that prevents water evaporation.

3. Wind Screens or Walls
We know that high-speed wind increases evaporation dramatically. Building a screen or wall to protect the pool from the prevailing wind can help reduce the evaporative effects on the pool while making the pool area more pleasant to use.

4. Avoid Water Features and Infinity Edges
If water loss is a concern for you, avoid water features and infinity edges when designing your pool. I do agree that they look awesome and give a really classy feel to your pool, so I wouldn’t judge you if you ignore this point.


It is quite normal to lose 2 to 4 inches of water every week. However, anything more than that means that you very likely have a leak and this will need to be checked by a professional.

To make sure that your water loss is within normal ranges, take the time to test how much water you are losing per week before doing anything.

If your water loss is within normal ranges, then you are dealing with normal water loss from evaporation. Many people choose to simply top up their pool weekly, but some of us no doubt will want to save time, money, and water by minimizing evaporation as much as possible.

Water evaporation is dependant on water temperature, air temperature, humidity, wind, pool usage, and surface area. We can reduce exposure to some of these factors by the use of solar pool covers, screen walls, and avoiding certain design features in a pool.

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