Pool Evaporation Calculator

Period	Evaporation (Gallons)	Depth (inches)
Hour	0	0
Day	0	0
Week	0	0
Month (30 days)	0	0

What is Pool Evaporation?

Pool evaporation is the process where water from your swimming pool changes from a liquid to a gas or vapor and is lost to the atmosphere. It’s a natural phenomenon driven by differences in vapor pressure between the pool water surface and the surrounding air, influenced by factors like temperature, humidity, wind, and the pool’s surface area. Understanding and estimating pool evaporation is crucial for pool owners to manage water levels, conserve water, and understand potential pool water loss, which can also be caused by leaks. This pool evaporation calculator helps quantify this loss.

Anyone with an outdoor or even a heated indoor pool should use a pool evaporation calculator to get an idea of how much water they might be losing naturally. It helps differentiate normal evaporation from potential leaks and aids in budgeting for water replacement and understanding pool maintenance costs.

A common misconception is that evaporation is negligible, especially in cooler or humid climates. However, even under moderate conditions, a significant amount of water can evaporate over time, particularly from large or heated pools. Another is that evaporation is the same day and night; it often reduces at night due to lower temperatures and higher humidity, but wind can still play a role. Our pool evaporation calculator considers these factors.

Pool Evaporation Formula and Mathematical Explanation

The pool evaporation calculator uses a simplified model based on principles seen in formulas like the Rohwer or Penman equations, adapted for typical pool conditions. The core idea is that evaporation is proportional to the difference between the saturation vapor pressure at the water surface (ew) and the actual vapor pressure of the air (ea), and is also influenced by wind speed.

The steps are generally:

Calculate the saturation vapor pressure at the water temperature (ew) and at the air temperature.
Calculate the actual vapor pressure of the air (ea) using the air temperature and relative humidity.
Determine the vapor pressure difference (ew – ea).
Apply a factor that includes wind speed to this difference to get the evaporation rate (e.g., inches or mm per day). Our pool evaporation calculator uses a formula like: `E (inches/day) = (C1 + C2 * Wind) * (ew – ea)`, where C1 and C2 are coefficients and wind is in mph, and vapor pressures are converted to inches of Mercury (inHg).
Multiply the rate by the pool surface area and time period to get the total volume lost.

Here’s a breakdown of the variables involved in the pool evaporation calculator:

Variable	Meaning	Unit	Typical Range
A	Pool Surface Area	sq ft or m²	100 – 1000 sq ft (10 – 100 m²)
Tw	Water Temperature	°F or °C	60 – 90 °F (15 – 32 °C)
Ta	Air Temperature	°F or °C	50 – 100 °F (10 – 38 °C)
RH	Relative Humidity	%	20 – 90 %
W	Wind Speed	mph or m/s	0 – 20 mph (0 – 9 m/s)
ew	Saturation Vapor Pressure at Tw	inHg or kPa	0.5 – 1.5 inHg
ea	Actual Vapor Pressure of Air	inHg or kPa	0.1 – 1.2 inHg
E	Evaporation Rate	inches/day or mm/day	0.05 – 0.5 inches/day
t	Time Period	days	1 – 30 days

Practical Examples (Real-World Use Cases)

Let’s see how the pool evaporation calculator works with some examples.

Example 1: Hot, Dry, and Windy Day

Imagine a 500 sq ft pool in a dry, hot, and windy area:

Pool Area: 500 sq ft
Water Temp: 85 °F
Air Temp: 95 °F
Humidity: 20%
Wind Speed: 10 mph
Time: 7 days

Using the pool evaporation calculator with these values, you might find an evaporation rate of around 0.35 inches/day, leading to a total loss of about 2.45 inches over 7 days, or approximately 760 gallons. This highlights the significant water loss under such conditions and the importance of saving pool water.

Example 2: Mild, Humid Day

Now consider a 300 sq ft pool in a milder, more humid setting:

Pool Area: 300 sq ft
Water Temp: 75 °F
Air Temp: 78 °F
Humidity: 70%
Wind Speed: 3 mph
Time: 7 days

The pool evaporation calculator would likely show a much lower rate, maybe 0.10 inches/day, resulting in 0.7 inches lost over a week, or about 130 gallons. This is still water loss, but considerably less than the first example.

How to Use This Pool Evaporation Calculator

Select Units: Choose between US/Imperial or Metric units first. This will adjust the input labels.
Enter Pool Surface Area: Input the area of your pool’s water surface in the specified units.
Enter Water Temperature: Input the average temperature of the pool water.
Enter Air Temperature: Input the average air temperature near the pool.
Enter Relative Humidity: Input the average relative humidity as a percentage.
Enter Wind Speed: Input the average wind speed across the pool surface.
Enter Time Period: Specify the number of days you want to estimate evaporation for.
View Results: The calculator automatically updates the “Total Evaporation,” “Evaporation Rate,” “Vapor Pressure Difference,” and “Total Evaporation Depth.” The table and chart also update.
Interpret Results: The primary result shows the total volume of water lost over the time period. The rate gives you daily loss, and the table breaks it down further. If your observed water loss is much higher than the calculated evaporation, you might want to investigate detecting pool leaks.

Use the results to understand your pool’s water needs and to differentiate between normal evaporation and potential leaks. Consistent use of a pool evaporation calculator can help in water management.

Key Factors That Affect Pool Evaporation Results

Several factors influence the rate of evaporation from a pool, as reflected in the pool evaporation calculator inputs:

Pool Surface Area: Larger surface areas expose more water to the atmosphere, increasing evaporation proportionally.
Water Temperature: Warmer water has a higher vapor pressure, leading to faster evaporation, especially if the air is cooler or drier. Heating your pool increases evaporation.
Air Temperature: Higher air temperature can hold more moisture, but it’s the difference between water and air temperature, and the humidity, that really drives evaporation.
Relative Humidity: Lower humidity means drier air, which can absorb more water vapor, increasing evaporation. High humidity reduces it.
Wind Speed: Wind blows away the moist air layer just above the water surface, replacing it with drier air, thus accelerating evaporation. Even light winds significantly increase evaporation compared to still air.
Sunlight Exposure: Direct sunlight heats the water, increasing its temperature and thus evaporation. While not a direct input in this simplified pool evaporation calculator, it influences water temperature. Using pool cover benefits include reducing sunlight exposure and wind effect.
Water Salinity/TDS: Higher levels of dissolved solids can slightly reduce the evaporation rate, but this effect is generally small for typical pool water compared to the other factors.

Frequently Asked Questions (FAQ)

How accurate is this pool evaporation calculator?: This calculator provides an estimate based on a simplified formula. Actual evaporation can vary due to microclimates around the pool, direct sun exposure, and how sheltered the pool is. It’s a good guide but not a precise measurement device.
Why is my pool losing more water than the calculator estimates?: If your pool is losing significantly more water than estimated, especially if the loss is more than 0.25-0.5 inches per day after accounting for splash-out, you might have a leak. Consider performing a bucket test or consulting a pool professional about detecting pool leaks.
How can I reduce pool evaporation?: Using a pool cover is the most effective way. It drastically reduces the water surface exposed to air and wind. Other methods include windbreaks (fences, hedges), maintaining lower water temperature, and reducing water features that increase aeration when not in use.
Does a heated pool evaporate more water?: Yes, heating a pool increases the water temperature, which raises the vapor pressure at the surface and significantly increases the evaporation rate, especially if the air is cooler and drier.
Does an indoor pool evaporate?: Yes, indoor pools also experience evaporation, especially if the air within the enclosure is not very humid or is well-ventilated. The pool evaporation calculator can still give an idea if you input the indoor air conditions, though indoor wind is usually minimal.
What is the normal amount of pool evaporation?: It varies greatly by location, season, and weather, but typically ranges from 0.1 to 0.4 inches per day. Hot, dry, windy areas can see more.
Does salt water evaporate slower than fresh water?: Salt and other dissolved solids do slightly reduce the evaporation rate, but the effect is usually small (a few percent) at typical pool salt concentrations compared to the impact of temperature, humidity, and wind.
How does the pool evaporation calculator handle units?: You can select either US/Imperial or Metric units. The calculator converts inputs and displays results accordingly. Make sure your input values match the selected unit system.

Related Tools and Internal Resources

Pool Water Loss Causes: Understand the different reasons your pool might be losing water, including evaporation and leaks.
Pool Maintenance Cost Calculator: Estimate the costs associated with maintaining your pool, including water replacement.
How to Save Pool Water: Tips and tricks for conserving water in and around your swimming pool.
Pool Cover Benefits: Learn about the advantages of using a pool cover, including reduced evaporation and energy savings.
Is My Pool Leaking?: A guide to help you determine if your pool has a leak beyond normal evaporation.
Water Loss Calculator: A general tool to calculate water loss for various applications.

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