Swimming Pool Evaporation Calculator – Estimate Water Loss

Swimming Pool Evaporation Calculator

Estimate the water lost from your swimming pool due to evaporation using our Swimming Pool Evaporation Calculator. Enter your pool’s dimensions and local conditions.

Pool Length (feet):

Enter the length of your pool at the water surface.

Pool Width (feet):

Enter the average width of your pool at the water surface. For non-rectangular pools, estimate an equivalent rectangular area or enter area directly below.

Or Pool Surface Area (square feet):

If your pool isn’t rectangular, enter the surface area here. This will override Length x Width.

Water Temperature (°F):

Average temperature of the pool water (33-100°F).

Air Temperature (°F):

Average air temperature around the pool (0-120°F).

Relative Humidity (%):

Average relative humidity (0-100%).

Wind Speed (mph):

Average wind speed just above the water surface (0-30 mph). Sheltered pools have lower effective wind.

Pool Environment:

Select the environment of your pool.

Hours Exposed per Day:

Number of hours per day the pool is uncovered or exposed to these conditions (1-24). If you use a cover, enter hours uncovered.

Evaporation Results

Enter values to see results

Water Surface Area: – sq ft

Evaporation per Week: – Gallons | – Inches

Evaporation per Month (30 days): – Gallons | – Inches

Saturation Vapor Pressure (Water): – inHg

Actual Vapor Pressure (Air): – inHg

Formula Used (Simplified): Evaporation (gallons/day) is estimated based on surface area, the difference between water surface vapor pressure (Pw) and air vapor pressure (Pa), wind speed, exposure, and hours exposed. Pw and Pa are calculated from temperatures and humidity. A larger difference, higher wind, and more exposure increase evaporation.

Evaporation vs. Wind Speed and Humidity (Gallons/Day)

Estimated daily evaporation at different wind speeds and humidity levels, based on current temperature and area inputs.

Evaporation Estimates at Different Wind Speeds

Wind Speed (mph)	Daily (Gallons)	Weekly (Gallons)	Monthly (Gallons)
–	–	–	–
–	–	–	–
–	–	–	–
–	–	–	–

Table showing how daily, weekly, and monthly water loss (gallons) changes with wind speed, keeping other factors constant.

What is a Swimming Pool Evaporation Calculator?

A Swimming Pool Evaporation Calculator is a tool used to estimate the amount of water lost from a swimming pool due to evaporation over a specific period. It takes into account various factors such as the pool’s surface area, water temperature, air temperature, relative humidity, wind speed, and the pool’s environment (indoor, outdoor, screened). By understanding the rate of evaporation, pool owners can anticipate water replacement needs, understand potential increases in chemical concentration, and take measures to reduce water loss, like using a pool cover. This is especially important in dry or windy climates where evaporation rates can be significant. Using a Swimming Pool Evaporation Calculator helps in water conservation and managing pool maintenance costs.

Anyone who owns or maintains a swimming pool, spa, or even a large pond can benefit from using a Swimming Pool Evaporation Calculator. It’s particularly useful for those in arid or windy regions, or those looking to conserve water and reduce the costs associated with refilling and rebalancing pool chemicals. A common misconception is that evaporation is negligible, but it can be the largest source of water loss for a pool, often exceeding losses from splashing or leaks.

Swimming Pool Evaporation Calculator Formula and Mathematical Explanation

The rate of evaporation from a pool surface is primarily driven by the difference in vapor pressure between the water surface and the surrounding air, as well as wind speed. While precise calculations can be very complex, a commonly used empirical formula adapted for pools is:

E_inches_per_hour = C * (Pw - Pa) * (1 + Wf * V) * EF

E_gallons_per_day = E_inches_per_hour * H * A / 12 * 7.48

Where:

E_inches_per_hour is the evaporation rate in inches per hour.
C is a coefficient (related to heat transfer, often around 0.0041 for inches/hr with P in inHg and V in mph).
Pw is the saturation vapor pressure of water at the pool surface temperature (inHg).
Pa is the actual vapor pressure of the air (saturation vapor pressure at air temperature * relative humidity) (inHg).
Wf is a wind factor coefficient (e.g., 0.0025).
V is the wind speed above the water surface (mph).
EF is an exposure factor (e.g., 1 for outdoor, 0.7 for screened, 0.4 for indoor).
E_gallons_per_day is the evaporation in gallons per day.
H is the number of hours the pool is exposed per day.
A is the surface area of the pool (sq ft).
The /12 * 7.48 converts cubic feet (Area * inches/12) to gallons.

The saturation vapor pressure (Ps) at a given temperature (T in Celsius) can be estimated using the Tetens equation: Ps(kPa) = 0.61078 * exp((17.27 * T) / (T + 237.3)), which is then converted from kPa to inHg (1 kPa ≈ 0.2953 inHg).

Variable	Meaning	Unit	Typical Range
A	Pool Surface Area	sq ft	100 – 1000
Tw	Water Temperature	°F	60 – 90
Ta	Air Temperature	°F	50 – 100
RH	Relative Humidity	%	20 – 90
V	Wind Speed	mph	0 – 15
Pw	Sat. Vapor Pressure (Water)	inHg	0.5 – 1.4
Pa	Actual Vapor Pressure (Air)	inHg	0.1 – 1.0
H	Hours Exposed	hours	1 – 24
EF	Exposure Factor	–	0.4 – 1.0

Table of variables used in the Swimming Pool Evaporation Calculator.

Practical Examples (Real-World Use Cases)

Example 1: Hot, Dry, and Windy Day

Imagine a 30ft x 15ft pool (450 sq ft) in Phoenix, Arizona. Water temp is 85°F, air temp is 100°F, humidity is 20%, and it’s a bit windy at 8 mph. The pool is outdoor and uncovered for 24 hours.

Area (A) = 450 sq ft
Water Temp = 85°F
Air Temp = 100°F
Humidity = 20%
Wind = 8 mph
Exposure = Outdoor (1.0), 24 hours

Using the Swimming Pool Evaporation Calculator, you might find an evaporation of around 1.5 to 2 inches per day, translating to roughly 420 to 560 gallons per day! This high rate is due to the large vapor pressure difference (hot water, very dry air) and the wind.

Example 2: Cool, Humid, Calm Day with a Cover Used

Consider the same 450 sq ft pool, but now in a coastal area. Water temp is 75°F, air temp is 70°F, humidity is 80%, and wind is low at 2 mph. The owner uses a cover for 12 hours a day, so it’s exposed for 12 hours.

Area (A) = 450 sq ft
Water Temp = 75°F
Air Temp = 70°F
Humidity = 80%
Wind = 2 mph
Exposure = Outdoor (1.0), 12 hours

The Swimming Pool Evaporation Calculator would show a much lower evaporation rate, perhaps 0.15 to 0.25 inches over the 12 exposed hours, leading to maybe 40-70 gallons lost during that time. The lower temperatures, high humidity, low wind, and reduced exposure time significantly reduce water loss.

How to Use This Swimming Pool Evaporation Calculator

Enter Pool Dimensions: Input the length and width of your pool. If it’s not rectangular, calculate the surface area and enter it directly into the “Pool Surface Area” field.
Input Temperatures: Enter the average water temperature and air temperature in degrees Fahrenheit.
Enter Humidity: Input the average relative humidity as a percentage.
Enter Wind Speed: Estimate the average wind speed just above the water surface in mph.
Select Pool Environment: Choose whether your pool is outdoor, screened, or indoor.
Hours Exposed: Enter how many hours per day the pool is uncovered or exposed to the elements.
Calculate: The calculator will automatically update, or you can click “Calculate”.
Read Results: The primary result shows daily evaporation in gallons and inches. Intermediate results show weekly and monthly estimates, area, and vapor pressures.
Analyze Chart and Table: The chart and table show how evaporation changes with wind and humidity, helping you understand the impact of these factors.

Use the results from the Swimming Pool Evaporation Calculator to understand how much water your pool is likely losing and when it’s highest. Consider using a pool cover, especially during periods of high evaporation (hot, dry, windy), to conserve water and reduce the need to add and rebalance chemicals. More details on pool water conservation can be found here.

Key Factors That Affect Swimming Pool Evaporation Calculator Results

Pool Surface Area: Larger pools have more surface area exposed to the atmosphere, leading to greater evaporation.
Water Temperature: Warmer water has a higher vapor pressure, increasing the driving force for evaporation.
Air Temperature & Humidity: The difference in vapor pressure between the water and the air is crucial. Hot, dry air (low humidity) creates a larger vapor pressure deficit compared to the water, increasing evaporation.
Wind Speed: Wind blows away the layer of moist air just above the water surface, maintaining a higher vapor pressure gradient and thus increasing evaporation. Even a light breeze significantly increases evaporation compared to still air.
Sunlight/Solar Radiation: Sunlight heats the water, increasing its temperature and vapor pressure, contributing to more evaporation. While not a direct input, it influences water temperature.
Pool Cover/Screening: A pool cover acts as a barrier, dramatically reducing evaporation (by up to 95%). Screening also reduces wind speed over the surface, lowering evaporation compared to an open pool. Learn about pool cover benefits.
Bather Load: Splashing and activity can slightly increase evaporation and water loss, though it’s usually less significant than the main environmental factors.

Understanding these factors helps in interpreting the results of the Swimming Pool Evaporation Calculator and taking effective measures.

Frequently Asked Questions (FAQ)

How much water does a pool lose to evaporation per day?: It varies greatly depending on the factors above, but typically between 0.1 to 0.5 inches per day for an uncovered outdoor pool, which can be hundreds of gallons for a large pool. Our Swimming Pool Evaporation Calculator gives a more precise estimate.
Does a heated pool evaporate more water?: Yes, heating the pool water increases its temperature and thus its vapor pressure, leading to a higher evaporation rate, especially if the air is cool and dry.
How effective is a pool cover in reducing evaporation?: Very effective. A good pool cover can reduce evaporation by 90-95% by acting as a barrier.
Does wind really make a big difference?: Yes, wind is a major factor. It removes the saturated air layer above the water, allowing more water to evaporate more quickly. Even a small increase in wind speed can significantly boost evaporation, as shown by the Swimming Pool Evaporation Calculator.
What about indoor pools?: Indoor pools still experience evaporation, but usually at a lower rate than outdoor pools because the air is more contained, often more humid, and there’s no wind or direct sun. However, the evaporated water needs to be managed by a dehumidification system. Our Swimming Pool Evaporation Calculator has an ‘Indoor’ option.
Is evaporation the only way pools lose water?: No, pools also lose water through splashing, backwashing filters, and potential leaks. However, evaporation is often the largest component of water loss for a well-maintained pool. Check for pool leak detection methods if you suspect a leak.
How can I tell if my pool is leaking or just evaporating?: The “bucket test” is a common method. Place a bucket filled with pool water on a pool step, mark the water levels inside and outside the bucket, and compare the drop after 24 hours. If the pool level drops more than the bucket level, you likely have a leak in addition to evaporation. The Swimming Pool Evaporation Calculator can give you an idea of expected evaporation.
Does salt water evaporate faster than fresh water?: Slightly slower. The presence of salt and other dissolved solids reduces the vapor pressure of the water very slightly, so a saltwater pool will evaporate marginally less than an identical freshwater pool under the same conditions. However, for practical purposes, the difference is often negligible compared to other factors.

Related Tools and Internal Resources

Pool Volume Calculator: Calculate the total volume of water in your pool, essential for chemical dosing and understanding water loss proportions.
Pool Chemical Calculator: Determine the amount of chemicals needed to balance your pool water after water loss and refilling.
Water Conservation Tips for Pool Owners: Learn more ways to save water in and around your swimming pool.