Pool Evaporation Calculator

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Pool Evaporation Calculator – Estimate Water Loss & Detect Leaks


Pool Evaporation Calculator

Accurately determine how much water your swimming pool is losing to evaporation vs. potential leaks.


Standard pool is approx. 15×30 ft (450 sq. ft)
Please enter a positive area.


Average ambient outdoor temperature.


Temperature of the pool water surface.


Current or average daily humidity.
Humidity must be between 0 and 100.


Average wind speed at the pool surface.


Daily Water Loss
0 Gallons
Weekly Loss
0 Gallons
Level Drop (Day)
0 Inches
Level Drop (Week)
0 Inches

Formula: This pool evaporation calculator uses a modified Dalton’s Law equation:
E = (0.0174 + 0.0114 × v) × (Pw – Pa), where ‘v’ is wind speed, ‘Pw’ is vapor pressure of water, and ‘Pa’ is vapor pressure of air.

Water Loss vs. Wind Speed

Figure 1: Comparison of daily gallon loss as wind speed increases (0 to 20 mph).

Factor Current Input Impact on Evaporation
Surface Area 450 sq ft Directly proportional
Temp Difference 5 °F Higher water temp increases loss
Humidity 50 % Dryer air accelerates evaporation
Wind Speed 5 mph Significantly increases rate

What is a Pool Evaporation Calculator?

A pool evaporation calculator is a specialized tool used by pool owners, technicians, and property managers to estimate the amount of water lost from a swimming pool due to environmental factors. Unlike a physical leak, evaporation is a natural process where water transforms from liquid to gas at the surface. Using a pool evaporation calculator helps distinguish between normal seasonal water loss and a potential structural or plumbing leak.

Every pool owner should use a pool evaporation calculator because understanding your baseline water loss saves money on chemicals and prevents unnecessary “leak hunts.” A common misconception is that pool covers stop 100% of evaporation or that pools shouldn’t lose any water at all; in reality, even in humid climates, a pool can lose significant volume daily.

Pool Evaporation Calculator Formula and Mathematical Explanation

The science behind our pool evaporation calculator is based on Dalton’s Law of Partial Pressures. The rate of evaporation is driven by the difference between the saturation vapor pressure at the water’s surface and the actual vapor pressure of the ambient air.

The simplified derivation used in this pool evaporation calculator is:

E = (C1 + C2 * v) * (Pw - Pa)

Where:

  • E: Evaporation rate (kg/m²/h)
  • v: Wind speed at the water surface (m/s)
  • Pw: Vapor pressure of saturated air at the water temperature (kPa)
  • Pa: Vapor pressure of the air (kPa)
  • C1, C2: Empirical constants (typically 0.0174 and 0.0114)
Variable Meaning Unit Typical Range
Water Temp Surface temperature °F / °C 75 – 90 °F
Air Temp Ambient temperature °F / °C 60 – 105 °F
Humidity Relative Air Humidity % 20 – 80 %
Wind Speed Air velocity at surface mph / m/s 0 – 15 mph

Practical Examples (Real-World Use Cases)

Example 1: The Desert Oasis (High Evaporation)

In Arizona, a pool owner has a 500 sq ft pool. The air is 100°F with only 10% humidity, and the water is 85°F. With a 5 mph breeze, the pool evaporation calculator would show a loss of approximately 55 gallons per day. Over a week, that’s nearly 400 gallons. Without this pool evaporation calculator, the owner might wrongly assume the pool has a structural crack.

Example 2: The Tropical Retreat (Low Evaporation)

In Florida, during a humid summer, the air is 90°F with 85% humidity. Even if the water is 85°F, the high moisture content in the air slows down the process. The pool evaporation calculator might only show a loss of 12 gallons per day. If this owner sees the water level dropping 2 inches a week, they know immediately they have a leak, not just evaporation.

How to Use This Pool Evaporation Calculator

  1. Enter Dimensions: Input your pool’s total surface area. If you don’t know it, multiply Length x Width for rectangular pools.
  2. Input Weather Data: Check your local weather app for the current air temperature, humidity, and wind speed.
  3. Check Water Temp: Use a pool thermometer to get an accurate surface temperature.
  4. Analyze Results: Look at the “Daily Water Loss” in gallons and inches. Compare this to your actual observed water level drop.
  5. The Bucket Test: If your observed loss is significantly higher than the pool evaporation calculator result, perform a “Bucket Test” to confirm a leak.

Key Factors That Affect Pool Evaporation Calculator Results

  • Surface Area: The larger the surface area, the more “room” there is for water molecules to escape. A 1,000 sq ft pool will always lose more water than a 400 sq ft pool under the same conditions.
  • Vapor Pressure Differential: The difference between water and air temperature is critical. Warm water in cool, dry air (like at night) evaporates very quickly.
  • Relative Humidity: Air acts like a sponge. If the humidity is high (80%+), the air is “full” and cannot accept much more evaporated water. In dry climates, the sponge is empty.
  • Wind Speed: Wind removes the “boundary layer” of saturated air just above the water surface, replacing it with dryer air and drastically accelerating loss.
  • Solar Radiation: While not a direct input in simple versions of a pool evaporation calculator, sunlight provides the latent heat needed for phase change from liquid to gas.
  • Pool Covers: A solid cover can reduce the results provided by a pool evaporation calculator by up to 95% by eliminating the air-water interface.

Frequently Asked Questions (FAQ)

How much evaporation is normal for a pool?

Typically, 1/4 to 1/2 inch of water per day is normal, depending on your climate and wind exposure. Our pool evaporation calculator helps you find your specific “normal.”

Does evaporation happen more at night?

Yes, often. When the air temperature drops but the pool water remains warm from the sun, the vapor pressure difference increases, causing high night-time evaporation.

Can a pool heater increase evaporation?

Absolutely. Heating your water increases its vapor pressure. Using a pool evaporation calculator while adjusting water temp shows how much extra water (and energy) you lose.

What is the “Bucket Test”?

Place a bucket of water on a pool step, matching the water levels inside and out. After 24 hours, if the pool level dropped more than the bucket level, you have a leak.

How does wind speed impact the pool evaporation calculator?

Wind is one of the most aggressive factors. Increasing wind from 0 to 10 mph can often triple the evaporation rate.

Does humidity stop evaporation?

No, but 100% humidity slows it to a near-halt. The pool evaporation calculator accounts for this via the vapor pressure of the air (Pa).

Why does my salt pool seem to evaporate faster?

Technically, salt slightly lowers vapor pressure, slowing evaporation, but the difference is negligible. Usually, it’s just weather-related.

Should I use a pool cover every night?

Yes. It is the single most effective way to stop the water loss calculated by our pool evaporation calculator.

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Pool Evaporation Calculator

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Pool Evaporation Calculator & Guide


Pool Evaporation Calculator

Calculate Pool Water Evaporation

Estimate the amount of water your pool loses due to evaporation based on environmental factors.





Enter the total surface area of your pool.


Average temperature of the pool water.


Average ambient air temperature.


Enter the average relative humidity (0-100).


Average wind speed over the pool surface.


Number of days to estimate evaporation for.



Results:

Enter values and click Calculate.

Formula Used (Simplified): Evaporation is primarily driven by the difference between water vapor pressure at the water surface and in the air, influenced by wind speed and surface area. The calculator uses established formulas (like a modified Penman or Buck equation for vapor pressures) to estimate this.

Chart: Estimated Daily Evaporation vs. Wind Speed

Day Daily Loss (Gallons) Cumulative Loss (Gallons)
Enter values to see table.

Table: Estimated Daily and Cumulative Water Loss

What is a Pool Evaporation Calculator?

A 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 considers factors like the pool’s surface area, water and air temperature, relative humidity, and wind speed. Evaporation is a natural process where water changes from a liquid to a gas or vapor, and it’s one of the primary ways swimming pools lose water, often more so than leaks or splash-out.

Pool owners, maintenance professionals, and water conservation enthusiasts use a pool evaporation calculator to understand the extent of water loss and to plan for refilling or implementing water-saving measures like pool covers. Misconceptions include believing that only heat causes evaporation (humidity and wind are also crucial) or that small pools don’t lose much water (per square foot, the rate can be similar).

Pool Evaporation Formula and Mathematical Explanation

The rate of evaporation from a pool surface is complex but can be estimated using formulas that relate to the difference in vapor pressure between the water surface and the surrounding air, modified by wind. A common approach involves:

  1. Calculating the saturation vapor pressure at the water surface temperature (Pw).
  2. Calculating the actual vapor pressure in the air based on air temperature and relative humidity (Pa).
  3. Using the difference (Pw – Pa) along with wind speed and surface area to estimate the evaporation rate.

A simplified formula for evaporation rate (E) in inches per day per square foot can look like:
E_rate = C * (Pw - Pa) * (1 + k * WindSpeed)
Where C and k are coefficients, Pw and Pa are vapor pressures (e.g., in inHg), and WindSpeed is in mph. The calculator uses more precise methods like the Buck equation to find Pw and Pa from temperatures.

Vapor Pressure Calculation (Buck Equation simplified for hPa):
Ps = 6.112 * exp((17.67 * T_c) / (T_c + 243.5))
Where T_c is temperature in °C. Pw is Ps at water temp, and Pa = (RH/100) * Ps at air temp.

The total volume lost is then E_rate * Area * Time * ConversionFactorToGallons.

Variables Used in Calculation
Variable Meaning Unit Typical Range
A Pool Surface Area sq ft or m² 100 – 1000 sq ft
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 – 15 mph
Pw Saturated Vapor Pressure at Water Surface inHg or hPa 0.5 – 1.5 inHg
Pa Actual Vapor Pressure in Air inHg or hPa 0.1 – 1.0 inHg
E Evaporation Rate/Total Loss Gallons/day or Liters/day Varies widely

Practical Examples (Real-World Use Cases)

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

Example 1: Hot, Dry, and Breezy Day

  • Pool Area: 600 sq ft
  • Water Temp: 85°F
  • Air Temp: 95°F
  • Humidity: 30%
  • Wind Speed: 10 mph
  • Time: 1 day

The calculator would show a significant daily water loss, potentially over 150-200 gallons per day, due to the high temperatures, low humidity, and moderate wind creating a large vapor pressure difference and aiding evaporation.

Example 2: Cool, Humid, and Calm Day

  • Pool Area: 400 sq ft
  • Water Temp: 70°F
  • Air Temp: 72°F
  • Humidity: 80%
  • Wind Speed: 2 mph
  • Time: 1 day

Here, the water loss would be much lower, perhaps 20-40 gallons per day, because the air is already quite moist (high humidity), the temperature difference is small, and the wind is minimal.

How to Use This Pool Evaporation Calculator

  1. Select Units: Choose between Imperial (ft², °F, mph) or Metric (m², °C, m/s) units at the top. The labels will update accordingly.
  2. Enter Pool Area: Input the surface area of your pool.
  3. Enter Temperatures: Provide the average water temperature and air temperature.
  4. Enter Humidity: Input the average relative humidity as a percentage.
  5. Enter Wind Speed: Input the average wind speed over the pool.
  6. Enter Time Period: Specify the number of days you want to estimate evaporation for.
  7. Calculate: Click the “Calculate” button.
  8. Review Results: The calculator will display the estimated evaporation rate per day (primary result) and the total water loss over the period, along with intermediate values like vapor pressures. The chart and table will also update.

Use the results to understand your pool’s water loss rate and decide if measures like using a pool cover are needed to conserve water and reduce pool maintenance costs.

Key Factors That Affect Pool Evaporation Results

  • Pool Surface Area: Larger pools have more surface area exposed to the air, leading to greater evaporation.
  • Water Temperature: Warmer water has a higher vapor pressure, meaning more water molecules are ready to escape into the air. Solar radiation is a primary driver of water temperature.
  • Air Temperature: While important for calculating humidity’s effect, it’s the difference between water and air conditions that drives evaporation more.
  • Relative Humidity: Low humidity means the air can hold more moisture, increasing the rate at which water evaporates from the pool to the air. High humidity slows it down.
  • Wind Speed: Wind blows away the moist air layer just above the pool surface, replacing it with drier air, which accelerates evaporation. Even a light breeze significantly increases evaporation compared to calm air.
  • Solar Radiation: Direct sunlight heats the pool water, increasing its temperature and thus the evaporation rate. While not a direct input, it strongly influences water temperature.
  • Pool Cover Usage: Using a pool cover is the most effective way to reduce evaporation, often by 90-95%, as it creates a physical barrier.
  • Atmospheric Pressure: Lower atmospheric pressure (at higher altitudes) can slightly increase evaporation, but this effect is less significant than the other factors for most residential pools.

Frequently Asked Questions (FAQ)

How much water does a pool lose to evaporation normally?
It varies greatly, but a typical uncovered pool might lose 0.25 to 0.5 inches (6-12 mm) of water per day, which can translate to 100-200 gallons or more for an average-sized pool, depending on conditions.
How can I reduce pool evaporation?
The most effective method is using a pool cover (solar cover, safety cover, or liquid cover). Windbreaks (fences, hedges) and maintaining slightly cooler water can also help.
Does a pool cover really stop evaporation?
Yes, a physical pool cover can reduce evaporation by 90-95% by creating a barrier between the water and the air.
Do indoor pools evaporate?
Yes, indoor pools also evaporate, though usually less than outdoor pools if the indoor air is humid. Proper ventilation and dehumidification are crucial for indoor pool areas to manage moisture.
Does saltwater pool evaporation differ from freshwater?
The presence of salt very slightly reduces the evaporation rate because salt lowers the water’s vapor pressure, but the effect is minimal for typical pool salt concentrations compared to environmental factors.
Does evaporation vary by time of year?
Yes, evaporation is usually higher in warmer, drier, and windier months (summer) and lower in cooler, more humid months (winter, if the pool is open).
How accurate is this pool evaporation calculator?
This calculator provides a good estimate based on established physical principles. However, actual evaporation can vary due to microclimates, direct sun exposure, and precise local conditions. It’s a guide, not an exact measurement.
Why does wind increase evaporation so much?
Wind removes the layer of moist air right above the water surface, replacing it with drier air. This maintains a steeper moisture gradient between the water and the air, encouraging more water to evaporate to try and saturate the new air.

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