Pool Water Evaporation Reference Chart
Estimated Daily Water Loss (Inches per Day) under typical wind speed (5 mph / 8 km/h).
| Water Temp \ Dew Point | 40°F (4.4°C) | 50°F (10.0°C) | 60°F (15.6°C) | 70°F (21.1°C) | 80°F (26.7°C) |
|---|---|---|---|---|---|
| 65°F (18.3°C) | 0.15 in | 0.09 in | 0.02 in | 0.00 in | 0.00 in |
| 70°F (21.1°C) | 0.20 in | 0.14 in | 0.07 in | 0.00 in | 0.00 in |
| 75°F (23.9°C) | 0.26 in | 0.20 in | 0.13 in | 0.05 in | 0.00 in |
| 80°F (26.7°C) | 0.33 in | 0.27 in | 0.20 in | 0.12 in | 0.00 in |
| 85°F (29.4°C) | 0.42 in | 0.36 in | 0.29 in | 0.21 in | 0.08 in |
| 90°F (32.2°C) | 0.52 in | 0.46 in | 0.39 in | 0.31 in | 0.19 in |
How Pool Evaporation is Calculated
Swimming pool evaporation is driven by the dynamic thermal physics of Dalton's Law. Water evaporates when the vapor pressure of the pool surface exceeds the vapor pressure of the surrounding ambient air. This easy 4-step framework outlines how to measure and compute daily pool water loss accurately.
- Establish Base Unit and Measurements
Decide whether you want to measure water depth loss in inches or centimeters to maintain consistency across all environmental variables.
- Gather Environmental and Water Metrics
Collect data for Pool Water Temperature, Air Dew Point Temperature, Wind Speed, and Atmospheric Air Pressure. These factors determine the vapor pressure deficit.
- Account for Wind Shelter and Water Agitation
Assess the exposure of your pool. Select your shelter type (Protected, Typical, or Minimal) and water movement activity (Still, Normal, or Excessive) to adjust the mass transfer wind function coefficient.
- Execute Formula and Analyze Daily Loss
Apply Dalton's Law mass transfer equation. Calculate the difference between saturation vapor pressure at water surface and actual air vapor pressure, then multiply by your wind factor to yield the definitive pool water loss in depth per day.
Pool Evaporation FAQs
Losing 1 inch of water per day is highly unusual for natural evaporation alone. While intense sun, low humidity, and high winds can accelerate evaporation, a drop of this magnitude strongly points to a physical structural or plumbing leak. It is highly recommended to perform a bucket test immediately to diagnose the true cause.
On average, a normal swimming pool will lose about 1/8 to 1/4 of an inch of water through evaporation every 24 hours. Under extreme microclimate conditions—such as very dry desert air, scorching heat, or constant high winds—daily evaporation can spike up to 1/2 of an inch, but rarely goes higher naturally.
It takes approximately 970 BTUs (British Thermal Units) of heat energy to evaporate 1 pound of water that is already at its boiling point (212°F / 100°C) under standard atmospheric pressure. If the water starts at room temperature, additional energy is required first to raise its temperature before Phase change (Latent Heat of Vaporization) begins.
Yes, absolutely. Winter evaporation is heavily driven by the temperature differential between warm pool water and cold air. If you heat your pool during winter or if dry, freezing arctic winds sweep over the surface, the vapor pressure deficit increases dramatically, easily causing a 1/2 inch drop (1/4 inch per day) in just two days.
The evaporation timeline for 8 ounces of water depends entirely on surface area, temperature, and airflow. Left in a standard drinking glass in a typical indoor room, it can take anywhere from several days to well over a week. However, if spilled flat across a wide concrete floor under a running fan, it will completely evaporate in less than an hour.
The most effective method is the 'Bucket Test'. Fill a 5-gallon plastic bucket with pool water and place it on a pool step, ensuring the water level inside the bucket matches the pool's water level exactly. Mark both levels. After 24 to 48 hours, compare the marks. If the pool water has dropped significantly lower than the water inside the bucket, you have a leak. If they dropped equally, it is purely evaporation.
The fastest way to accelerate evaporation is to maximize three specific metrics simultaneously: maximize the thermal energy (boil the water), maximize the exposed surface area (spread the water into a shallow, ultra-wide sheet), and continuously introduce dry air movement across the top using a fan to strip away the saturated boundary layer.
