The amount of water that evaporates from a swimming pool varies depending on multiple factors such as wind, temperature, humidity, sunlight exposure, and the pool’s surface area. On average, pools lose about a quarter of an inch (approximately 6 mm) of water per day due to evaporation, but this can fluctuate significantly with environmental conditions

Typical Evaporation Rates

• Average daily loss: About 1/4 inch (6 mm) per day is common in many climates

• Annual loss: For a 400-square-foot pool, evaporation can amount to around 10,000 to nearly 20,000 gallons per year, depending on climate and conditions

• Seasonal variation: Evaporation can be faster in hot, dry, and windy weather, and somewhat counterintuitively, it can also be significant in cooler seasons when the air is dry and temperature differences between water and air increase

Factors Affecting Evaporation

• Wind: Wind increases evaporation by removing the humid air layer above the pool surface, potentially doubling evaporation rates with even a gentle breeze

• Temperature: Higher air and water temperatures accelerate evaporation, with hotter days causing more water loss

• Humidity: Low humidity increases evaporation because dry air absorbs more water vapor from the pool surface

• Sunlight exposure: Pools exposed to direct sunlight evaporate more water; using a pool cover can drastically reduce evaporation

• Pool surface area: Larger pools lose more water due to greater surface area

Example Calculation

For a 32 ft by 13 ft pool losing about half an inch of water per day in summer, the weekly evaporation loss can be around 909 gallons, or roughly 3,636 gallons per month

Summary

• A typical pool loses about 1/4 inch of water daily to evaporation.
• This translates to thousands of gallons annually depending on pool size and climate.
• Evaporation rates increase with wind, heat, low humidity, and sunlight.
• Using pool covers and windbreaks can significantly reduce evaporation losses.

This understanding helps pool owners anticipate water loss and maintain proper water levels and chemical balance