Pool Evaporation Calculator

Estimate water loss from your swimming pool due to environmental factors.

What is a Pool Evaporation Calculator?

A pool evaporation calculator is a specialized tool designed to estimate the amount of water lost from a swimming pool due to evaporation. Evaporation is a natural process where water turns into vapor and dissipates into the atmosphere. For pool owners, this process can lead to significant water loss over time, impacting water bills, chemical balance, and equipment performance. This calculator uses key environmental data—such as pool size, water and air temperature, humidity, and wind speed—to provide a scientific estimate of daily water loss, helping to differentiate between normal evaporation and a potential pool leak.

Pool Evaporation Formula and Explanation

The calculation is based on a modified version of the formula developed by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE), which is an industry standard for estimating evaporation from water surfaces. The formula considers the difference in vapor pressure between the pool water and the surrounding air, along with the effect of wind.

The core formula is:

Evaporation Rate (lb/hr) = (A / L) * (Pw – Pa) * (95 + 0.425 * v)

This result is then converted from pounds per hour to gallons per day for a more practical measurement.

Formula Variables

Variable	Meaning	Unit	Typical Range
A	Pool Surface Area	Square Feet (ft²)	100 – 1000
L	Latent Heat of Vaporization	Btu/lb	~1050
Pw	Saturation Vapor Pressure at Water Temperature	inches of Mercury (inHg)	0.5 – 1.5
Pa	Vapor Pressure of Air at Dew Point	inches of Mercury (inHg)	0.2 – 1.0
v	Air Velocity over Surface	feet per minute (ft/min)	0 – 1000

Practical Examples

Example 1: Hot, Dry, and Windy Day

Imagine a pool in a hot, arid climate like Phoenix, Arizona, during the summer.

• Inputs: Pool Area: 500 sq ft, Water Temp: 88°F, Air Temp: 102°F, Humidity: 15%, Wind Speed: 10 mph.
• Calculation: The large temperature difference and low humidity create a high vapor pressure differential. The strong wind accelerates the process.
• Results: The evaporation calculator pool would predict a high water loss, potentially over 0.5 inches or 150+ gallons per day.

Example 2: Mild, Humid, and Calm Day

Now consider a pool in a coastal area like Miami, Florida, on a typical day.

• Inputs: Pool Area: 500 sq ft, Water Temp: 84°F, Air Temp: 88°F, Humidity: 75%, Wind Speed: 3 mph.
• Calculation: The high humidity means the air is already saturated with moisture, significantly reducing the vapor pressure difference. The gentle wind has a minimal effect. For more information, you might want to read about factors affecting chemical balance.
• Results: The calculator would show a much lower water loss, likely around 0.15 inches or 45-55 gallons per day.

How to Use This Evaporation Calculator for Pools

1. Enter Pool Area: Input the surface area of your pool. If you don’t know it, you can approximate by multiplying the length by the width. Ensure you select the correct units (square feet or meters).
2. Set Temperatures: Provide the water temperature and the ambient air temperature. Use a pool thermometer for accuracy. Ensure the unit (°F or °C) is consistent.
3. Input Humidity: Enter the relative humidity as a percentage. You can get this from a local weather forecast.
4. Add Wind Speed: Enter the average wind speed. Calm days have low wind, while breezy days are higher. A nearby weather station can provide this.
5. Review Results: The calculator instantly shows the estimated gallons of water lost per day. It also displays intermediate values like dew point and vapor pressures, which are key to the calculation.

Key Factors That Affect Pool Evaporation

Several environmental factors influence how quickly water evaporates from your pool. Understanding them can help you manage water loss. For a deeper dive, consider our guide on seasonal pool care.

• Pool Surface Area: The larger the pool, the more surface is exposed to the elements, leading to greater evaporation.
• Temperature (Water and Air): The rate of evaporation increases as the temperature difference between the pool water and the air grows. Warmer water evaporates faster.
• Humidity: Low relative humidity accelerates evaporation. Dry air acts like a sponge, pulling moisture from the pool. In contrast, when the air is already moist (high humidity), evaporation slows down significantly.
• Wind Speed: Wind blowing across the pool’s surface strips away the layer of moist air that sits just above the water, replacing it with drier air and speeding up evaporation.
• Sunlight Exposure: Direct sunlight heats the water, increasing its temperature and thereby increasing the evaporation rate.
• Pool Usage: Splashing and activity in the pool increase the water surface area temporarily and can contribute to higher evaporative loss. Learning about pool safety covers can be beneficial.

Frequently Asked Questions

1. How much water evaporation from a pool is normal?

A typical pool can lose about a quarter-inch of water per day due to evaporation. However, this can double on hot, windy, and dry days. Our evaporation calculator pool provides a precise estimate for your specific conditions.

2. How can I tell if my pool is leaking or just evaporating?

Use this calculator to get a baseline for your daily evaporation. Then, perform a “bucket test”: place a bucket on your pool step, fill it with pool water to match the pool’s water level, and mark both levels. After 24 hours, if the pool water has dropped more than the water in the bucket, you likely have a leak. We have a detailed guide on detecting pool leaks.

3. Does a salt water pool evaporate faster?

No, the salt content does not significantly change the rate of evaporation. The water evaporates, but the salt remains behind, which is why you occasionally need to add fresh water, but not salt, to a saltwater pool.

4. How can I reduce pool evaporation?

The most effective method is using a pool cover (solar, liquid, or safety). A cover can reduce evaporation by up to 95%. Other methods include adding windbreaks (fences/shrubs), reducing water temperature slightly, and turning off water features like waterfalls when not in use.

5. Why does the calculator need dew point?

Dew point is the temperature at which air becomes saturated with moisture. It’s used to calculate the actual vapor pressure of the air (Pa), which is a critical component of the ASHRAE evaporation formula.

6. Does the time of day affect evaporation?

Yes, evaporation is often highest in the afternoon when air temperatures are high and humidity can be lower. It also occurs at night if the water is significantly warmer than the air.

7. Can this calculator be used for indoor pools?

While the physics are similar, this evaporation calculator pool is optimized for outdoor pools where wind and sun are major factors. Indoor pools have different ventilation and humidity control systems that require a different calculation model.

8. How accurate is this calculator?

This tool uses a proven, industry-standard formula to provide a reliable scientific estimate. However, it’s an estimate, as micro-climates and specific pool features can have an effect. It is an excellent starting point for understanding your pool’s water loss.

Related Tools and Internal Resources

For more on maintaining your pool and managing costs, check out these resources:

• Pool Volume Calculator: Accurately determine the amount of water your pool holds.
• Pool Chemical Calculator: Ensure your water is balanced and safe for swimming.
• Our Blog Post: “Top 10 Ways to Save Water and Money on Your Pool”