Pool Evaporation Calculator

Estimate how much water your pool loses to evaporation daily and monthly. Enter your pool’s details and local weather conditions to distinguish between natural water loss and potential leaks.

Estimated Daily Water Loss

—

—

Monthly Loss

—

Depth Lost Per Day

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 the natural process of evaporation. It helps pool owners differentiate between normal water loss and a potential leak. By inputting key variables such as pool surface area, water and air temperatures, humidity, and wind speed, the calculator provides a reliable estimate of daily, weekly, or monthly water loss in gallons or liters. Understanding this is crucial for efficient pool water management and conservation.

Most pools can lose up to a quarter-inch of water per day purely from evaporation. This tool uses established physics formulas to model this process, giving you the knowledge to maintain proper water levels, balance chemicals correctly, and save money on water bills.

Pool Evaporation Formula and Explanation

The calculation is based on principles of thermodynamics and fluid dynamics. The primary driver of evaporation is the difference in vapor pressure between the pool water surface and the surrounding air. A simplified but effective formula used by many calculators, including this one, is derived from the work of Willis Carrier and others:

Evaporation Rate = C * A * (Pw - Pa) * (1 + 0.098 * W)

This formula is adapted for web use, where variables are calculated as follows:

Evaporation Formula Variables

Variable	Meaning	Unit (Auto-Inferred)	Typical Range
A	Pool Surface Area	sq ft or m²	200 – 1000
Pw	Saturation vapor pressure at the water’s temperature	inHg or kPa	0.5 – 1.5
Pa	Actual vapor pressure of the air (based on air temp and humidity)	inHg or kPa	0.3 – 1.2
W	Wind Speed	mph or kph	0 – 20
C	An empirical constant	Unitless	~0.00013

The calculator determines the vapor pressures using scientific approximations and then calculates the total volume of water lost over the given surface area, converting it into user-friendly units like gallons or liters per day. A pool volume calculator can help you understand the total amount of water you’re managing.

Practical Examples

Example 1: Hot, Dry, and Windy Day

Consider a standard pool in a dry climate like Arizona during the summer.

• Inputs:
  • Pool Area: 500 sq ft
  • Water Temp: 82°F
  • Air Temp: 95°F
  • Humidity: 20%
  • Wind Speed: 10 mph
• Results: The calculator would predict a significant water loss, likely around 40-50 gallons per day. The large temperature difference and low humidity create a steep vapor pressure gradient, which is amplified by the wind.

Example 2: Cool, Humid, and Calm Day

Now, let’s take the same pool but in a humid climate like Florida on a calm day.

• Inputs:
  • Pool Area: 500 sq ft
  • Water Temp: 78°F
  • Air Temp: 80°F
  • Humidity: 85%
  • Wind Speed: 2 mph
• Results: The evaporation would be minimal, perhaps only 5-10 gallons per day. The high humidity means the air is already saturated with moisture, reducing its capacity to absorb more water from the pool. Learning about the cost of pool evaporation can motivate homeowners to take preventative measures.

How to Use This Pool Evaporation Calculator

1. Select Your Unit System: Choose between Imperial (°F, feet, mph) or Metric (°C, meters, kph). The labels and calculations will adjust automatically.
2. Enter Pool Surface Area: Input the total area of your pool’s water surface. If you don’t know it, you can calculate it for simple shapes (Length x Width).
3. Input Temperatures: Provide the average temperature of the pool water and the ambient air. The greater the difference, the higher the evaporation.
4. Set Relative Humidity: Enter the average daily humidity as a percentage. Lower humidity leads to more evaporation.
5. Enter Wind Speed: Input the average wind speed. Even a light breeze significantly increases water loss.
6. Review Your Results: The calculator instantly displays the estimated daily water loss in the main result panel, along with monthly loss and the equivalent water depth lost per day.

Key Factors That Affect Pool Evaporation

Several factors work together to determine your pool’s evaporation rate.

• Surface Area: A larger pool has more surface exposed to the air, resulting in greater evaporation.
• Temperature (Air and Water): Evaporation increases when water temperature is high and/or when air temperature is high. The temperature difference between the water and air is a key driver.
• Humidity: This is one of the most critical factors. Dry air acts like a sponge, pulling moisture from your pool. In areas with low humidity, evaporation rates are much higher.
• Wind Speed: Wind blows away the layer of moist air sitting just above the pool’s surface, replacing it with drier air and accelerating evaporation.
• Sunlight Exposure: Direct sunlight heats the water, increasing its temperature and thus the rate of evaporation. A pool in full sun will lose more water than a shaded one.
• Pool Usage: Splashing and water features (fountains, waterfalls) increase the surface area of the water exposed to air, which can increase evaporation. Exploring ways to reduce pool evaporation is a smart step for any owner.

Frequently Asked Questions (FAQ)

1. How much water loss is normal for a pool?

A loss of about 1/4 inch (6mm) per day is considered average, but it can be much higher in hot, dry, and windy conditions. Our calculator helps you determine the normal rate for your specific environment.

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

Use this calculator to get a baseline for evaporation. Then, perform a “bucket test.” Place a bucket filled with pool water on your pool steps and mark the water level in both the bucket and the pool. After 24 hours, compare the water loss. If the pool has lost significantly more water than the bucket, you likely have a leak.

3. How do I reduce pool evaporation?

The most effective method is using a pool cover (solar, liquid, or solid). It can reduce evaporation by up to 95%. Planting windbreaks like shrubs and using a salt water system (which has slightly different properties) can also help.

4. Does a heated pool evaporate more water?

Yes, absolutely. The higher the water temperature relative to the air temperature, the faster the evaporation rate. This is especially true overnight when the air cools down but the water remains warm.

5. Does the calculator account for rainfall?

No, this calculator only estimates water *loss* from evaporation. It does not account for water added to the pool from rain or from you refilling it.

6. Why are the units for vapor pressure ‘inHg’ or ‘kPa’?

Vapor pressure is a scientific measurement of pressure. Inches of Mercury (inHg) is a common unit in the Imperial system for atmospheric pressure, while kilopascals (kPa) is the standard metric unit. The calculator uses these internally for accuracy.

7. Can I use this for my hot tub?

Yes, the principles are the same. Simply enter the surface area and higher water temperature of your hot tub. The evaporation rate will be significantly higher than a pool’s due to the high temperature.

8. How accurate is this pool evaporation calculator?

This calculator uses a scientifically-backed formula to provide a very good estimate. However, it’s a model and can’t account for every unique variable, such as intermittent shade or complex wind patterns. It is an excellent tool for understanding expected loss. You can compare its output with our guide on how to calculate pool water loss manually.

Related Tools and Internal Resources

Managing your pool involves more than just tracking water levels. Here are some other resources that can help you maintain a perfect pool: