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Evaporation Rate Calculator

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How to use the water evaporation rate calculatorHow to calculate evaporation rateDaily evaporation rate formulaFAQs

If you have ever wondered how fast water evaporates and what affects the speed of evaporation, then the evaporation rate calculator is perfect for you.

The following article delves into the theory behind evaporation rate calculations and provides practical examples using the evaporation rate calculator. It covers:

  • How to use the evaporation rate calculator;
  • How to calculate evaporation rates;
  • The daily evaporation rate formula; and
  • How fast does pool water evaporate?

OK, let's dive in! (As long as all the water hasn't evaporated.)

How to use the water evaporation rate calculator

Let's see how you can use our evaporation rate calculator to see how fast your garden pond is going to evaporate into thin air.

  1. First, we need to know the surface area of your pond — let's say you have a 20 square-foot pond (1.86 m²). The evaporation rate is proportional to the surface area.

  2. Next, enter the average air speed in your garden. You can get this from today's weather report. Today, it says there is a 5 mph light breeze (8 km/h). The stronger the wind, the faster the evaporation rate.

  3. The calculator can accept two sets of inputs: either temperature and relative humidity or the current and maximum humidity ratios. The first set of data is easy to look up, so let's go with using temperature and humidity for now.

  4. Looking again at today's weather report, you see that it's 68 ºF (20 ºC) with a relative humidity of 70%. Nice 😎

  5. And that's it. We have answered what the evaporation rate is: 1.2 lb per hour (0.55 kg per hour). Since 1 liter of water weighs approximately 1 kilogram (see water density calculator), that's 0.55 liters per hour (0.15 US gallons per hour).

    That might seem more than you expected, but it will be less at night when it is colder and more humid. For example, for 50 °F (10 °C) and 90% relative humidity, the evaporation rate is only 0.2 lb/hr (0.1 kg/hr).

If you know the current and maximum humidity ratios, you can open up the last section of the calculator and enter these values to obtain a result for the evaporation rate.

Another application of the calculator is as a pool evaporation rate calculator, so you know how much to top up your pool in the summer. You might also find our pool volume calculator useful for calculating how much water is in your pool to begin with.

How to calculate evaporation rate

The evaporation rate of water can be calculated using the following formula:

gh=(25+19×v)×A×(XsX)\footnotesize g_\mathrm h = (25\! +\! 19\!\times\! v)\!\times\! A \!\times\! (X_\mathrm s\! -\! X)


  • ghg_\mathrm h — Evaporation rate in kg per hour;
  • vv — Velocity of air above the surface (m/s);
  • AA — Surface area of the body of water (m²);
  • XsX_\mathrm s — Maximum humidity ratio of saturated air (given the same temperature as the water's surface); and
  • XX — Current humidity ratio of the air (kg/kg).

The last two variables measure how much water the air can "hold" as water vapor. The current humidity ratio is the weight of water per unit weight of air (for example, how many kilograms of water vapor in a kilogram of air). This is also known as the specific humidity.

The maximum humidity ratio is the maximum amount of water the air can hold before it condenses out into liquid water (e.g., it starts raining). This value changes dramatically with air temperature, but we can fit an empirical formula to the data for a typical temperature range (0–30 °C, 32–86°F):

Xs=3.733×103+3.2×104×T +3×106×T2+4×107×T3\footnotesize \begin{split} X_s = 3.733\! \times\! 10^{-3} + 3.2\! \times\! 10^{-4} \! \times \! T\ +\\ 3\! \times \! 10^{-6}\! \times \! T^2 + 4\! \times \! 10^{-7} \! \times \! T^3 \end{split}

where TT is the air temperature at the water's surface in degrees Celsius.

Using this formula allows you to enter temperature instead of having to know the maximum humidity ratio. However, for temperatures below 0 °C (32 °F) and above 30 °C (86 °F), its accuracy can not be guaranteed, because the formula is derived from maximum humidity ratio data given in this psychrometic chart at 100% relative humidity.

The remaining input, relative humidity, measures what percentage of the maximum humidity is the current humidity.

💡 Learn more about relative humidity and its relationship with temperature and dew point with our relative humidity calculator.

Daily evaporation rate formula

The evaporation rate formula given above is for the amount of water that evaporates in an hour. To obtain the daily evaporation rate, we can simply multiply by the 24 hours in a day:

gd= 24×(25+19×v d) ×A×(XsdXd)\footnotesize \begin{aligned} g_\mathrm d =&\ 24\! \times\! (25\! +\! 19\!\times\! v_\mathrm{\ \overline{d}}) \\ &\ \times A\! \times\! (X_\mathrm{s\overline{d}}\! -\! X_\mathrm{\overline{d}}) \end{aligned}

Note that you now need to use the average wind velocity v dv_\mathrm{\ \overline{d}}, maximum humidity ratio XsdX_\mathrm{s\overline{d}}, and current humidity ratio XdX_\mathrm{\overline{d}} over a day to get an accurate result.

In the calculator, you can change the second unit of the evaporation rate from per hour to per day. Then, make sure you use the daily average air speed, temperature, and relative humidity to find out what evaporation rate is per 24 hours.


Why does water evaporate?

Water is made up of molecules that move at different speeds, and some have enough energy to overcome the intermolecular forces of the other water molecules and escape the water's surface. The water molecule is then floating in the air, where it is now a gas called water vapor.

How do I calculate evaporation rate of water?

To calculate the evaporation rate of a body of water, follow these steps:

  1. Multiply the wind speed (m/s) by 19 and add 25.
  2. Multiply by the surface area of the water (m²).
  3. Multiply by the maximum humidity ratio subtracted from the current humidity ratio (kg/kg).
  4. You'll then have the water's evaporation rate in kilograms per hour.

How fast does pool water evaporate on a warm summer's day?

Around 22 pounds of water an hour (10 kg/h). This assumes a pool size of 30 by 15 feet, a light breeze of 3 mph, a temperature of 82 °F, and 80% relative humidity. If these conditions lasted all afternoon (6 hours), you'll have to top up your pool by around 16 gallons (60 liters).

To see how fast your pool's water will evaporate, visit Omni Calculator's evaporation rate calculator.

How do environmental factors affect evaporation rates?

The main environmental factor when it comes to the evaporation rate of water is relative humidity. If the air is already saturated with water, then no more water can evaporate into it without immediately falling back into the water.

Wind across the surface of the water is also significant, as it brings in fresh, dry air from the environment. With no wind, the relative humidity close to the water's surface will increase and slow the evaporation rate.

Use temperature and relative humidity

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