Free Evaporation Rate Calculator

Understanding Evaporation Rate: A Practical Guide

A water evaporation calculator provides a quick way to estimate water loss from open surfaces such as ponds, pools, and lakes. This article explains the theory behind evaporation rate calculations, walks through realistic usage examples, and presents the central formula that drives the estimate. Whether you manage a garden pond or a large reservoir, understanding how wind, humidity, and temperature affect evaporation can help you plan water usage more accurately.

Using the Water Evaporation Calculator

To get started, enter the surface area of the water body. For a small pond with an area of 20 ft² (1.86 m²), the calculator needs either temperature and relative humidity or the current and maximum humidity ratios. The simplest option is to take readings from a local weather report. Suppose the current conditions are 68 °F (20 °C) with 70 % relative humidity. The tool then returns an hourly evaporation rate of roughly 1.2 lb/hr (0.55 kg/hr). Because 1 liter of water weighs about 1 kilogram, this loss equals 0.55 L/hr (0.15 US gal/hr). At night, when the air cools and humidity rises, the rate drops significantly — for example, at 50 °F (10 °C) and 90 % RH, the estimate falls to approximately 0.2 lb/hr (0.1 kg/hr).

If you already know the humidity ratio values (current and maximum), you can bypass the temperature‑humidity route and directly input those numbers. This second mode is useful when working from psychrometric data or specialized sensors.

Applications as a Pond, Pool, or Lake Evaporation Calculator

The same tool doubles as a pool evaporation rate calculator and a pond evaporation calculator. In hot weather, pool owners can use it to decide how much water must be added to keep the level steady. For larger bodies of water, such as a lake, the lake evaporation calculator function handles extensive surface areas; accurate wind and humidity data become especially important. The underlying physics remains identical across scales: evaporation is proportional to area, wind speed, and the difference between the air’s current moisture content and its maximum possible content.

The Evaporation Rate Formula

The core equation for hourly evaporation $g_h$ (kg/hr) is:

$$g_h = (25 + 19\,v) \cdot A \cdot (X_s - X)$$

Where:

• $v$ = wind speed over the water surface (m/s)
• $A$ = surface area (m²)
• $X_s$ = maximum (saturation) humidity ratio (kg/kg) at the water‑surface temperature
• $X$ = actual humidity ratio of the surrounding air (kg/kg)

The term $X_s - X$ reflects how much additional vapor the air can absorb. A larger gap accelerates evaporation, while a gap close to zero means the air is nearly saturated and evaporation slows.

Maximum Humidity Ratio and its Estimation

$X_s$ rises quickly with temperature. For typical outdoor conditions in the range 0–30 °C (32–86 °F), $X_s$ can be approximated with an empirical polynomial fitted from standard psychrometric data at 100 % relative humidity. Outside this temperature window the approximation loses accuracy, so results below 0 °C or above 30 °C should be treated as rough estimates.

Relative humidity (RH) is defined as the ratio of the current humidity ratio to the maximum humidity ratio:

$$RH = \frac{X}{X_s} \times 100\%$$

Thus, if you have temperature and relative humidity, you can first estimate $X_s$ and then calculate $X$.

Daily Evaporation Rate

To obtain the daily total, multiply the hourly rate by 24:

$$g_d = g_h \times 24$$

For a reliable daily figure, use daily average wind speed, temperature, and relative humidity. The calculator allows you to toggle between per‑hour and per‑day outputs, making it easy to switch time scales.

Key Environmental Drivers

Relative humidity is the most influential factor: when the air is already moist, further evaporation is limited. Wind is the second major driver because it flushes away the humid layer just above the water and brings in drier air, sustaining a strong concentration gradient. Temperature affects the saturation capacity of the air, while surface area determines the total exposed volume for vapor exchange. All these elements are integrated into the water evaporation calculator (also called a lake evaporation calculator for large basins), giving you a data‑driven answer for water management.