Evaporation Rate Calculator

Imagine holding a wet sponge in a hot, dry, windy room versus a cool, humid, still one. The sponge dries out far faster in the first case — not because the water is hotter, but because the surrounding air is hungry for moisture. Evaporation is fundamentally a competition between the vapor pressure at the liquid surface and the vapor pressure in the air above it.

This calculator estimates that competition using a form of the vapor pressure deficit equation. The air temperature determines how much water vapor the air can hold at maximum (saturation vapor pressure). The relative humidity tells us how much it actually holds. The gap between those two values — the deficit — is what drives evaporation. A bigger gap means faster evaporation. Wind accelerates the process by continuously replacing the saturated air layer directly above the water with drier air from elsewhere.

The surface area simply scales the total volume lost. A pool twice as large loses twice as much water per day under identical conditions.

Use this calculator when you need to plan water top-ups for pools, ponds, or ornamental water features, estimate storage losses from open tanks or reservoirs, assess solvent evaporation in a lab or industrial tray, or size ventilation for a space containing evaporating liquids. It is also useful for irrigation planning when an uncovered water source feeds a drip system — knowing daily losses helps size the tank correctly.

This calculator is not appropriate for pressurized systems, for flow-through channels where the water surface is constantly renewing, or for calculating evapotranspiration from soil and plant surfaces (which requires the Penman-Monteith method and additional inputs). It is also not suitable for cryogenic liquids, highly viscous liquids, or liquids with complex multicomponent vapor pressures like fuel blends.

If you are working on a regulatory or engineering submission, treat these results as a screening estimate only. Field measurement using an evaporation pan or lysimeter will always be more accurate for site-specific conditions. This tool gives you the right order of magnitude to make confident operational decisions.

The most common mistake is entering the weather-report wind as the air speed. Weather stations measure wind well above the ground, where it runs much faster than the air just above a water surface. Enter the speed at the surface: 0 to 0.3 m/s indoors, and outdoors the breeze at the water's edge. A 15 km/h weather wind typed in as 4 m/s gives about four times the still-air estimate.

A second frequent error is confusing air temperature with water temperature. In practice, a pool in afternoon sun may have water that is 5-8 degrees cooler than the air above it. This tool uses a combined temperature as a working approximation. When precision matters, use the water surface temperature, which is the actual driver of vapor pressure at the evaporation interface. Using air temperature on a hot sunny day will overestimate evaporation.

A third mistake is applying this model to boiling or near-boiling liquids. The vapor pressure relationship changes dramatically above around 80 degrees Celsius, and at 100 degrees the liquid boils rather than evaporates. The formula breaks down at these temperatures. Similarly, applying it to frozen surfaces (sublimation from ice) requires an entirely different approach. Always check that your inputs sit within the valid range of 0-99 degrees Celsius.

The core calculation is built on the Magnus formula for saturation vapor pressure. For temperature T in Celsius, saturation vapor pressure eSat in hPa is computed as: eSat = 6.1078 times e raised to the power of (17.27 times T divided by T plus 237.3). Actual vapor pressure eAct equals eSat multiplied by relative humidity divided by 100. The vapor pressure deficit VPD equals eSat minus eAct.

Evaporation uses the empirical equation published by The Engineering ToolBox (Evaporation from a Water Surface): evaporation in kg per hour equals Theta times the surface area times (xs minus x), where Theta = 25 + 19 times the air speed just above the water in meters per second, xs is the humidity ratio of saturated air at the water temperature and x is the humidity ratio of the air. The humidity ratio comes from vapor pressure: x = 0.622 times e divided by (1013.25 minus e), with e in hPa. Multiply by 24 for a day, and by the liquid factor for liquids other than water.

Because 1 kg of water occupies approximately 1 liter, the mass rate converts directly to a volume rate in liters per square meter per day. Multiplying by surface area gives total daily volume loss. Depth loss per day (in millimeters) equals the volumetric rate per unit area, since 1 liter per square meter equals exactly 1 millimeter depth.

The coefficient Theta = 25 + 19v is empirical, published by The Engineering ToolBox for water surfaces with air moving above them, and v is the air speed just above the surface. That suits pools, tanks and trays better than open lakes, where outdoor models such as Penman use wind measured at a set height. The humidity ratios assume sea-level pressure (1013.25 hPa). The model also sets the water temperature equal to the air temperature; in dry heat, evaporation cools the water below the air, so the result there is an upper bound.