Evaporation Rate (Water) Calculator

Estimate water evaporation rate and surface-level loss from area, air speed, temperature, humidity, or humidity ratios in metric and US units.

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Evaporation Rate Formula

The following empirical Sprenger-style correlation gives a rough screening estimate for evaporation from an exposed, maintained liquid-water surface. It is not a universal evaporation law, a boiling or ice model, or a substitute for a validated heat/ventilation design. The weather-input implementation supports liquid-water surface temperatures from 0 to 50 °C and air temperatures from −40 to 50 °C, with relative humidity referenced to liquid water; these bounds constrain the saturation-pressure approximation, not the accuracy of the evaporation correlation.

• Where gh is the evaporation rate (kg/h)
• v is the velocity of air above the surface (m/s)
• A is the water surface area (m^2)
• Xs is the maximum humidity ratio of saturated air (at the same temperature as the water surface)
• X is the current humidity ratio of the air

To calculate the evaporation rate, multiply 19 by the velocity of the air, add 25 to the result, multiply by the surface area, then multiply by the surface-saturated humidity ratio minus the ambient humidity ratio, both in kg water vapor per kg dry air. Equal ratios give zero modeled net evaporation; a negative difference indicates condensation conditions outside this evaporation-only calculation.

Evaporation Rate Definition

An evaporation rate is the mass of water transferred to air per unit time. Daily volume and surface-level loss here assume constant conditions and water density of 1 kg/L; they are approximate. The surface temperature is held at the entered value, so this model does not calculate evaporative cooling or heat availability.

Evaporation Rate Example

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