Evaporation Rate Calculator
Calculate how quickly a body of water will evaporate given wind speed and humidity using the evaporation rate calculator.
Evaporation rate10.866 kg/hour260.784 kg a day, which for water is the same number of litres
Evaporation coefficient θ27.85 kg/(m²·h)25 + 19v — still air gives 25, and even a light breeze raises it sharply
Humidity ratio at the water surface24.0596 g/kgsaturated at the WATER temperature of 28 °C, not the air temperature
Humidity ratio of the air11.8671 g/kg60% at 25 °C
Driving difference12.1925 g/kgevaporation is driven by this GAP, which is why warm water in dry air disappears so much faster than cool water in humid air
Depth lost per day8.1495 mma litre per square metre is one millimetre
Heat carried away7.3949 kWeach kilogram takes about 2.45 MJ of latent heat with it. Evaporation is the dominant heat loss from an open pool, well ahead of conduction and radiation
Cooling per square metre231.09 W/m²
Rate in still air9.754 kg/hour10.23% less than with the airflow entered — sheltering the surface matters more than most people expect
A cover stops nearly all of itmass transfer, not temperaturecovering the surface halts the vapour transfer entirely, and the heat loss goes with it — worth far more than dropping the water temperature a degree or two
The formula
ṁ = (25 + 19v) A (x_s − x)
Driven by a difference, not a temperature
Evaporation depends on the gap between the humidity ratio saturated at the WATER temperature and the actual humidity ratio of the air. Warm water in dry air evaporates fast; cool water in humid air barely at all. If the air is wetter than the water surface can support, the flow reverses and dew condenses — the same equation with the sign flipped.
Air movement dominates
The standard engineering coefficient is 25 + 19v, so still air gives 25 and even a gentle half-metre-per-second breeze raises it by nearly forty percent. Wind strips away the saturated layer sitting immediately above the surface and replaces it with drier air.
Evaporation is also the dominant heat loss from an open pool, well ahead of conduction and radiation. Every kilogram leaving takes about 2.45 MJ of latent heat with it, which is why a cover pays for itself so quickly — it stops the mass transfer, and the heat loss goes with it.