About this tool
The Water Vapor Pressure Calculator computes both the saturation vapor pressure (es) — the maximum pressure water vapor can exert at a given temperature — and the actual vapor pressure (e) present in the air at a given relative humidity. It also reports the vapor-pressure deficit (VPD = es − e), a driver of plant transpiration, evaporation, and drying rates widely used in horticulture and greenhouse control.
Saturation vapor pressure uses the Alduchov–Eskridge improved Magnus form: es = 6.1094 · exp( 17.625·T/(T+243.04) ) in hPa, with T in °C. The actual vapor pressure is then e = es · RH/100, and the deficit is VPD = es − e. These are the standard relations behind dew-point, humidity, and evapotranspiration models.
Everything is calculated in your browser with no server round-trip. Enter the temperature (°C or °F) and relative humidity, then choose the output unit — hectopascals/millibars (hPa), kilopascals (kPa), or millimeters of mercury (mmHg, where 1 hPa ≈ 0.75006 mmHg). Relative humidity must lie between 0 and 100%.