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Water Vapor Pressure Calculator

Compute saturation and actual water-vapor pressure plus vapor-pressure deficit from temperature and relative humidity via the Magnus equation.

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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%.

Frequently asked questions

What is saturation vapor pressure?
It is the partial pressure of water vapor when air is fully saturated at a given temperature — the ceiling on how much vapor the air can hold. It rises exponentially with temperature, which is why warm air can carry so much more moisture.
What is vapor-pressure deficit (VPD)?
VPD is saturation vapor pressure minus actual vapor pressure (es − e). It measures how 'thirsty' the air is; higher VPD pulls water from leaves and wet surfaces faster, making it a key variable in greenhouse and crop management.
Which units can I get?
Hectopascals (hPa, identical to millibars), kilopascals (kPa = hPa/10), and millimeters of mercury (mmHg), where 1 hPa ≈ 0.75006 mmHg. Pick the unit your instruments or standards use from the selector.
How is actual vapor pressure obtained?
It is the saturation vapor pressure scaled by relative humidity: e = es · RH/100. So at 50% RH the actual vapor pressure is exactly half the saturation value for that temperature.

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