VPD Calculator and Chart

Vapor pressure deficit, commonly shortened to VPD, describes the drying power of air around a leaf. It is the difference between the vapor pressure at saturation near the leaf surface and the actual water-vapor pressure in the surrounding air. This calculator uses the Tetens saturation vapor pressure equation for both temperatures. Actual air vapor pressure is air saturation pressure multiplied by relative humidity. Leaf VPD is then leaf saturation pressure minus actual air vapor pressure, reported in kilopascals. VPD combines temperature and humidity into a value that is often more useful for crop management than either measurement alone. At low VPD, air is close to saturation and transpiration can slow. Prolonged condensation and wet surfaces may also increase disease risk. At high VPD, the atmosphere can pull water from leaves rapidly. Plants may close stomata to conserve water, reducing carbon dioxide uptake and growth even when the root zone contains moisture. A moderate value can support transpiration, nutrient movement, and gas exchange when light, irrigation, and root health are also appropriate. Leaf temperature improves the estimate because leaves are not always at air temperature. Strong light can warm a leaf, while transpiration can cool it. Measure canopy leaf temperature with a properly used infrared thermometer or thermal sensor and use a representative reading rather than an unusually bright, shaded, wet, or damaged leaf. Measure relative humidity and air temperature close to canopy height. Sensors near a heater, humidifier, wall, intake, or lamp may not represent the crop environment. Calibrate sensors and consider several locations in a large growing space. There is no universal ideal VPD. Targets depend on species, growth stage, light intensity, carbon dioxide, rooting medium, irrigation strategy, and the source used. Seedlings and propagation crops often operate at lower deficits than established vegetative or flowering plants. The broad status bands shown here are educational, not crop prescriptions. Avoid making abrupt environmental changes solely to chase one number. Review trends over the light and dark periods, ensure plants can replace transpired water, and follow crop-specific production guidance. VPD does not directly measure soil moisture, stomatal conductance, nutrient uptake, disease risk, or plant stress, so combine it with observation and reliable environmental controls.