Vapor Pressure Deficit (VPD) Calculator

VPD is the difference between the amount of moisture the air could hold when saturated and the amount it actually holds — in other words, the drying power of the air around a plant. It is calculated as the saturation vapor pressure at the leaf temperature minus the actual vapor pressure of the air, and is usually expressed in kilopascals (kPa). Higher VPD means drier air and faster transpiration; lower VPD means humid air and slower water loss.

Transpiration cools leaves, so an actively growing leaf is typically a couple of degrees cooler than the surrounding air. Because saturation vapor pressure is evaluated at the evaporating surface, VPD should use the leaf temperature. This calculator lets you set a leaf-temperature offset (a default of −2 °C is common); set it to 0 to get the plain 'air VPD'. The cooler the leaf relative to the air, the lower the VPD.

As a rule of thumb for greenhouse and indoor growing: roughly 0.4–0.8 kPa during propagation (seedlings and clones), 0.8–1.2 kPa during vegetative growth, and 1.2–1.6 kPa during flowering or fruiting. Below about 0.4 kPa the air is too humid and invites mold and disease; above about 1.6 kPa it is too dry and plants close their stomata to conserve water. Optimal values vary by crop and cultivar.

The saturation vapor pressures behind the VPD come from the IAPWS-95 formulation — the international reference standard, accurate to better than ±0.025%. Most VPD tools use a simpler approximation; using IAPWS-95 means the underlying pressures are as accurate as it gets, and the same engine is validated against reference values across this site.