VPD Calculator (Vapor Pressure Deficit)

Relative humidity tells you how full the air is as a percentage, but plants respond to how hard the air pulls water out of the leaf. That pull is the vapor pressure deficit (VPD): the gap between how much water vapor the air could hold at saturation and how much it actually holds, measured in kilopascals (kPa). The same 60% humidity is gentle at 18 C (VPD about 0.83 kPa) and much drier at 30 C (about 1.70 kPa), because warm air can hold far more vapor.

This calculator finds saturation vapor pressure with the Tetens equation used in FAO-56: e° = 0.6108 x exp(17.27T / (T + 237.3)), with T in Celsius. At 25 C that gives 3.168 kPa. Air VPD is e°(air) x (1 - RH/100), so 25 C at 60% humidity is 1.27 kPa.

Growers target a range rather than a single number. Common guideline bands are 0.4 to 0.8 kPa for unrooted cuttings and seedlings, 0.8 to 1.2 kPa for early vegetative growth, 1.2 to 1.5 kPa for late veg and early flower, and 1.5 to 1.6 kPa late in flower, when drier air helps prevent bud rot. These are horticultural rules of thumb, and species differ: leafy greens often do best near 0.8 kPa, while tomatoes tolerate 1.0 to 1.5 kPa.

Below about 0.4 kPa, transpiration nearly stops, calcium delivery to new growth suffers, and fungal pathogens such as botrytis and powdery mildew thrive. Above about 1.6 kPa, many plants partly close their stomata to save water, which also cuts CO2 uptake and slows growth. To lower VPD, raise humidity or lower temperature; to raise it, dehumidify or add a little heat.