VPD Calculator

VPD is the difference between how much moisture air CAN hold (at saturation) versus how much it CURRENTLY holds. Measured in kilopascals (kPa) or millibars (mb). Example: If air at 75°F can hold 30 mb but only has 12 mb, VPD = 18 mb (1.8 kPa). VPD drives transpiration - plants lose water faster when VPD is high. Unlike humidity, VPD has consistent target ranges regardless of temperature. Low VPD (<0.5 kPa) slows transpiration. High VPD (>1.5 kPa) stresses plants.

Optimal VPD varies by growth stage: Seedlings: 0.3-0.8 kPa (low VPD prevents wilting). Vegetative: 0.8-1.1 kPa (balances transpiration and CO2 uptake). Flowering/Fruiting: 0.8-1.0 kPa (reduces bud rot and blossom end rot). Common guideline: aim for 0.8-1.2 kPa for most crops. Too low (<0.5): slow growth, mold risk. Too high (>1.5): excessive transpiration, wilting, nutrient lockout. Use environmental controls to maintain target. Different species have different optimal ranges.

VPD directly controls stomatal opening and transpiration rate. Low VPD: stomata partially close → less CO2 uptake → slower photosynthesis. High VPD: stomata wide open → high transpiration → risk of cavitation (air bubbles in xylem). Stomatal conductance correlates linearly with photosynthesis up to optimal VPD, then drops. VPD also affects: nutrient uptake (mass flow depends on transpiration), leaf temperature (evaporative cooling), humidity around leaves. Managing VPD is managing plant water stress and growth rate simultaneously.

Measure with: psychrometer (wet/dry bulb), digital hygrometer, or sensor package. For leaf-level VPD: use leaf temperature (IR thermometer) vs air temp. Control by: increasing humidity (fogging, pebble trays, inline humidifier) to lower VPD. Increase ventilation/heating to raise VPD. Leaf surface VPD is usually higher than ambient in high light. Target VPD changes through stages: higher in veg, lower in flower. Commercial growers use environmental controllers ($200-1,000) to maintain stable VPD.