VPD Calculator (Vapor Pressure Deficit)

Vapor pressure deficit (VPD) is the difference between the amount of moisture in the air and the amount of moisture that can be held by the air at saturation. It's measured in pressure units like kilopascals (kPa) and helps determine how much water plants will transpire.

VPD directly affects plant transpiration rates. Low VPD means high humidity, which can slow transpiration and nutrient uptake. High VPD means low humidity, which can stress plants and cause excessive water loss. The ideal VPD promotes healthy transpiration and optimal growth.

VPD is calculated using the saturation vapor pressure at leaf temperature minus the actual vapor pressure of the air. The formula involves temperature and relative humidity measurements, with leaf temperature typically being 2-3°F cooler than air temperature.

Propagation/Seedling stage: 0.4-0.8 kPa, Vegetative stage: 0.8-1.2 kPa, Flowering stage: 1.0-1.4 kPa. Young plants need lower VPD (higher humidity) while mature flowering plants can handle higher VPD (lower humidity).

Both are important. Air temperature is what you measure with standard thermometers, but leaf temperature is what matters for VPD calculations. Leaves are typically 2-3°F cooler than air temperature due to transpiration cooling.

To raise VPD: lower humidity (use dehumidifier) or raise temperature. To lower VPD: raise humidity (use humidifier) or lower temperature. Small adjustments in either direction can significantly impact VPD.

Use the leaf temperature offset feature in the calculator. Typically, leaves are 2-3°F cooler than air temperature due to transpiration. Start with -2°F offset and adjust based on your growing conditions and plant response.

VPD considers both temperature and humidity together, giving a more complete picture of the plant's environment. Two rooms with the same humidity but different temperatures will have very different VPD values and will affect plants differently.