VPD calculator
Estimate vapor pressure deficit from air temperature, leaf temperature offset, and relative humidity.
Use the result as a starting point.
VPD describes the drying pressure between a leaf and the surrounding air. Use it with crop stage, airflow, root health, and irrigation rather than chasing one number.
Formula: VPD = saturation vapor pressure at leaf temperature minus actual vapor pressure of the air.
Leaf temperature is often a little cooler than room air under LED lighting.
How to use this tool
Use VPD to describe the drying pressure between a leaf and the surrounding air, then combine it with crop stage, airflow, roots, and irrigation.
- Enter air temperature in Fahrenheit, the leaf temperature offset, and relative humidity from 0 to 100 percent.
- Enter the values, check the units, and select Calculate.
- Read the result as a planning estimate, then compare it with the crop and system.
Formula or reference method
VPD = saturation vapor pressure at leaf temperature - saturation vapor pressure at air temperature x relative humidity.
Worked example
At 75 F air, a 2 F cooler leaf, and 60 percent RH, this calculator returns about 0.99 kPa using the Magnus approximation.
Understanding the result
Lower VPD means slower transpiration; higher VPD means a stronger drying demand. A useful range depends on crop and growth stage.
Assumptions and limits
Leaf temperature is an estimate unless measured. Air movement, root health, light, and irrigation can change plant response.
Common mistakes
Do not use room temperature as leaf temperature automatically or chase one VPD number while ignoring wilting and root-zone conditions.