Vapor Pressure Deficit Calculator
Enter the air temperature and relative humidity to find the vapor pressure deficit, the gap between how much water vapor the air holds and how much it could hold at that temperature. The result is an air-temperature estimate in kilopascals.
Last updated: September 26, 2026
Frequently asked questions
What is vapor pressure deficit?
Vapor pressure deficit (VPD) is the difference between the amount of water vapor the air could hold when saturated and the amount it actually holds. It is expressed as a pressure, usually in kilopascals, and describes how strongly the air pulls water from surfaces.
Which formula does this calculator use?
It estimates saturation vapor pressure with SVP = 0.6108 × exp(17.27 T / (T + 237.3)), where T is the air temperature in °C, then computes VPD = SVP × (1 − RH/100). Fahrenheit input is converted to Celsius first.
Why is VPD about zero at 100% humidity?
At 100% relative humidity the air is saturated, so the actual vapor pressure equals the saturation vapor pressure and the difference is zero.
Is this air VPD or leaf VPD?
It is air VPD, based only on air temperature and humidity. Leaf VPD needs the leaf surface temperature, which can differ from the air temperature, so do not treat the two as the same number.
What VPD should I aim for?
There is no universal target. Suitable ranges depend on the species, growth stage, and growing conditions, so consult guidance specific to your plants rather than a single number.
How does VPD relate to the Water Potential Calculator?
VPD describes the drying power of the air, while water potential describes the energy state of water in plant tissue or solution. They are different quantities, but both help describe water movement between plants and their surroundings.