How to use the vapor pressure deficit calculator
- Enter the air temperature and choose Celsius or Fahrenheit.
- Enter the relative humidity as a percent.
- Set how much cooler the leaf is than the air, in the same degrees, or 0 to ignore it.
- Read the VPD of the air and of the leaf in kPa, millibar and psi, plus the saturation and actual vapor pressures.
Formula
SVP = 0.6108 x exp(17.27 T / (T + 237.3)), T in degrees C, SVP in kPa
Actual vapor pressure = SVP of air x relative humidity / 100
Air VPD = SVP of air - actual vapor pressure
Leaf VPD = SVP at leaf temperature - actual vapor pressure
The deficit is how much more water vapor the air could hold at that temperature. It is larger when the air is warm and dry, and zero at 100 percent humidity.
25 degrees C at 60 percent humidity
At 25 C the air can hold 3.1678 kPa of vapor. At 60 percent humidity it holds 1.9007 kPa, so the air VPD is 1.2671 kPa, which is 12.671 millibar or 0.1838 psi. A leaf 2 degrees cooler, at 23 C, can hold 2.8094 kPa, so the leaf VPD is 0.90877 kPa.
At 20 C and 50 percent humidity with no leaf offset the VPD is 1.1691 kPa. The same air entered as 77 F, 60 percent and a 3.6 F leaf offset gives the same 0.90877 kPa for the leaf, which shows the unit conversion working.
Why growers watch VPD
Plants lose water through their leaves faster when the air is dry and warm. Relative humidity alone does not capture that, since the same humidity means a larger deficit at a higher temperature. VPD combines the two, and many growers track it along with light and nutrients. The level a crop prefers depends on the species and the stage of growth, so check guidance for your own plants.
Related tools are the daily light integral calculator, the dew point calculator, the EC to ppm calculator and the growing degree days calculator.
The leaf offset
A leaf that is transpiring is often a little cooler than the air around it, and a leaf in strong light can be warmer. An infrared thermometer pointed at the leaf gives its real temperature, which you can turn into an offset. The 2 degree default is only an example that you can change.
Assumptions and limits
- Saturation vapor pressure follows the Tetens equation, which is close to other formulas within a fraction of a percent in this range.
- Air temperature runs from -10 to 60 degrees C (14 to 140 F), and humidity from 1 to 100 percent.
- The leaf temperature is the air temperature minus the offset you enter, and the humidity is that of the air.
- The page does not say what VPD any plant needs.
Frequently asked questions
How do I calculate VPD?
Find the saturation vapor pressure at the air temperature, multiply by the relative humidity to get the actual pressure, and subtract. At 25 C and 60 percent that is 3.168 - 1.901 = 1.267 kPa.
What is the difference between air VPD and leaf VPD?
Air VPD uses the air temperature for the saturation pressure. Leaf VPD uses the leaf temperature instead, which is what the plant actually experiences at its surface.
What units is VPD in?
It is a pressure. Growers usually quote kPa, and 1 kPa equals 10 millibar or 0.145 psi. The page shows all three.
Why is VPD zero at 100 percent humidity?
At 100 percent the air already holds all the vapor it can at that temperature, so there is no deficit left to fill.