Vapor Pressure Deficit Calculator (VPD for Greenhouses and Grow Rooms)
The vapour pressure deficit of the air, and at the leaf, from the air temperature and relative humidity — the number growers use to judge how hard plants are transpiring.
Results
The dashed line marks the value you entered.
What this tool does
Vapour pressure deficit is the gap between how much water vapour the air could hold and how much it holds, in kilopascals. Relative humidity alone misleads, because the same 60 % is far drier air when hot. A low deficit leaves plants unable to transpire and encourages mould; a high one makes them lose water faster than the roots supply it. Many growers aim for roughly 0.8 to 1.2 kPa for leafy growth, lower for seedlings and cuttings. A leaf is usually a degree or two cooler than the air, so the leaf figure is the one the plant feels.
Formula
e_s(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa · VPD = e_s(T) − e_a = e_s(T) (1 − RH ÷ 100)
Variables
| Symbol | Meaning | Unit |
|---|---|---|
ta | Air temperature | °C |
rh | Relative humidity | % |
lo | Leaf temperature below the air temperature | °C |
V | Vapour pressure deficit of the air | kPa |
VL | Vapour pressure deficit at the leaf | kPa |
ES | Saturation vapour pressure of the air | kPa |
EA | Actual vapour pressure of the air | kPa |
Worked example
- Air temperature25 °C
- Relative humidity60 %
- Leaf temperature below the air temperature2 °C
- Vapour pressure deficit of the air1.27 kPa
- Vapour pressure deficit at the leaf0.91 kPa
- Saturation vapour pressure of the air3.168 kPa
- Actual vapour pressure of the air1.901 kPa
Limitations
- The result is an estimate based only on the values you type. Real situations often include factors this calculator does not know about.
- The default values are typical reference figures, not measurements of your situation. Replace them with your own data whenever you have it.