VPD calculator (vapor pressure deficit)
This VPD calculator works out the vapor pressure deficit – how far the air is from saturation – from temperature and relative humidity. At 77 °F (25 °C) and 60 % RH the air could hold 3.17 kPa of water vapor but holds 1.90 kPa, so the VPD is 1.27 kPa. Growers use it to judge how strongly plants transpire.
Example: Temperature unit: °F · Air temperature 77 · Relative humidity 60 % → Vapor pressure deficit: VPD 1.27 kPa (humidity deficit 9.2 g/m³). Source: FAO Irrigation and Drainage Paper 56 (Allen et al. 1998), chapter 3: saturation vapour pressure (eq. 11), actual vapour pressure (eq. 19), vapour pressure deficit (read 09.10.2026). Updated: .
How it is calculated
VPD = es − ea. The saturation vapor pressure is es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa with T in °C (FAO-56, equation 11). The actual vapor pressure is ea = es × RH ÷ 100 (equation 19). FAO’s own example: es is 2.064 kPa at 18 °C and 3.168 kPa at 25 °C.
VPD chart
| Temperature | RH | VPD |
|---|---|---|
| 20 °C (68 °F) | 70 % | 0.70 kPa |
| 22 °C (71.6 °F) | 65 % | 0.93 kPa |
| 25 °C (77 °F) | 60 % | 1.27 kPa |
| 28 °C (82.4 °F) | 75 % | 0.94 kPa |
| 30 °C (86 °F) | 50 % | 2.12 kPa |
Warm air holds much more water: at the same 60 % RH the deficit rises quickly with temperature. That is why growers lower the humidity as plants mature or raise it for seedlings – the right target depends on the crop and is not set by this calculator.
Leaf VPD
Leaves are often a little cooler than the air. Enter the difference: at 77 °F air, 60 % RH and leaves 2 °C (3.6 °F) cooler, the leaf VPD is 0.91 kPa instead of 1.27 kPa. An infrared thermometer gives the leaf temperature.
Humidity deficit in g/m³
The same deficit can be expressed as grams of water per cubic meter of air, using the ideal gas law with the molar mass of water (18.0153 g/mol) and the gas constant from NIST. At 25 °C saturated air holds 23.0 g/m³; at 60 % it holds 13.8 g/m³, a deficit of 9.2 g/m³.
Related: the dew point calculator shows when condensation starts, and the heat index calculator how warm humid air feels.
Frequently asked questions
How do you calculate VPD?
VPD = es − ea. Take the saturation vapor pressure at the air temperature (FAO-56 equation 11) and subtract es × RH ÷ 100. At 25 °C and 60 % RH: 3.17 − 1.90 = 1.27 kPa.
What is leaf VPD?
The deficit between the saturation pressure at leaf temperature and the vapor pressure of the air. Leaves 2 °C cooler than 25 °C air at 60 % give 0.91 kPa.
What is a good VPD?
That depends on the crop and the growth stage. The calculator gives the value; take the target from your crop guide.
Is VPD the same as relative humidity?
No. The same 60 % RH means a VPD of 0.94 kPa at 20 °C but 1.70 kPa at 30 °C – the temperature matters as much as the humidity.
What unit is VPD in?
Usually kPa; 1 kPa = 10 hPa = 10 mbar. The table also shows the deficit in g/m³.
Which formula does the VPD calculator use?
The FAO-56 equation es = 0.6108 × exp(17.27 T ÷ (T + 237.3)), the same used for crop evapotranspiration worldwide.
Sources and legal basis
- FAO Irrigation and Drainage Paper 56 (Allen et al. 1998), chapter 3: saturation vapour pressure (eq. 11), actual vapour pressure (eq. 19), vapour pressure deficit (read 09.10.2026)
- NIST – molar gas constant R (CODATA) and molecular weight of water 18.0153 g/mol (Chemistry WebBook) (read 09.10.2026)
As of: · Editorial policy
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