VPD calculator (leaf and air) and humidity deficit
Get the VPD (vapor pressure deficit) and humidity deficit HD from air temperature and relative humidity or dew point. Add the leaf temperature offset to get the leaf VPD too. A temperature × RH VPD chart is below.
What VPD and humidity deficit mean
VPD (vapor pressure deficit) is the difference between the most water vapor pressure the air can hold at its temperature (the saturation vapor pressure) and the vapor pressure it actually has. The higher it is, the harder the air pulls water and the faster leaves transpire; the lower it is, the slower they transpire. It is given in kPa: 1 kPa = 10 hPa = 10 mbar.
Humidity deficit (HD) says the same thing as an amount of water: the grams of vapor that 1 m³ of air could still take up before it saturates (g/m³), or the same per kilogram of dry air (g/kg). Korean and Dutch greenhouse practice often uses HD, while most other growing guides use VPD.
Relative humidity alone does not tell you how dry the air is. At the same 70 % RH, the VPD is about 0.70 kPa at 20 °C but about 1.27 kPa at 30 °C, nearly twice as much.
VPD chart (temperature × relative humidity)
Air VPD in kPa (leaf at air temperature). The darker the cell, the higher the VPD (drier air). The shading steps are the general-guide ranges in the legend below; the right range differs by crop and growth stage. Swipe sideways if the table is wide.
Shading steps (air VPD, general guide)
- < 0.4 kPaVery humid
- 0.4 ~ 0.8 kPaHumid
- 0.8 ~ 1.2 kPaModerate
- 1.2 ~ 1.6 kPaDry
- ≥ 1.6 kPaVery dry
A range often cited in Korean greenhouse sources: HD 3 to 8 g/m³, VPD 0.5 to 1.2 kPa.
| Temperature | RH 40 % | RH 45 % | RH 50 % | RH 55 % | RH 60 % | RH 65 % | RH 70 % | RH 75 % | RH 80 % | RH 85 % | RH 90 % |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 15 °C | 1.02 | 0.94 | 0.85 | 0.77 | 0.68 | 0.60 | 0.51 | 0.43 | 0.34 | 0.26 | 0.17 |
| 16 °C | 1.09 | 1.00 | 0.91 | 0.82 | 0.73 | 0.64 | 0.55 | 0.45 | 0.36 | 0.27 | 0.18 |
| 17 °C | 1.16 | 1.07 | 0.97 | 0.87 | 0.78 | 0.68 | 0.58 | 0.48 | 0.39 | 0.29 | 0.19 |
| 18 °C | 1.24 | 1.14 | 1.03 | 0.93 | 0.83 | 0.72 | 0.62 | 0.52 | 0.41 | 0.31 | 0.21 |
| 19 °C | 1.32 | 1.21 | 1.10 | 0.99 | 0.88 | 0.77 | 0.66 | 0.55 | 0.44 | 0.33 | 0.22 |
| 20 °C | 1.40 | 1.29 | 1.17 | 1.05 | 0.94 | 0.82 | 0.70 | 0.58 | 0.47 | 0.35 | 0.23 |
| 21 °C | 1.49 | 1.37 | 1.24 | 1.12 | 1.00 | 0.87 | 0.75 | 0.62 | 0.50 | 0.37 | 0.25 |
| 22 °C | 1.59 | 1.45 | 1.32 | 1.19 | 1.06 | 0.93 | 0.79 | 0.66 | 0.53 | 0.40 | 0.26 |
| 23 °C | 1.69 | 1.55 | 1.41 | 1.26 | 1.12 | 0.98 | 0.84 | 0.70 | 0.56 | 0.42 | 0.28 |
| 24 °C | 1.79 | 1.64 | 1.49 | 1.34 | 1.19 | 1.04 | 0.90 | 0.75 | 0.60 | 0.45 | 0.30 |
| 25 °C | 1.90 | 1.74 | 1.58 | 1.43 | 1.27 | 1.11 | 0.95 | 0.79 | 0.63 | 0.48 | 0.32 |
| 26 °C | 2.02 | 1.85 | 1.68 | 1.51 | 1.35 | 1.18 | 1.01 | 0.84 | 0.67 | 0.50 | 0.34 |
| 27 °C | 2.14 | 1.96 | 1.78 | 1.61 | 1.43 | 1.25 | 1.07 | 0.89 | 0.71 | 0.54 | 0.36 |
| 28 °C | 2.27 | 2.08 | 1.89 | 1.70 | 1.51 | 1.32 | 1.13 | 0.95 | 0.76 | 0.57 | 0.38 |
| 29 °C | 2.40 | 2.20 | 2.00 | 1.80 | 1.60 | 1.40 | 1.20 | 1.00 | 0.80 | 0.60 | 0.40 |
| 30 °C | 2.55 | 2.34 | 2.12 | 1.91 | 1.70 | 1.49 | 1.27 | 1.06 | 0.85 | 0.64 | 0.42 |
| 31 °C | 2.70 | 2.47 | 2.25 | 2.02 | 1.80 | 1.57 | 1.35 | 1.12 | 0.90 | 0.67 | 0.45 |
| 32 °C | 2.86 | 2.62 | 2.38 | 2.14 | 1.90 | 1.67 | 1.43 | 1.19 | 0.95 | 0.71 | 0.48 |
| 33 °C | 3.02 | 2.77 | 2.52 | 2.27 | 2.01 | 1.76 | 1.51 | 1.26 | 1.01 | 0.76 | 0.50 |
| 34 °C | 3.19 | 2.93 | 2.66 | 2.40 | 2.13 | 1.86 | 1.60 | 1.33 | 1.06 | 0.80 | 0.53 |
| 35 °C | 3.38 | 3.10 | 2.81 | 2.53 | 2.25 | 1.97 | 1.69 | 1.41 | 1.13 | 0.84 | 0.56 |
Frequently asked questions
What is VPD?
It is the difference between the saturation vapor pressure of the air and its actual vapor pressure, in kPa. For example, the saturation vapor pressure at 25 °C is about 3.17 kPa, so at 50 % RH the actual vapor pressure is 1.58 kPa and the VPD is about 1.58 kPa. The higher it is, the drier the air and the faster plants transpire.
How is humidity deficit (HD) different from VPD?
Both say how far the air is from saturation. VPD is a pressure difference (kPa); HD is a difference in the amount of water vapor (g/m³ or g/kg). The same VPD gives a different HD at a different temperature, so this calculator shows both. At 20 °C and 70 % RH, VPD is about 0.70 kPa and HD about 5.2 g/m³.
Why enter the leaf temperature?
What drives transpiration is the vapor pressure difference between the inside of the leaf (taken as saturated at leaf temperature) and the outside air: the leaf VPD. Leaves are usually slightly cooler than the air because transpiration cools them, and they can be warmer under strong sun. Enter the difference between leaf temperature measured with an infrared thermometer and air temperature to get the leaf VPD. If you do not know it, leave it at 0.
How do I convert VPD between kPa, hPa and mbar?
1 kPa = 10 hPa = 10 mbar, so 0.8 kPa is 8 hPa (8 mbar). 1 kPa is about 0.145 psi or about 0.295 inHg.
What is the right VPD?
It depends on the crop and growth stage. A common general guide in growing treats below 0.4 kPa as very humid and above 1.6 kPa as very dry, and Korean greenhouse sources often cite VPD 0.5 to 1.2 kPa and HD 3 to 8 g/m³ as an example range. The band and table on this page only show such general guides; follow the growing guidelines for your crop.
Equations and sources
The same equations as the calculator engine. Below 0.01 °C the saturation pressure uses eq. 5 over ice. p = pressure in Pa.
| Item | Equation | Notes · basis |
|---|---|---|
| Saturation vapor pressure | ln es = C8/T + C9 + C10·T + C11·T² + C12·T³ + C13·ln T | Hyland-Wexler, ASHRAE 2017 chapter 1 eq. 6 (over water, 0.01 °C and above). T = t + 273.15 K |
| Actual vapor pressure | ea = RH / 100 · es(t) = es(td) | From relative humidity, or the saturation pressure at the dew point |
| Air VPD | VPD = es(t) − ea | Leaf at air temperature |
| Leaf VPD | VPD_leaf = es(t + ΔT) − ea | Air inside the leaf is taken as saturated at leaf temperature. ΔT = leaf − air temperature |
| Absolute humidity | ρv = W / v, W = 0.621945 · ea / (p − ea) | v = specific volume (ASHRAE eq. 26). Mass of vapor per volume of air |
| Humidity deficit | HD = ρv,s − ρv [g/m³], Ws − W [g/kg] | s = saturation at the same temperature and pressure |
Sources
- ASHRAE Handbook Fundamentals 2017 (SI), chapter 1 Psychrometrics: eqs. 5 and 6 saturation pressure, eq. 20 humidity ratio, eq. 26 specific volume, definition of absolute humidity (water vapor density)
- Hyland, R. W. and Wexler, A. (1983), Formulations for the thermodynamic properties of the saturated phases of H2O from 173.15 K to 473.15 K, ASHRAE Transactions 89(2A)
- Korean greenhouse range example (HD 3 to 8 g/m³, VPD 0.5 to 1.2 kPa), in Korean: SmartBizN, iocrops blog
Last updated 2026-11-01.