What is the VPD of my grow room?
Type the air temperature and the relative humidity. The VPD calculator gives the vapour pressure deficit in kPa, with the saturation and actual vapour pressures, and the leaf VPD if you also type the leaf temperature.
- Vapour pressure deficit (air)
- 1.267 kPa
At 77 °F and 60% relative humidity, the vapour pressure deficit is 1.267 kPa.
- Saturation vapour pressure
- 3.168 kPa
- Actual vapour pressure
- 1.901 kPa
Vapour pressure deficit (air): 1.267 kPa. At 77 °F and 60% relative humidity, the vapour pressure deficit is 1.267 kPa.
Vapour pressure deficit (air) by relative humidity
How to calculate
Computes the vapour pressure deficit (VPD) of the air from its temperature and relative humidity, with the saturation and actual vapour pressures, and the leaf VPD when you type a leaf temperature.
Example with the default inputs (Air temperature 77 °F, Relative humidity 60%): At 77 °F and 60% relative humidity, the vapour pressure deficit is 1.267 kPa.
Method: e°(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa; eₐ = e°(T_air) × RH ÷ 100; VPD = e°(T_air) − eₐ; leaf VPD = e°(T_leaf) − eₐ.
- Temperatures are converted to °C before the formula. The formula is FAO-56’s Tetens form over water.
- The leaf VPD assumes the air inside the leaf is saturated at the leaf temperature.
- Temperatures run from −40 °C to 60 °C (−40 °F to 140 °F).
Worked examples
Each example is checked against the calculator on every build.
- Air temperature 76.1 °F, Relative humidity 50% gives Saturation vapour pressure 3.075 kPa, Vapour pressure deficit (air) 1.537 kPa, Actual vapour pressure 1.537 kPa.Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03)
- Air temperature 59 °F, Relative humidity 80% gives Saturation vapour pressure 1.705 kPa, Vapour pressure deficit (air) 0.341 kPa.Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03)
- Air temperature 77 °F, Relative humidity 60%, Leaf temperature 73.4 °F gives Vapour pressure deficit (air) 1.267 kPa, Leaf VPD 0.909 kPa.Source: FAO Irrigation and Drainage Paper 56, Crop evapotranspiration, chapter 3, Air humidity (equation 11: e°(T) = 0.6108 exp[17.27 T ÷ (T + 237.3)] kPa; equation 19: eₐ = e° × RH ÷ 100; the vapour pressure deficit is the saturation minus the actual vapour pressure; e°(24.5 °C) = 3.075 kPa and e°(15 °C) = 1.705 kPa), https://www.fao.org/4/x0490e/x0490e07.htm (retrieved 2026-10-03)
How it works
With the air temperature T_air and the leaf temperature T_leaf in °C, and the relative humidity RH in percent:
- Saturation vapour pressure e°(T) = 0.6108 × exp(17.27 × T ÷ (T + 237.3)) kPa (FAO-56 equation 11).
- Actual vapour pressure eₐ = e°(T_air) × RH ÷ 100 (FAO-56 equation 19).
- Vapour pressure deficit (air) = e°(T_air) − eₐ = e°(T_air) × (100 − RH) ÷ 100.
- Leaf VPD = e°(T_leaf) − eₐ, shown only when you type a leaf temperature.
Rules. Temperatures may be typed in °C or °F and are converted to °C first (°C = (°F − 32) × 5 ÷ 9). Each temperature is from −40 °C to 60 °C (−40 °F to 140 °F). Relative humidity is from 0 to 100%. The leaf temperature is optional.
Output format. Pressures show in kPa with 3 decimals. The raw outputs are in pascals (1 kPa = 1,000 Pa), and the raw temperature inputs are in kelvin (°C + 273.15).
Worked examples by hand
FAO’s 24.5 °C at 50% relative humidity. e° = 0.6108 × exp(17.27 × 24.5 ÷ 261.8) = 0.6108 × exp(1.6162) = 3.075 kPa, the value FAO gives. eₐ = 3.075 × 0.50 = 1.537 kPa, and VPD = 3.075 − 1.537 = 1.537 kPa.
15 °C at 80%. e° = 0.6108 × exp(17.27 × 15 ÷ 252.3) = 1.705 kPa (FAO’s value). VPD = 1.705 × 0.20 = 0.341 kPa.
Air at 25 °C and 60%, leaves at 23 °C. e°(25) = 3.168 kPa, so eₐ = 3.168 × 0.60 = 1.901 kPa and the air VPD = 3.168 × 0.40 = 1.267 kPa. e°(23) = 2.809 kPa, so the leaf VPD = 2.809 − 1.901 = 0.909 kPa.
Other questions people ask
What is VPD?
The vapour pressure deficit is how much more water vapour the air could hold at its temperature: the saturation vapour pressure minus the vapour pressure the air has now. FAO calls it an accurate indicator of how fast the air can take up water from plants and soil.
How is VPD calculated?
First the saturation vapour pressure, e° = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa with T in °C. The air holds e° × RH ÷ 100 of that. VPD is the difference: e° × (1 − RH ÷ 100). At 25 °C and 60% that is 3.168 × 0.40 = 1.267 kPa.
What is leaf VPD?
The deficit between the inside of the leaf and the air. The air in a leaf is taken as saturated at the leaf temperature, so leaf VPD = e°(leaf temperature) − the air’s actual vapour pressure. Leaves cooler than the air have a lower VPD.
Why does VPD matter for plants?
VPD sets how hard the air pulls water out of leaves: a higher VPD means faster transpiration, a lower one slower. FAO uses it for this reason in crop water use. Growers change the heat and humidity to keep VPD in the range their crop and its stage need.
Can VPD be negative?
The air VPD cannot: relative humidity is at most 100%, so the air holds at most the saturation amount. A leaf VPD can be negative when the leaf is much cooler than the air and the air is humid, which means water can condense on the leaf.
What unit is VPD in?
Kilopascals (kPa). 1 kPa is 10 hPa, or 10 millibars, or about 0.145 psi. Grow charts give VPD in kPa.