VPD Calculator (vapour pressure deficit from temperature & humidity)

Understanding the VPD calculator — concept and principle

Vapour pressure deficit (VPD) measures how "thirsty" the air is: the gap between how much water vapour it could hold and how much it actually holds. Greenhouse growers, indoor gardeners and plant scientists use it to manage transpiration. This calculator gives air VPD, leaf VPD (using leaf temperature), the saturation and actual vapour pressures and the dew point.

Saturation minus actual vapour pressure: 1.27 kPa for the air, 0.91 kPa at the cooler leaf.

How it is calculated

es(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa, T in °C

ea = es(air) × RH ÷ 100

Air VPD = es(air) − ea

Leaf VPD = es(leaf) − ea

Dew point: solve es(Td) = ea

Temperatures may be entered in °C or °F; RH is relative humidity in %.

The principle

Warm air can hold more water vapour than cold air, and the saturation vapour pressure rises roughly exponentially with temperature. Relative humidity alone hides this: 60 % RH at 30 °C means much drier air for a leaf than 60 % at 15 °C. VPD expresses the actual drying power in kPa. Inside a leaf the air spaces are close to saturation at leaf temperature, so the driving force for water to leave the leaf is es(leaf) − ea. Leaves are often a little cooler than air because of evaporation, which makes leaf VPD smaller than air VPD.

Worked example

Default inputs: air 25 °C, RH 60 %, leaf 23 °C.

es(25 °C) = 0.6108 × exp(17.27 × 25 ÷ 262.3) = 0.6108 × 5.187 = 3.168 kPa.
ea = 3.168 × 0.60 = 1.901 kPa.
Air VPD = 3.168 − 1.901 = 1.27 kPa.
es(23 °C) = 2.810 kPa, so leaf VPD = 2.810 − 1.901 = 0.91 kPa.
Dew point, where es = 1.901 kPa, = 16.7 °C (62.1 °F); the leaf is above the dew point.

Each point is the calculator re-run with only “Air temperature” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Air temperature	Air VPD (kPa)
6.25	0.38
12.5	0.58
18.8	0.87
25	1.27
31.2	1.82
37.5	2.58
50	4.93

How to read the result

Higher VPD pulls water from leaves faster; very high VPD can make plants close their stomata, which slows growth, while very low VPD reduces transpiration and nutrient flow and favours fungal disease. Recommended ranges differ by crop and growth stage, so use guidance for your crop. If the leaf is at or below the dew point, condensation can form on it.

Common mistakes

Using air temperature when leaf temperature is quite different, for example under strong lamps.
Measuring humidity in a different spot from temperature.
Comparing relative humidity values at different temperatures as if they meant the same dryness.
Mixing °F and °C between air and leaf.

Keep in mind: Results depend on accurate sensors and placement. VPD targets are guidelines; plant response also depends on light, CO₂, root conditions and variety.

Frequently asked questions

What units is VPD in?

Usually kilopascals (kPa); 1 kPa = 10 hPa = 10 mbar.

Why measure leaf temperature?

Water evaporates from inside the leaf, so the leaf's own temperature sets the saturation pressure there.

Can VPD be negative?

Air VPD cannot be below zero, but leaf VPD can be when the leaf is colder than the dew point, meaning condensation.

How do I raise VPD in a greenhouse?

Warm the air or reduce humidity by ventilating or dehumidifying.

Sources: Allen R. G., Pereira L. S., Raes D. & Smith M., "Crop evapotranspiration", FAO Irrigation and Drainage Paper No. 56 (1998), Chapter 3

Checked 9 Nov 2026. Allen R.G., Pereira L.S., Raes D. & Smith M., “Crop evapotranspiration — Guidelines for computing crop water requirements”, FAO Irrigation and Drainage Paper No. 56 (1998), Chapter 3: saturation vapour pressure e°(T) = 0.6108·exp[17.27T/(T + 237.3)] kPa, actual vapour pressure from relative humidity, vapour pressure deficit e_s − e_a.

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