Greenhouse VPD Calculator: Air and Leaf Vapor Pressure Deficit in kPa

Use this when you are adjusting greenhouse heating, venting or misting and want to know how strongly the air pulls water vapor from your crop. Enter air temperature and relative humidity for air VPD, and add a measured leaf temperature to see leaf VPD, which is the deficit between the leaf surface and the surrounding air. A negative leaf VPD is shown as-is because it means the leaf is below the dew point of the air.

Air VPD Versus Leaf VPD

Relative humidity alone does not tell you how dry the air feels to a plant: 60 % RH at 15 °C and 60 % RH at 30 °C are very different drying conditions. Vapor pressure deficit (VPD) puts temperature and humidity on one scale, in kPa, as the gap between the water vapor the air could hold when saturated and what it actually holds.

Air VPD evaluates that gap at air temperature. Leaf VPD evaluates it at the leaf surface, because water evaporates from the saturated tissue inside the leaf, which sits at leaf temperature. Both use the same actual vapor pressure of the air, so the only difference between them is the temperature used for the saturation term.

Why Measured Leaf Temperature Matters

Leaves are rarely at air temperature. Under strong sun they can run warmer than the air; at night, under a clear sky or next to a cold glazing surface, they can run several degrees cooler. A 2 °C cooler leaf in 25 °C, 60 % RH air cuts the deficit from 1.267 to 0.909 kPa. An infrared thermometer or leaf thermocouple aimed at sunlit, mature leaves gives a much better leaf VPD than assuming leaf = air.

Leaf temperature responds to the climate you run. Night heating set points, which you can cost with the greenhouse heating cost calculator, keep leaves above the dew point. Ventilation removes moist air and lowers humidity; the greenhouse fan sizing calculator estimates the airflow needed. Closing vents to hold enriched CO₂ also holds humidity in, so check VPD when you plan with the greenhouse CO₂ enrichment calculator.

Reading the Sign of the Result

The leaf − air difference is signed. A negative difference means the leaf is cooler than the air and has a smaller deficit; a positive one means the leaf is warmer. A negative leaf VPD means the saturation vapor pressure at the leaf is below the actual vapor pressure of the air, so the leaf is colder than the dew point and water can condense on it. In 25 °C, 60 % RH air the dew point is about 16.7 °C, so a leaf at 15 °C gives a leaf VPD of about −0.195 kPa.

The calculator keeps that negative number rather than clamping it to zero. It is a diagnostic indicator for condensation risk, not a model of how much water condenses or how long leaves stay wet.

Inputs Explained

Air temperature: Greenhouse air temperature at canopy height, in °C or °F (-10 to 60 °C, 14 to 140 °F). It sets the saturation vapor pressure of the air.
Relative humidity: Relative humidity (0 to 100 %) read at the same place and time as the air temperature. Together they give the actual vapor pressure of the air.
Leaf temperature (optional): Measured leaf surface temperature, for example from an infrared thermometer. If you leave it blank the calculator assumes the leaf is at air temperature and labels the result as an assumption.
Target VPD minimum and maximum (optional): A VPD range in kPa from your own crop and growth-stage source. The calculator only compares against a target you enter; it does not apply a universal optimum zone.

How This Calculator Works

Based on: FAO-56 Eq. 11 saturation vapor pressure, es(T) = 0.6108 × exp(17.27 T / (T + 237.3)) kPa; ea = es(T_air) × RH / 100; VPD = es(T) − ea

Best for: Checking greenhouse air and leaf transpiration demand from one temperature and humidity reading

Check locally: Compare results against crop- and stage-specific VPD guidance from your extension service or crop advisor

Units supported: Metric (°C, %, kPa), Imperial (°F, %, kPa)

Formula

es(T) = 0.6108 × exp(17.27 × T / (T + 237.3)): Saturation vapor pressure in kPa at temperature T in °C (FAO-56 Eq. 11). °F inputs are converted to °C first.
ea = es(T_air) × RH / 100: Actual vapor pressure of the air in kPa, from the definition of relative humidity (FAO-56 Eq. 10).
VPD_air = es(T_air) − ea: Air vapor pressure deficit in kPa (FAO-56 "vapour pressure deficit, es − ea").
VPD_leaf = es(T_leaf) − ea: Leaf vapor pressure deficit in kPa: the same ea, with saturation evaluated at the leaf temperature. T_leaf = T_air when no leaf temperature is entered.
Leaf − air difference = VPD_leaf − VPD_air: Signed; negative when the leaf is cooler than the air.

Source

FAO Irrigation and Drainage Paper 56, Chapter 3: Meteorological data (opens in a new tab), Food and Agriculture Organization of the United Nations (Allen et al., 1998) — Eq. 10 (relative humidity), Eq. 11 (saturation vapour pressure) and the vapour pressure deficit definition

Assumptions

Air temperature and relative humidity describe the same air, read at canopy height at the same moment.
The air inside the leaf is saturated at leaf temperature, so leaf VPD uses es(T_leaf) with the vapor pressure of the surrounding air.
If no leaf temperature is entered, the leaf is assumed to be at air temperature and leaf VPD equals air VPD; the result is labelled as an assumption.
The FAO-56 saturation equation applies over water. Below 0 °C the calculator still uses it, which slightly overstates saturation vapor pressure compared with ice.
Inputs are limited to −10 to 60 °C (14 to 140 °F) for air and leaf temperature and 0 to 100 % relative humidity. Results are rounded only for display.

Worked Example

Greenhouse air at 25 °C and 60 % relative humidity, with a measured leaf temperature of 23 °C

1. Saturation vapor pressure of the air
es(25 °C) = 0.6108 × exp(17.27 × 25 / 262.3) = 3.168 kPa
2. Actual vapor pressure
ea = 3.168 × 60 / 100 = 1.901 kPa
3. Air VPD
3.168 − 1.901 = 1.267 kPa
4. Leaf VPD
es(23 °C) = 2.809 kPa, so leaf VPD = 2.809 − 1.901 = 0.909 kPa

Air VPD 1.267 kPa, leaf VPD 0.909 kPa: the cooler leaf has a 0.358 kPa smaller deficit than the air

How to Interpret Your Results

Condition	What It Means
Leaf VPD lower than air VPD	The leaf is cooler than the air, so the vapor pressure gradient from leaf to air is smaller than the air reading suggests.
Leaf VPD higher than air VPD	The leaf is warmer than the air, for example under strong radiation, so transpiration demand at the leaf is higher than the air VPD shows.
Leaf VPD below zero	The leaf surface is below the dew point of the air and water can condense on it. Treat this as a diagnostic warning, not a full condensation model.
Compared with your target	Below, within or above refers only to the range you entered. Choose that range from a source for your crop and growth stage.

Limitations and Cautions

The result describes one point in time and one location. Greenhouse temperature and humidity vary by height, bench and hour, so take several readings before changing set points.
A negative leaf VPD is a diagnostic flag for possible condensation. It does not predict the amount of water, the duration of leaf wetness or disease risk.
VPD describes the drying power of the air, not actual transpiration. Stomatal response, light, air movement and root-zone water also control how much water the crop loses.
No crop or growth-stage targets are built in. Any target you enter comes from your own source, and the heatmap colors show magnitude only.
Sensor error carries through: a ±3 % RH sensor at 25 °C shifts air VPD by about ±0.1 kPa, and an IR leaf reading depends on emissivity and what the sensor sees.

Common Mistakes to Avoid

Treating air VPD as leaf VPD

Leaves are often a few degrees warmer or cooler than the air. Measure leaf temperature when the difference matters.

Reading temperature and humidity in different places

Take both readings at canopy height from the same shaded, aspirated sensor so they describe the same air.

Using one VPD range for every crop and stage

Appropriate VPD depends on the crop, growth stage and production goal. Enter a sourced target rather than a generic chart value.

References

FAO Irrigation and Drainage Paper 56, Chapter 3: Meteorological data (opens in a new tab), Food and Agriculture Organization of the United Nations (Allen et al., 1998) — saturation vapour pressure (Eq. 11), relative humidity (Eq. 10) and vapour pressure deficit; its worked example also gives e°(25 °C) = 3.168 kPa

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