VPD Calculator

How strongly the surrounding air can dry a crop, and how that differs from the vapour-pressure gradient at a measured leaf temperature.

Air conditions and plant conditions

Air temperature and relative humidity together give the actual vapour pressure of the surrounding air: how much water vapour it holds. Both results on this page start from that figure.

Air vapour pressure deficit (Air VPD) compares that actual vapour pressure with the saturation vapour pressure at air temperature. It describes the drying capacity of the surrounding air. It is calculated from air temperature and relative humidity and does not account for leaf temperature.

Plant-to-air vapour pressure difference compares the same actual vapour pressure with the saturation vapour pressure at measured leaf temperature. It assumes the air spaces inside the leaf are saturated, so it estimates the vapour-pressure gradient between the inside of the leaf and the surrounding air. Because leaf and air temperature can differ, this plant-centred value may be meaningfully different from Air VPD. It is also commonly described as leaf-to-air VPD or leaf VPD.

One is a deficit of the air, the other a difference between leaf and air. Both are sometimes abbreviated to VPD, but they use different temperatures and are not interchangeable.

Plant-to-air vapour pressure difference is a primary atmospheric driving force for transpiration and influences stomatal response. It does not by itself determine stomatal opening, transpiration rate or plant stress. Plant response also depends on radiation and leaf energy balance, air movement and boundary-layer resistance, stomatal conductance, irrigation and root-zone water availability, root-zone temperature and oxygen, crop stage and crop load, and plant health and acclimation.

Use the result as one part of a climate assessment rather than as a universal setpoint. This calculator supports a plant-centred interpretation by comparing surrounding-air conditions with a measured leaf temperature.

Vapour Pressure Difference and Plant Empowerment

Air VPD describes the surrounding air. Plant Empowerment focuses instead on Vapour Pressure Difference, which also takes leaf temperature into account: it compares the vapour pressure inside the leaf with that of the surrounding air. On this page that is the plant-to-air vapour pressure difference.

In that approach, Absolute Humidity helps describe the moisture balance of the greenhouse, and Vapour Pressure Difference adds a more plant-centred view of the gradient the crop experiences. Plant Empowerment also puts weight on trends rather than single values, and on the climate at the leaf itself: the canopy microclimate, the boundary layer of still air around the leaf, airflow and the energy balance, rather than steering to one number.

Its published guidance gives two broad indicators: below about 0.2 kPa transpiration slows and the plant becomes less active, and above about 2.0 kPa stomata can begin to close to limit water loss. The calculator uses them to interpret the plant-to-air result, as process indicators rather than crop setpoints.

below 0 kPa

Condensation possible. The surrounding-air vapour pressure exceeds saturation vapour pressure at leaf temperature.

0 to below 0.2 kPa

Very low gradient. Limited outward vapour-pressure gradient; transpiration may slow.

0.2 to 2.0 kPa

Transpiration gradient. An outward gradient is present. Actual transpiration still depends on stomata, energy, airflow and water availability.

above 2.0 kPa

Very high gradient. Strong evaporative demand; stomatal restriction and water-stress risk may increase.

Broad process guidance based on Plant Empowerment principles, not a universal crop setpoint. Source: Understanding absolute humidity in greenhouse climate control, Plant Empowerment. Vaerens is not affiliated with Plant Empowerment.

Relationship with Humidity Deficit

Air VPD and Humidity Deficit both describe the moisture deficit of the surrounding air. Air VPD expresses the deficit as a pressure difference in kPa. Humidity Deficit expresses it as a moisture quantity, commonly in g/m³ or g/kg. Humidity Deficit is widely used in greenhouse horticulture, while Air VPD is especially common in North American and indoor-growing contexts.

They describe closely related air conditions but are not numerically interchangeable: they use different units and, depending on the Humidity Deficit convention, the conversion also depends on temperature and sometimes atmospheric pressure.

How to measure plant conditions

Measure air temperature and relative humidity as close to the canopy as practical. Measure the temperature of representative, actively transpiring leaves. Conditions at the leaf surface can differ from readings elsewhere in the growing space because of radiation, airflow and the leaf boundary layer.

A handheld infrared leaf-temperature measurement is a snapshot. Repeat measurements at different times, canopy positions and changing conditions when assessing a crop or a climate strategy.

• Measure air temperature and RH at canopy level.
• Use a representative, actively transpiring leaf rather than an exceptional hot or cold leaf.
• Take the air and leaf readings at about the same place and time.
• Repeat measurements across canopy locations and as conditions change.
• Interpret the result alongside radiation, airflow, irrigation and plant observations.

Greenhouses and controlled environments

The same distinction applies in greenhouses, vertical farms, indoor farms and growth chambers. In controlled environments with artificial lighting, leaf and air temperatures may sometimes be closer than in strongly sunlit greenhouse conditions. They should still be treated as separate measurements because transpiration, airflow, lighting, HVAC operation and local humidity can create meaningful differences.

Air VPD is useful for

• Comparing general atmospheric drying conditions
• Building temperature/RH climate tables
• Monitoring controlled environments where leaf temperature is unavailable
• Comparing climate setpoints between periods or growing areas
• Using broad crop-stage reference ranges cautiously

Plant-to-air vapour pressure difference is useful for

• Including the plant’s measured temperature
• Understanding how radiation and cooling affect the plant–air gradient
• Comparing different canopy positions
• Investigating condensation risk
• Adding plant-level context to a greenhouse or vertical-farm climate assessment