VPD Calculator (Vapor Pressure Deficit)

How to use this vapor pressure deficit calculator

What this calculator does

This calculator estimates vapor pressure deficit, or VPD, in kilopascals. VPD is the difference between the saturation vapor pressure at the crop canopy and the actual vapor pressure of the surrounding air. Growers use it as a practical indicator of the atmosphere's evaporative demand and its likely influence on transpiration. It does not diagnose plant health, prescribe an irrigation schedule, or establish a universal crop target; cultivar, growth stage, light, airflow, root-zone moisture, and sensor placement all matter.

When to use it

Use it when checking a greenhouse or indoor growing room, comparing climate settings before and after humidification, reviewing canopy-temperature measurements from an infrared sensor, or translating dew-point or psychrometer readings into a single pressure-based indicator. Michigan State University Extension explains why growers often find VPD more useful than relative humidity alone when evaluating plant water loss.

How to calculate

1. The calculator opens with a demonstration: relative humidity 55%, air temperature 22 °C, and canopy temperature 20 °C. The example workbook is immediately available from Download Excel.
2. Choose Calculate using. Select relative humidity, dew point, or wet-bulb measurements according to your instruments.
3. Select the Temperature unit. Changing between °C and °F converts current temperature entries rather than merely relabeling them.
4. Enter the required measurements. Keep I know the canopy temperature checked when a leaf or canopy reading is available; otherwise clear it and the model will set canopy temperature equal to air temperature.
5. Read Canopy VPD first, then use the secondary pressures and the breakdown table to understand how the value was formed.
6. Select Download Excel to export the current inputs and typed results. Reset clears the demonstration and all calculated output, so export is disabled until a complete valid state is entered again.

Input guide

Calculate using is required and accepts one of three methods. “Relative humidity and air temperature” expects a percent and is usually the simplest sensor workflow. “Dew point and air temperature” uses dew point as the direct moisture measurement; the U.S. National Weather Service describes dew point as the temperature at which air becomes saturated. “Wet-bulb and dry-bulb temperature” estimates actual vapor pressure using a standard-pressure psychrometric relationship.

Temperature unit is required and accepts °C or °F. Enter ordinary decimals such as 22, 22.5, 72, or 72.5; commas and scientific notation are rejected to avoid ambiguous interpretation. Air temperature is required from – 50 to 80 °C equivalent. Raising air temperature while humidity stays fixed generally raises air VPD because saturation pressure increases rapidly with temperature.

Relative humidity is required only in the relative-humidity method and accepts 0 to 100%. For example, 55 means 55%, not 0.55. Higher humidity lowers actual pressure deficit. Dew point is required only in the dew-point method and must not exceed air temperature. Wet-bulb temperature is required only in the wet-bulb method and must not exceed dry-bulb air temperature; the FAO provides the underlying wet-bulb vapor-pressure relationship.

I know the canopy temperature is a required state choice. When checked, Canopy temperature becomes required from – 50 to 80 °C equivalent. A realistic example is 20 °C when air is 22 °C. A cooler canopy lowers leaf saturation pressure and therefore lowers canopy VPD. When the box is clear, canopy temperature is assumed equal to air temperature and the primary result becomes the air VPD.

Output guide

Canopy VPD is the primary estimate in kPa: leaf saturation pressure minus actual air vapor pressure. Zero means the leaf-air system is saturated under the model; a negative value can occur when a cool canopy is exposed to air whose vapor pressure exceeds saturation at the leaf temperature, indicating condensation potential rather than drying demand. Air VPD compares saturation and actual pressure at air temperature, so it remains useful when canopy temperature is unknown.

Actual air vapor pressure is the modeled moisture pressure in the air. Leaf saturation pressure is saturation vapor pressure at canopy temperature. Calculated relative humidity repeats the entered humidity in RH mode and derives it from dew point or wet-bulb data in the other modes. The header pills summarize the active method, air temperature, and RH. The Pressure breakdown table lists the air saturation pressure, actual air vapor pressure, leaf saturation pressure, and the final canopy VPD, including the temperature basis and pressure value for each row.

Worked example

With 22 °C air, 55% RH, and a 20 °C canopy, the Tetens equation gives leaf saturation pressure of about 2.338 kPa. Air saturation pressure at 22 °C is about 2.644 kPa, so actual air vapor pressure is 2.644 × 0.55 = 1.454 kPa. The canopy VPD is therefore 2.338 – 1.454 = 0.884 kPa, displayed as 0.88 kPa. The corresponding air VPD is about 1.19 kPa.