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VPD Calculator

Calculate air or leaf vapor pressure deficit from temperature and relative humidity for greenhouse and horticulture checks.

Optional. Leave blank to use air temperature.

Result

1.2671 kPa

Air VPD is 1.2671 kPa at 25 C air temperature and 60% relative humidity

Quick Answer

A VPD calculator finds the pressure gap that drives water loss from a leaf. At 25°C and 60% RH, SVP is 3.168 kPa. Air VP is 1.901 kPa. VPD is 1.267 kPa. This pressure value is reported in kilopascals. Add leaf temp for a leaf-based result.

Vapor Pressure Deficit Explained: What VPD Measures

Vapor pressure deficit, or VPD, is the gap between full air moisture and the water vapor now in the air; the tool first finds saturation vapor pressure, or SVP, from air temperature. The tool then uses relative humidity, or RH, to find air vapor pressure, called air VP, and air VPD is SVP minus air VP. If you add leaf temperature, the tool finds leaf SVP and subtracts the same air VP; this leaf result may differ because leaf temperature may be above or below air temperature. Growers and students use VPD to read the drying force near a plant. The value can aid a climate check, but plant needs still vary by crop, stage, light, air flow, and sensor site.

The VPD Formula And How Air And Leaf Values Differ

Saturation vapor pressure uses the Tetens/FAO equation. VPD is the difference between saturation vapor pressure and actual air vapor pressure.

  • eₛ(T) = 0.6108 × exp(17.27T ÷ (T + 237.3)): saturation vapor pressure in kPa.
  • eₐ = eₛ(Tair) × (RH ÷ 100): actual air vapor pressure in kPa.
  • VPDair = eₛ(Tair) − eₐ
  • VPDleaf = eₛ(Tleaf) − eₐ (when leaf temperature is supplied)

How To Use A Greenhouse VPD Calculator In Four Steps

Inputs

  • Air temperature: the measured greenhouse air temperature in Celsius.
  • Relative humidity: the measured moisture ratio as a percentage.
  • Leaf temperature: an optional surface temperature in Celsius.

Steps

  1. Enter the air temperature.
  2. Enter relative humidity from 0% to 100%.
  3. Add leaf temperature when a reliable leaf reading is available.
  4. Read saturation pressure, actual pressure, and VPD in kPa.

VPD Example With Air Temperature And Humidity Shown

Air temperature = 25°C, relative humidity = 60%, no separate leaf temperature.

  1. Find saturation vapor pressure: eₛ = 0.6108 × exp((17.27 × 25) ÷ (25 + 237.3)) = 3.1677777175 kPa.
  2. Find actual vapor pressure: eₐ = 3.1677777175 × (60 ÷ 100) = 1.9006666305 kPa.
  3. Subtract actual pressure from saturation pressure: VPD = 3.1677777175 − 1.9006666305 = 1.2671110870 kPa.

Saturation vapor pressure is 3.168 kPa, actual vapor pressure is 1.901 kPa, and VPD is 1.267 kPa. Each value is rounded to three decimal places.

When VPD Helps Guide Greenhouse Humidity Decisions

Use VPD to read air temp and RH on one pressure scale. A leaf-temp reading also helps. The leaf may be warmer or cooler than the air.

For other science tools, see the science calculators hub or look up relevant terms in the glossary.

Assumptions

  • Temperature values use degrees Celsius in the equation.
  • Relative humidity represents the air near the measured plants.
  • Air temperature stands in for leaf temperature when the leaf field is blank.
  • The result uses kilopascals.
  • Sensors provide representative readings for the crop area.

Limitations

  • VPD is a calculated estimate, not a direct measure of plant water status.
  • A single reading may miss changes across a canopy or greenhouse zone.
  • Crop targets vary with species, cultivar, stage, light, and airflow.
  • Local crop guidance and calibrated sensors should inform climate decisions.

In Practice

A common error is using RH alone as a fixed guide. RH shifts when air temp shifts, so the same RH can give a new VPD. Take air temp and RH at the same site and time. Call a value leaf VPD only when you have a leaf-temp reading.

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Related Guides

Frequently Asked Questions About Vapor Pressure Deficit

What Is The Difference Between Air And Leaf VPD?

Air VPD uses SVP at air temperature, while leaf VPD uses SVP at leaf temperature. Both use the same air VP. The leaf result changes when leaf temp differs from air temp at the chosen sensor point.

Does Higher Relative Humidity Always Lower VPD?

Higher relative humidity lowers VPD when the air temperature stays the same. More of the air's water limit is filled, so the gap gets small. A change in air temperature can still change VPD at the same RH.

Why Is VPD Reported In Kilopascals Instead Of Percent?

VPD is a vapor pressure gap, so kPa is the correct reporting unit. RH is a percent, while VPD is the gap between two pressure values. Keeping kPa with the result helps keep the two measures apart.

Can Air Temperature Replace Leaf Temperature For VPD?

Air temp can stand in for leaf temp when no leaf reading exists. The tool then sets both temps equal, so leaf VPD and air VPD match. A leaf-temp reading gives a more direct leaf check.

Is One VPD Target Correct For Every Greenhouse Crop?

No single VPD target fits every crop, growth stage, and grow room. Crop type, growth stage, light, air flow, and sensor site can shift a useful range. Check crop-specific extension advice before you change climate controls.

Sources

Last updated: . Reviewed for accuracy against the formula shown above.