VPD Calculator (Vapor Pressure Deficit)

Calculate vapor pressure deficit in kPa from air temperature and relative humidity, with an optional leaf-temperature offset for grow-room and greenhouse work.

Quick Facts

Method
Tetens equation: SVP = 0.6108 · e^(17.27T/(T+237.3)); VPD = SVP × (1 − RH/100)
Leaf offset lowers the leaf-surface saturation pressure; set it to 0 for air VPD.

Your Results

Calculated
Vapor pressure deficit
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Leaf-to-air drying demand
Saturation vapor pressure
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SVP at leaf/air temperature
Actual vapor pressure
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Water vapor in the air
Growth stage fit
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Typical target zone

Ready

Enter temperature and humidity, then calculate.

About the VPD Calculator (Vapor Pressure Deficit)

Vapor pressure deficit (VPD) is the difference between how much water vapor the air could hold when saturated and how much it actually holds at a given temperature and humidity. It is the true measure of the "drying power" of the air around a plant, and it drives transpiration — the pull of water from the roots, up through the plant, and out of the leaf stomata. Because VPD combines temperature and humidity into a single physically meaningful number (kilopascals, kPa), growers and greenhouse managers use it instead of relative humidity alone, which is misleading on its own because the same 50% RH means a very different drying demand at 18 °C than at 30 °C.

The formula this calculator uses

The calculation has three steps. First, saturation vapor pressure (SVP) is found with the Tetens equation:

SVP = 0.6108 × exp( 17.27 × T / (T + 237.3) )

where T is temperature in °C and SVP comes out in kPa. Second, the actual vapor pressure (AVP) is the saturation value scaled by relative humidity: AVP = SVPair × (RH / 100). Third, the deficit is the gap between the leaf's saturation pressure and the air's actual vapor content:

VPD = SVPleaf − AVPair

When you set the leaf-temperature offset to 0, leaf and air temperature are equal and the expression simplifies to VPD = SVP × (1 − RH/100). Leaves that are transpiring are usually a couple of degrees cooler than the surrounding air, so a common horticultural adjustment is a leaf offset of about −2 °C, which lowers the leaf's saturation pressure and therefore the calculated VPD.

Why VPD matters more than humidity

At high VPD the air pulls water out of leaves faster than the roots can supply it; stomata close to protect the plant, photosynthesis slows, and tip burn or wilting can follow. At very low VPD the air is already near saturation, so transpiration nearly stops — nutrients (especially calcium) stop moving with the water flow, condensation forms, and mold and mildew take hold. Steering VPD keeps the plant in the productive middle where water and nutrients move steadily.

Reference targets by growth stage

  • Clones and seedlings: ~0.4–0.8 kPa — low drying demand protects young, poorly rooted plants.
  • Vegetative growth: ~0.8–1.1 kPa — steady transpiration and vigorous growth.
  • Flowering / fruiting: ~1.2–1.5 kPa — higher demand tightens plants and discourages mold.
  • Danger zones: below ~0.4 kPa risks fungal disease and poor nutrient uptake; above ~1.5 kPa risks stomatal closure and water stress.

Worked example

At 25 °C air temperature and 50% relative humidity with no leaf offset: SVP = 0.6108 × exp(17.27 × 25 / 262.3) = 3.169 kPa. AVP = 3.169 × 0.50 = 1.585 kPa. VPD = 3.169 − 1.585 = 1.58 kPa — a value squarely in the late-vegetative / flowering range.

Frequently Asked Questions

What is a good VPD for plants?
Most crops do well between about 0.8 and 1.2 kPa at the leaf surface. Aim lower (0.4–0.8 kPa) for clones and seedlings, around 0.8–1.1 kPa for vegetative growth, and up to about 1.2–1.5 kPa during flowering or fruiting. Staying below ~0.4 kPa invites mold and stalls nutrient uptake; going above ~1.5 kPa can force stomata shut and stress the plant.
Should I use air temperature or leaf temperature?
For the most accurate "leaf VPD," use leaf temperature, which you can read with an infrared thermometer. Transpiring leaves typically run about 2 °C cooler than the air, so entering a leaf-temperature offset of −2 gives a realistic estimate without a separate sensor. Set the offset to 0 to compute plain air VPD from a standard thermometer and hygrometer.
Why not just use relative humidity?
Relative humidity ignores temperature. Air at 50% RH and 18 °C (VPD ~1.0 kPa) has far less drying power than air at 50% RH and 30 °C (VPD ~2.1 kPa), even though the humidity reading is identical. VPD folds both variables into one number that reflects what the plant actually experiences.

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