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Biology

VPD Calculator (Vapor Pressure Deficit)

Calculate air and leaf vapour pressure deficit from temperature and humidity.

Your inputs

Change any value — everything updates instantly.

Room or greenhouse temperature.

From a hygrometer.

Leaves are often 1–3 °C cooler than air.

Try an example

Leaf VPD

0.91 kPa

This means the air pulls water from leaves with a "drying power" of 0.91 kPa.

humid 0dry 2 kPa
LeafWater vapourYour VPD
0.91 kPa

What you are seeing

The leaf releases water droplets into the air.

Faster, more droplets mean a higher VPD — yours is 0.91 kPa.

The gauge shows where you sit from too humid to too dry.

Air VPD

1.27 kPa

Using air temperature only.

Air saturation pressure

3.168 kPa

Most water the air could hold.

Actual vapour pressure

1.901 kPa

Water the air holds now.

Leaf saturation pressure

2.809 kPa

Water pressure inside the leaf.

Result explanation: Is the air too wet or too dry?

Leaf VPD 0.91 kPa.

Real-life comparison: Many growers aim for roughly 0.8–1.2 kPa in vegetative growth and 1.0–1.5 kPa in flowering.

Low, normal or high? Comfortable range for many crops.

Graph and details

10335578100-0.361.072.49Relative humidity (%)Leaf VPD (kPa)

Leaf VPD against humidity

How to read this chart: Higher humidity lowers VPD. The red dot is your room.

What this calculator does

Uses temperature and humidity to find how strongly air draws water from leaves.

What your result means

0.91 kPa leaf VPD.

Raise humidity or lower temperature to reduce VPD.

Values below zero mean water may condense on leaves.

The formulas

Compute saturation pressure at leaf and air temperatures, subtract the air's actual vapour.

1. Vapour pressure deficit

SVP(T) = 0.6108 × e^(17.27T ÷ (T + 237.3)); VPD = SVP(leaf) − SVP(air) × RH ÷ 100

Symbols

SVP
Saturation vapour pressure (kPa)
RH
Relative humidity (%)
T
Temperature (°C)

Why this formula works

Water moves from the saturated leaf interior to less-saturated air.

Unit check

kPa − kPa = kPa ✓

Worked example

  1. Air 25 °C, 60% RH, leaf 23 °C
  2. SVP air 3.168, leaf 2.809 kPa
  3. VPD = 2.809 − 1.901 = 0.91 kPa

Common mistakes

  • Using air temperature for the leaf.
  • Entering RH as a decimal.

Working for your numbers

  1. SVP(air 25 °C) = 3.1678 kPa
  2. Actual = 3.1678 × 60% = 1.9007 kPa
  3. SVP(leaf 23 °C) = 2.8094 kPa
  4. VPD = 2.8094 − 1.9007 = 0.9088 kPa

Leaf VPD is 0.91 kPa.

Frequently asked questions

Why use leaf temperature?

Leaves are usually cooler than air, which lowers true VPD.

How do I measure leaf temperature?

An infrared thermometer pointed at the leaf surface.

What if VPD is negative?

The air is wetter than the leaf — condensation and disease risk rise.

Is there one perfect value?

No. Targets differ by crop and stage.

Symbols and assumptions

VPD
Vapour pressure deficit (kPa)
  • FAO-56 Tetens saturation formula.
  • Sea-level pressure.

Sources and review

Reviewed 2026-09-25 · Next review 2027-09-25

FAO-56 Crop evapotranspiration, Chapter 3Food and Agriculture Organization (FAO)

Important limitation. Educational estimate. Crop targets vary.

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