VPD Calculator: How to Calculate Vapor Pressure Deficit

To calculate vapor pressure deficit (VPD), subtract the actual vapor pressure of the air from the saturation vapor pressure at the same temperature. Enter air temperature, relative humidity, and—when calculating leaf VPD—leaf temperature; the calculator returns VPD in kilopascals (kPa), a measure of how far the air is from saturation.

Formula

VPD is the difference between saturation vapor pressure and actual vapor pressure: VPD = SVP − AVP. For air VPD, saturation and actual vapor pressure are calculated at air temperature. For leaf VPD, saturation vapor pressure is calculated at leaf temperature, while actual vapor pressure is estimated from air temperature and relative humidity.

• SVP is saturation vapor pressure, in kPa.
• AVP is actual vapor pressure, in kPa.
• t is air temperature, in degrees Celsius (°C).
• tl is leaf temperature, in °C.
• rh is relative humidity, entered as a percentage from 0 to 100.

The calculator uses the commonly applied saturation-pressure approximation SVP = 0.6108 × exp(17.27 × T / (T + 237.3)). This gives SVP in kPa when T is in °C. Since 0.6108 kPa equals 610.8 Pa, the first output expression uses the equivalent pascal form and divides by 1,000 to return kPa. Air AVP is SVP at air temperature multiplied by rh/100. The leaf calculation is SVP at leaf temperature minus that same air AVP.

Worked Example

With the calculator’s default values, air temperature is 25°C, relative humidity is 60%, and leaf temperature is 25°C.

1. Calculate saturation vapor pressure at 25°C: 0.6108 × exp(17.27 × 25 / (25 + 237.3)) ≈ 3.17 kPa.
2. Calculate actual vapor pressure: 3.17 × 0.60 ≈ 1.90 kPa.
3. Subtract actual pressure from saturation pressure: 3.17 − 1.90 ≈ 1.27 kPa air VPD.
4. Because leaf and air temperatures are both 25°C in this example, leaf SVP is also about 3.17 kPa, so leaf VPD is likewise 1.27 kPa.

The displayed result is rounded to two decimal places. Calculations use the unrounded intermediate values, so rounding each step by hand can cause a difference of a hundredth.

What VPD Measures

VPD means vapor pressure deficit. It describes the difference between the moisture the air could hold at saturation and the water vapor it actually contains, expressed as a pressure. A VPD of zero means the air is saturated under the calculation’s assumptions. A larger positive value indicates a larger gap between saturation and actual vapor pressure.

VPD is not the same as relative humidity. Relative humidity is a percentage comparing current water vapor with the saturation amount at a particular temperature. VPD is a pressure difference, and its value depends on temperature as well as humidity. Consequently, two environments with the same relative humidity can have different VPDs if their temperatures differ.

How to Use and Read the Calculator

Enter air temperature in °C and relative humidity as a percentage, not as a decimal. For example, type 60 for 60% humidity. Enter leaf temperature in °C if you want the leaf-based result; the default makes it equal to air temperature. The two VPD outputs answer related but different questions:

• Air VPD uses air temperature for the saturation calculation.
• Leaf VPD uses leaf temperature for saturation, while air humidity determines actual vapor pressure.

Leaf temperature can differ from air temperature because of radiation, airflow, and other environmental conditions. If you do not have a leaf-temperature measurement, the air VPD result is the straightforward estimate; setting leaf temperature equal to air temperature makes both results match. The outputs are kPa. To convert kPa to pascals, multiply by 1,000; to convert to hectopascals (hPa) or millibars (mbar), multiply by 10.

Why VPD Changes and When It Is Useful

Warming air raises its saturation vapor pressure. At the same relative humidity, warmer air therefore generally has a larger VPD. Increasing relative humidity raises actual vapor pressure and reduces the deficit. This is why temperature and humidity should be considered together rather than interpreting either reading alone.

VPD is used in plant science, greenhouse and indoor-environment monitoring, and meteorology to describe atmospheric drying demand. It can help explain why identical humidity readings do not necessarily represent identical conditions. The leaf-based form is useful when leaf temperature is known, because water exchange at a leaf depends on the vapor-pressure difference between the leaf surface and surrounding air. VPD by itself does not determine plant response: species, light, airflow, water availability, and other conditions also matter.

Common Errors and Limits

• Mixing units: The formula expects temperatures in °C, not Fahrenheit, and humidity as a percentage. Convert Fahrenheit first: °C = (°F − 32) × 5/9.
• Entering 0.60 for 60%: This calculator expects 60. Entering 0.60 treats humidity as 0.60% and produces a very different result.
• Confusing air VPD with leaf VPD: They match only when leaf and air temperatures are equal. A leaf-temperature error directly affects the leaf estimate.
• Overinterpreting precision: Sensors have measurement uncertainty, and the formula is an approximation. Two decimal places are useful for reading but do not imply that the real environment is known that precisely.
• Using unusual input conditions: The approximation is intended for common environmental temperatures. For specialized work, use a method appropriate to the temperature range and account for pressure and measurement details where relevant.

For ordinary inputs from 0% to 100% relative humidity, air VPD should be zero or positive. A negative result can signal inconsistent readings, an input error, or a situation outside the simple calculation’s assumptions. The formula estimates vapor pressure from temperature and relative humidity; it does not measure airflow, leaf wetness, or the actual rate of evaporation.

Frequently Asked Questions

What is the formula for VPD?
VPD = saturation vapor pressure − actual vapor pressure. For air, calculate both at air temperature; for leaf VPD, calculate saturation pressure at leaf temperature and subtract air actual vapor pressure.

Can I calculate VPD from temperature and relative humidity?
Yes. Air temperature and relative humidity are enough to estimate air VPD. Leaf VPD also needs leaf temperature, or an assumption that leaf and air temperatures are equal.

What units are used for VPD?
VPD is commonly reported in kilopascals (kPa). One kPa equals 1,000 Pa, 10 hPa, or 10 mbar.

Why is VPD different when relative humidity stays the same?
Because saturation vapor pressure changes with temperature. At a fixed relative humidity, a temperature change changes the pressure difference that VPD represents.

Is VPD the same as humidity?
No. Relative humidity is a percentage of saturation, while VPD is the pressure difference between saturation and actual water vapor. Both depend on temperature, but they describe different quantities.

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in t | Air temperature | C | 25 | -
in rh | Relative humidity | % | 60 | 0
in tl | Leaf temperature | C | 25 | -
out vpd | Air VPD | kPa | (610.8 * exp(17.27 * t / (t + 237.3)) * (1 - rh / 100)) / 1000 | 2
out leafvpd | Leaf VPD | kPa | (610.8 * exp(17.27 * tl / (tl + 237.3)) - (rh / 100) * 610.8 * exp(17.27 * t / (t + 237.3))) / 1000 | 2
out svp | Air saturation vapor pressure | kPa | 0.6108 * exp(17.27 * t / (t + 237.3)) | 2