How to Calculate VPD from Temperature and Relative Humidity

Your greenhouse thermometer reads 28°C and your humidity meter shows 55%. The plants look fine. But you have no idea whether they are actually in stress or thriving — because the two numbers alone do not tell you that. The number you need is Vapor Pressure Deficit (VPD), and once you start tracking it, everything about plant environment control clicks into place.

Use the calculator below to get your VPD, dew point, and full atmospheric picture in one shot:

What Is Vapor Pressure Deficit?

Vapor Pressure Deficit (VPD) is the difference between how much water vapor the air could hold at a given temperature and how much it is actually holding right now. It is measured in kilopascals (kPa) or occasionally millibars (mbar).

Think of it this way: the air is like a sponge. VPD tells you how much "suction" that sponge still has — how much more moisture it can pull from a wet surface (like a plant leaf). High VPD means the air is very dry relative to its capacity and is aggressively pulling moisture from leaves. Low VPD means the air is nearly saturated and barely pulling at all.

VPD (kPa) = Saturation Vapor Pressure − Actual Vapor Pressure
VPD = e_s(T) − e_s(T) × (RH / 100)
VPD = e_s(T) × (1 − RH / 100)
Where:
e_s(T) = 0.6112 × exp(17.625 × T / (243.04 + T))
T = air temperature in °C
RH = relative humidity in %

The e_s(T) formula is the Magnus-Tetens equation — the industry standard used by NOAA and every professional weather service. Our calculator uses the Alduchov & Eskridge (1996) coefficients accurate to ±0.35°C.

Step-by-Step VPD Calculation

Example: 28°C air temperature, 55% relative humidity.

Step 1 — Calculate saturation vapor pressure:

e_s(28) = 0.6112 × exp(17.625 × 28 / (243.04 + 28))
        = 0.6112 × exp(493.5 / 271.04)
        = 0.6112 × exp(1.820)
        = 0.6112 × 6.172
        ≈ 3.771 kPa

Step 2 — Calculate actual vapor pressure:

e = e_s(T) × (RH / 100)
  = 3.771 × (55 / 100)
  = 3.771 × 0.55
  ≈ 2.074 kPa

Step 3 — Subtract to get VPD:

VPD = 3.771 − 2.074 = 1.697 kPa

At 28°C / 55% RH, VPD ≈ 1.70 kPa — right in the sweet spot for most flowering cannabis plants and high-transpiration crops.

The VPD Reference Table

This table shows pre-calculated VPD values (kPa) across common greenhouse temperature and humidity combinations. Bold values represent the optimal range for most crops.

Bold values = optimal zone for most cultivars (1.2–1.6 kPa vegetative, 1.4–2.0 kPa flowering).

Why VPD Matters More Than Relative Humidity Alone

Growers who only track relative humidity are flying partially blind. Here is the core problem:

A grow room at 25°C / 65% RH has a VPD of 1.08 kPa — fine for vegetative growth. The same 65% RH at 30°C has a VPD of 1.47 kPa — starting to stress plants. The same 65% RH at 35°C has a VPD of 1.96 kPa — excessive transpiration, wilting risk.

Identical RH. Completely different plant stress levels. Relative humidity tells you how full the bucket is; VPD tells you how hard the air is pulling water out of your plants.

VPD Zones: What Each Range Means for Plants

How to Lower or Raise VPD

VPD is too high (air too dry, plants stressed):

• Increase relative humidity with a humidifier
• Slightly lower temperature
• Increase airflow around plants (paradoxically, this can reduce leaf-to-air VPD by equalising temperatures)

VPD is too low (air too moist, pathogen risk):

• Decrease relative humidity with a dehumidifier
• Raise temperature slightly
• Increase exhaust ventilation

Use the Relative Humidity Calculator above to model different temperature and humidity combinations and find the VPD that works for your target crop stage.

Frequently Asked Questions

What is the ideal VPD for cannabis? Most cannabis cultivators target 0.8–1.2 kPa during vegetative growth and 1.2–1.8 kPa during early-to-mid flowering. Late flowering often benefits from slightly higher VPD (1.5–2.0 kPa) to reduce bud moisture and mold risk. These are general benchmarks — cultivar-specific variation exists.

Is VPD the same as vapor pressure? No. Vapor pressure is the actual partial pressure of water vapor in the air. VPD is the difference between the maximum possible vapor pressure at the current temperature (saturation vapor pressure) and the actual vapor pressure. VPD is always a positive number and reaches zero only at 100% relative humidity.

Why do some VPD charts look different from others? Some charts use leaf temperature (typically 2°C lower than air temperature) while others use air temperature. Charts for cannabis cultivation often use leaf temperature. Charts for meteorology and HVAC use air temperature. Always check which temperature basis a chart uses before applying it.

What units is VPD measured in? Most agricultural and horticultural literature uses kilopascals (kPa). Meteorology sometimes uses millibars (mbar) or hectopascals (hPa), where 1 kPa = 10 hPa = 10 mbar. The calculator on this page outputs VPD in kPa via the Saturation and Actual Vapor Pressure stats — subtract to get VPD, or see our output directly.

Does altitude affect VPD? Yes. At high altitudes, total atmospheric pressure is lower, which affects how quickly plants can transpire even at the same VPD value. The absolute transpiration rate depends on both VPD and total pressure. Our altitude and humidity guide covers this in detail.

Can I calculate VPD without knowing relative humidity? Yes — if you know both dry-bulb and wet-bulb temperature (from a sling psychrometer), you can calculate actual vapor pressure using the psychrometric equation, then subtract from saturation vapor pressure to get VPD. Our calculator supports dry-bulb + wet-bulb input directly. See the wet bulb vs dry bulb guide for more on this method.