VPD Calculator

What is Vapor Pressure Deficit (VPD)?

Vapor Pressure Deficit (VPD) is the "hidden parameter" master growers use to maximize yield. While temperature and humidity tell you about the room, VPD tells you about the plant's experience.

Think of VPD as the drying power of the air. It measures the difference between the moisture inside the leaf (100% saturated) and the air around it. This pressure difference drives transpiration—the process of water moving from roots to leaves, carrying vital nutrients.

Propagation

0.4 - 0.8 kPa

Clones and seedlings have weak roots. High humidity (low VPD) reduces stress and prevents them from drying out.

Vegetative

0.8 - 1.2 kPa

Roots are established. Higher VPD encourages faster transpiration, pulling more nitrogen and water for rapid leafy growth.

Flowering

1.2 - 1.6 kPa

Plants need maximum transport. Higher transpiration helps cool flowers. Lower humidity prevents mold/rot in dense buds.

How to use this Calculator

1
Enter Air Temp & Humidity: Use readings from a sensor placed at canopy height.
2
Set Leaf Temperature Offset: Leaves cool themselves via evaporation. Under LED/HPS lights, they are typically 1-2°C cooler than the air. If unmeasured, use -2°C as a safe default.
3
Analyze the Result: The calculator shows your VPD in kPa. Use the color-coded status to see if you are in the "Danger Zone" or the "Sweet Spot".

Why not just use RH%?

Relative Humidity changes with temperature. 60% RH at 80°F is much "drier" to a plant than 60% RH at 60°F. VPD accounts for both, giving you a true measure of evaporation potential.

The Dew Point Factor

We also calculate Dew Point. If temperatures drop to this level (e.g., at night), water will condense on your leaves, guaranteeing mold. Always keep night temps above the Dew Point.

Frequently Asked Questions

What is VPD (Vapor Pressure Deficit)?

VPD is the difference (deficit) between the amount of moisture currently in the air and the maximum amount the air can hold at that temperature. It drives transpiration (how plants drink). Too low = plants choke (mold). Too high = plants sweat too much (wilt).

What is the best VPD for flowering?

For the flowering stage, the ideal VPD range is typically between 1.2 kPa and 1.6 kPa. This encourages strong water uptake for bud development while keeping humidity low enough to prevent bud rot.

What is the best VPD for vegetative growth?

In the vegetative stage, plants prefer a slightly more humid environment. The ideal VPD range is 0.8 kPa to 1.2 kPa (or up to 1.1 kPa). This maximizes stomatal opening for fast photosynthesis.

What is Leaf Temperature Offset?

Leaves cool themselves by evaporating water (transpiration), so they are usually 1-3°C cooler than the surrounding air. Using an infrared thermometer (IR gun) is the best way to find your exact offset. A common default is -2°C.

Why is my VPD too high?

High VPD means the air is too dry or too hot. Your plants are losing water faster than they can drink. To lower VPD: increase humidity (add a humidifier) or lower the temperature (increase AC/ventilation).

Why is my VPD too low?

Low VPD means the air is saturated (too humid). Transpiration stops, and nutrient uptake stalls. Risk of mold increases. To raise VPD: decrease humidity (dehumidifier) or raise the temperature.

How does temperature affect VPD?

Hot air can hold more water than cold air. If you raise the temperature without adding water, the air's 'thirst' increases, raising VPD. If you lower temperature, the air's capacity shrinks, lowering VPD.

Is VPD better than just checking RH%?

Yes. Relative Humidity (RH) is relative. 60% humidity at 60°F is very different for a plant than 60% at 85°F. VPD combines temp and humidity into a single 'stress' number that tells you exactly how the plant 'feels'.

What is the formula for VPD?

VPD = SVP_leaf - VP_actual. We first calculate Saturation Vapor Pressure (SVP) for the leaf temperature, then subtract the Actual Vapor Pressure of the air. It's measured in Kilopascals (kPa).

Does this calculator work for cannabis?

Absolutely. This tool is designed with ranges (0.8-1.5 kPa) that are standard for cannabis cultivation and indoor gardening.