VPD Calculator (Vapor Pressure Deficit)
Instantly calculate the Vapor Pressure Deficit (VPD) of your grow tent or greenhouse. Keep your cannabis or indoor crops transpiring perfectly.
Calculator Tool
What is Vapor Pressure Deficit?
Vapor Pressure Deficit, or VPD, is the ultimate metric for measuring the environmental pressure placed on indoor crops like cannabis, tomatoes, and leafy greens. It measures the difference (the deficit) between the amount of moisture currently in the air and how much moisture the air can hold when fully saturated.
By dialing in your VPD, you control the stomata (the pores on plant leaves). A perfect VPD ensures the plant is breathing aggressively—pulling water and nutrients up from the roots—without drying out or suffocating.
VPD (kPa) = Saturation Vapor Pressure - Actual Vapor Pressure
Because the math involves complex exponential temperature conversions, we built this instantaneous calculator to give you real-time data on your grow room environment.
VPD Growth Stages
Different stages of plant life require different ideal VPD targets:
Clones / Early Veg (0.4 - 0.8 kPa)
Low pressure. High humidity prevents young cuttings without established root zones from drying out.
Late Veg / Early Flower (0.8 - 1.2 kPa)
Medium pressure. Stomata are wide open, driving aggressive vegetative growth and nutrient uptake.
Mid / Late Flower (1.2 - 1.5 kPa)
High pressure. Drier air forces maximum transpiration while simultaneously protecting dense flowers from botrytis (bud rot).
Frequently Asked Questions
What is a good VPD for propagation and clones?
During propagation, early veg, or when cloning, you want a very low VPD—typically between 0.4 and 0.8 kPa. This prevents the delicate, rootless plants from drying out while they establish themselves.
What is the optimal VPD for flowering cannabis?
In the mid to late flowering stages, optimal VPD sits between 1.2 and 1.5 kPa. This higher deficit encourages the plant to transpire more water, drawing nutrients up from the roots, while also keeping the room dry enough to prevent bud rot or powdery mildew.
Why is VPD more accurate than Relative Humidity (RH)?
Relative humidity only tells you how much moisture the air holds relative to its capacity at a given temperature. VPD (Vapor Pressure Deficit) factors in temperature directly to tell you the exact 'drying power' of the air. Warmer air holds exponentially more water than colder air, meaning a room at 80°F/60% RH has a completely different transpiration pressure than a room at 70°F/60% RH.