What is Vapor Pressure Deficit (VPD)?
VPD measures the difference between the water vapor pressure in the air and the maximum water vapor pressure at saturation. In controlled growing environments, VPD is the primary driver of plant transpiration — the mechanism plants use to move water and dissolved nutrients from the root zone to the leaves.
Getting VPD right is more precise than targeting temperature and humidity separately. Two grow rooms can have the same temperature but very different VPDs depending on humidity, leading to completely different transpiration rates.
VPD = SVP(T_leaf) − (RH/100) × SVP(T_air)SVP(T) = 0.6108 × exp(17.27 × T / (T + 237.3)) [kPa]Ideal VPD Ranges by Growth Stage
| Stage | VPD Range | Notes |
|---|---|---|
| Seedling / Clone | 0.4–0.8 kPa | Measured at leaf surface |
| Vegetative | 0.8–1.2 kPa | Measured at leaf surface |
| Flowering / Fruiting | 1–1.6 kPa | Measured at leaf surface |
Calculating VPD is only as good as your temperature and humidity readings. These sensors are the most accurate options for indoor cultivation:
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Frequently Asked Questions
What is VPD in indoor growing?
VPD (Vapor Pressure Deficit) is the difference between the amount of moisture in the air and how much moisture the air can hold at saturation. In growing environments, VPD drives transpiration — the rate at which plants move water from roots to leaves. A VPD that is too low causes poor nutrient uptake and mold risk; too high causes wilting and stress.
What is the ideal VPD for vegetative growth?
The ideal VPD for vegetative growth is 0.8–1.2 kPa, with 1.0 kPa as the target. At this range, plants transpire efficiently, driving nutrient uptake without causing excess stress.
What is the ideal VPD for flowering?
During flowering, the ideal VPD is 1.0–1.6 kPa. Higher VPD in late flowering accelerates resin development and reduces mold risk, but should not exceed 1.6 kPa to avoid heat stress.
Why does leaf temperature matter for VPD?
VPD is calculated using leaf temperature, not just air temperature. Leaves are typically 1–3°C cooler than the surrounding air due to transpiration. Using only air temperature overestimates VPD. Our calculator uses a configurable leaf offset (-2°C by default).
How do I lower VPD in my grow room?
To lower VPD, increase humidity (humidifier) or lower temperature. A 5°C drop in temperature reduces VPD by approximately 0.3–0.5 kPa. Alternatively, raising humidity from 50% to 70% at 26°C reduces VPD from ~1.2 to ~0.7 kPa.