Enter your grow tent temperature and humidity to calculate Vapour Pressure Deficit — and find out if your plants are in the ideal range.
| Stage | VPD Range | Ideal | Temp (°C) | RH % |
|---|---|---|---|---|
| 🌱 Clone / Seedling | 0.4 – 0.8 kPa | 0.6 kPa | 20–25°C | 70–80% |
| 🌿 Vegetative | 0.8 – 1.2 kPa | 1.0 kPa | 22–28°C | 55–70% |
| 🌸 Early Flower | 1.0 – 1.5 kPa | 1.25 kPa | 22–26°C | 45–55% |
| 🍁 Late Flower | 1.2 – 1.6 kPa | 1.4 kPa | 20–24°C | 35–45% |
VPD (Vapour Pressure Deficit) measures the difference between the amount of moisture in the air and how much moisture the air can hold at saturation. Plants use this pressure difference to pull water and nutrients up through their roots via transpiration. Too low VPD (high humidity) and plants can't transpire properly — growth slows and mould risk increases. Too high VPD (low humidity) and plants close their stomata to conserve water — photosynthesis stops and heat stress sets in. The sweet spot varies by growth stage.
For vegetative growth, target a VPD of 0.8–1.2 kPa with 1.0 kPa being ideal. At 25°C, this corresponds to roughly 55–65% relative humidity. During veg, plants are building structure and can handle slightly higher humidity than in flower. Keeping VPD in this range maximises leaf expansion, stem thickness, and root development.
Canadian winters produce extremely dry indoor air — forced-air heating drops indoor humidity to 20–30% RH, pushing VPD dangerously high for clones and seedlings. Canadian summers bring the opposite problem — high outdoor humidity seeps into grow rooms and drives VPD too low. This seasonal swing means Canadian growers typically need both a humidifier and a dehumidifier, or a smart controller that manages both. This is why AC Infinity's integrated climate control systems are so popular with Canadian growers.
True VPD is calculated using leaf surface temperature, not air temperature — and leaf temperature is typically 1–3°C cooler than the surrounding air due to transpiration cooling. Using air temperature alone overestimates your VPD slightly. With good airflow (inline fan moving air across the canopy), leaf temps run about 1–2°C below air temp. With poor airflow or a hot light, the gap can be 3°C or more. This calculator uses a 2°C offset by default, which suits most well-ventilated grow tents.
AC Infinity's CONTROLLER 69 PRO is the best single purchase for VPD control — it reads temperature and humidity in real time and auto-adjusts your inline fan, humidifier, and dehumidifier to maintain your target. Pair it with a CLOUDLINE T4 (2×2 to 3×3 tents) or T6 (4×4 and larger) inline fan, and add a CLOUDFORGE humidifier for dry Canadian winters. AC Infinity ships quickly to Canada and offers solid warranty support — a big advantage over some overseas brands.
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