VPD Chart for Cannabis Growing: Temperature & Humidity Guide
Nail your VPD and watch growth rates double — the science behind environment optimization
All Growth Stages2026 UpdatedCalculator IncludedPrintable Chart
VPD (Vapor Pressure Deficit) is the difference between how much moisture the air currently holds versus how much it could hold at saturation. It is the most accurate way to set temperature and humidity targets for your grow room — far more useful than "keep humidity at 50%" because it accounts for the relationship between temperature AND humidity together.
Why VPD Matters At low VPD (high humidity): plants close stomata, transpiration stops, growth slows. At high VPD (low humidity): plants transpire rapidly, risk wilting, and nutrient uptake becomes erratic. The sweet spot drives maximum transpiration without stress.
VPD Target Ranges by Growth Stage
| Stage | VPD Target | Temp (°F) | RH Range |
|---|
| Seedling / Clone | 0.4–0.8 kPa | 75–80°F | 65–80% |
| Early Veg | 0.8–1.0 kPa | 76–80°F | 60–70% |
| Late Veg | 1.0–1.2 kPa | 77–82°F | 55–65% |
| Early Flower (Week 1–4) | 1.0–1.3 kPa | 77–80°F | 50–60% |
| Peak / Late Flower | 1.2–1.6 kPa | 75–80°F | 40–50% |
| Final Flush / Harvest | 1.5–2.0 kPa | 70–75°F | 35–45% |
How to Calculate VPD
VPD (kPa) = SVP × (1 − RH/100), where SVP (Saturation Vapor Pressure) is calculated from leaf temperature (usually 2–3°F below air temp). The formula:
Quick Calculation
SVP = 0.6108 × e^(17.27 × T / (T + 237.3)) where T is in Celsius
VPD = SVP × (1 − RH/100)
Example: 78°F (25.6°C) at 55% RH → SVP = 3.29 kPa → VPD = 3.29 × 0.45 = 1.48 kPa (ideal for peak flower)
Temperature & Humidity Dial-In Table (Most Common)
| Temp | 40% RH | 50% RH | 60% RH | 70% RH |
|---|
| 72°F | 1.34 kPa | 1.12 kPa | 0.89 kPa | 0.67 kPa |
| 75°F | 1.48 kPa | 1.24 kPa | 0.99 kPa | 0.74 kPa |
| 78°F | 1.64 kPa | 1.37 kPa | 1.10 kPa | 0.82 kPa |
| 80°F | 1.76 kPa | 1.47 kPa | 1.17 kPa | 0.88 kPa |
| 82°F | 1.87 kPa | 1.56 kPa | 1.25 kPa | 0.94 kPa |
Equipment for VPD Control
- Temperature: AC Infinity CLOUDLINE fan with controller + portable A/C in summer. Mini-split A/C for large rooms.
- Humidity (raise): Inkbird humidifier + controller. Ultrasonic humidifiers work well in small tents.
- Humidity (lower): Quest or Anden dehumidifier for large rooms. AC Infinity climate controller handles both in tents.
- Monitoring: Inkbird IBS-TH2 or Govee sensors for logging temperature + humidity 24/7.
Frequently Asked Questions
What is the ideal VPD for cannabis in flower?
Cannabis in peak flower (weeks 4–7) performs best at VPD 1.2–1.6 kPa. This is typically achieved at 77–80°F with 45–55% relative humidity. In the final 2 weeks before harvest, many growers push VPD to 1.5–2.0 kPa by dropping humidity to 35–40% to stress the plant slightly and enhance resin production.
Is VPD the same as humidity?
No — VPD is a much more precise measurement than relative humidity alone. RH is a percentage of the air's current moisture capacity, but that capacity changes with temperature. VPD accounts for both temperature and humidity, giving you a single number that describes how much "thirst" the air has. Two rooms at 55% RH can have very different VPDs if their temperatures differ.
What happens if VPD is too low?
Low VPD (below 0.6 kPa) means the air is nearly saturated — plants close their stomata, transpiration stops, CO2 uptake plummets, and growth slows dramatically. It also creates the perfect conditions for botrytis (bud rot) and powdery mildew in flower. Always keep VPD above 0.8 kPa in veg and above 1.0 kPa in flower.
What VPD should I target for seedlings and clones?
Young seedlings and fresh clones have minimal root systems and cannot handle high transpiration rates. Keep VPD at 0.4–0.8 kPa by maintaining 75–80°F and 65–80% relative humidity. A propagation dome helps maintain these conditions. As roots establish over 5–7 days, gradually lower humidity and raise VPD toward vegetative targets.
