VPD Calculator
Calculate Vapor Pressure Deficit (VPD) for optimal plant growth. Get precise VPD readings for your grow room at different growth stages.
Calculate VPD
Leaf temperature is typically 2-4°F cooler than air temperature
VPD Results
Enter values to calculate VPD
VPD Chart Reference
| Temperature | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH |
|---|---|---|---|---|---|
| 68°F (20°C) | 1.40 | 1.17 | 0.94 | 0.70 | 0.47 |
| 72°F (22°C) | 1.59 | 1.32 | 1.06 | 0.79 | 0.53 |
| 75°F (24°C) | 1.79 | 1.49 | 1.19 | 0.89 | 0.60 |
| 78°F (26°C) | 2.01 | 1.68 | 1.34 | 1.01 | 0.67 |
| 82°F (28°C) | 2.27 | 1.89 | 1.51 | 1.13 | 0.76 |
| 86°F (30°C) | 2.54 | 2.12 | 1.69 | 1.27 | 0.85 |
Understanding VPD (Vapor Pressure Deficit)
What is VPD?
Vapor Pressure Deficit (VPD) is the difference between the amount of moisture in the air and how much moisture the air can hold when saturated. It's a critical metric for plant health because it directly affects transpiration rates, nutrient uptake, and overall plant growth.
Why VPD Matters for Plants
VPD determines how much water vapor plants release through their stomata (transpiration). When VPD is optimal:
- Nutrient uptake improves - Plants transport nutrients from roots to leaves more efficiently
- Photosynthesis increases - Optimal stomata opening allows better COâ‚‚ absorption
- Growth accelerates - Plants can focus energy on growth instead of stress response
- Disease resistance - Proper airflow and humidity prevent mold, mildew, and pathogens
VPD by Growth Stage
Seedling/Clone Stage: 0.4-0.8 kPa
Young plants need higher humidity (65-75% RH) and lower VPD to prevent excessive water loss. Keep temperatures around 72-75°F (22-24°C). Lower VPD allows fragile root systems to establish without stress.
Vegetative Stage: 0.8-1.2 kPa
Established plants can handle higher VPD as roots are developed. Aim for 55-65% RH with temperatures of 75-80°F (24-27°C). This range promotes vigorous vegetative growth and strong stem development.
Flowering/Fruiting Stage: 1.0-1.5 kPa
Lower humidity (45-55% RH) reduces mold risk on dense flowers/fruits. Maintain 75-82°F (24-28°C). Higher VPD encourages resin production and prevents bud rot while maximizing nutrient delivery to flowering sites.
How to Calculate VPD
The VPD formula uses leaf temperature (typically 2-4°F cooler than air) and relative humidity:
Where T = leaf temperature in Celsius, RH = relative humidity %, e = Euler's number (2.71828)
Adjusting Your Grow Environment
If VPD is Too Low (< Target Range)
- • Increase temperature - Raise by 2-5°F to increase SVP
- • Decrease humidity - Use a dehumidifier or improve ventilation
- • Increase airflow - Add circulation fans to reduce leaf boundary layer
- • Reduce watering frequency - Let medium dry slightly between waterings
Low VPD can cause slow growth, mold/mildew, and nutrient deficiencies due to reduced transpiration.
If VPD is Too High (> Target Range)
- • Decrease temperature - Lower by 2-5°F to decrease SVP
- • Increase humidity - Use a humidifier or misting system
- • Add water reservoirs - Open water trays increase ambient humidity
- • Reduce light intensity - Dimming lights can lower leaf temperature
High VPD stresses plants with excessive transpiration, causing wilting, leaf curl, and stunted growth.
Leaf Temperature vs Air Temperature
Leaf temperature is typically 2-4°F (1-2°C) cooler than ambient air temperature due to transpirational cooling. This offset is crucial for accurate VPD calculations. Factors affecting leaf temperature:
- Light intensity - Stronger lights increase leaf temperature
- Airflow - More circulation increases cooling
- Humidity - Lower humidity increases transpiration and cooling
- Plant health - Stressed plants may have different leaf temps
For precise VPD management, use an infrared thermometer to measure actual leaf surface temperature. Our calculator uses a standard 2°F offset by default, which you can adjust based on your setup.
VPD Monitoring Tools
To maintain optimal VPD in your grow room:
- Thermometer/Hygrometer combo - Digital units with min/max memory track fluctuations
- Infrared thermometer - Measure leaf surface temperature for precise VPD calculations
- Environmental controllers - Automate temperature, humidity, and ventilation based on VPD targets
- Data loggers - Record conditions over time to identify patterns and optimize schedules
Common VPD Mistakes to Avoid
- Ignoring growth stage - Each stage needs different VPD ranges; don't use one-size-fits-all
- Using air temp instead of leaf temp - Always account for 2-4°F leaf cooling offset
- Sudden changes - Adjust VPD gradually over 2-3 days to avoid shocking plants
- Not monitoring at canopy level - Measure temp/humidity where the plants are, not at wall height
- Forgetting nighttime VPD - Lights-off period still affects plant health; maintain appropriate ranges
- Overreacting to short-term fluctuations - Focus on daily averages, not momentary spikes
Frequently Asked Questions
What is the ideal VPD for cannabis plants?
For cannabis: Seedlings/clones 0.4-0.8 kPa (70-75% RH, 72-75°F), Vegetative 0.8-1.2 kPa (55-65% RH, 75-80°F), Flowering 1.0-1.5 kPa (45-55% RH, 75-82°F). Lower humidity during flowering prevents mold on dense buds.
How do I measure leaf temperature for VPD calculations?
Use an infrared thermometer pointed at the leaf surface from about 6 inches away. Measure multiple leaves at canopy level and average the readings. If you don't have an IR thermometer, subtract 2-4°F from air temperature as a general approximation.
Why is my VPD different at different heights in my grow space?
Temperature and humidity stratify in enclosed spaces. Heat rises, creating warmer, drier conditions at the top and cooler, more humid conditions at the bottom. Always measure at canopy level where your plants are actively growing. Use circulation fans to minimize stratification.
Can VPD be too perfect? Should I vary it?
While consistency is good, small natural fluctuations are beneficial. Plants evolved with day/night cycles and weather changes. Aim to stay within target ranges but don't stress over minor variations. Some growers intentionally vary VPD slightly (±0.1-0.2 kPa) to strengthen plants.
What should nighttime VPD be?
Nighttime VPD should be slightly lower than daytime (about 0.2-0.4 kPa lower) since stomata partially close and transpiration slows. For flowering plants, keep nighttime VPD around 0.8-1.0 kPa to prevent mold while maintaining some transpiration. Avoid going below 0.4 kPa at night.
How does COâ‚‚ supplementation affect VPD?
CO₂ enrichment (1200-1500 ppm) allows plants to tolerate higher VPD levels because enhanced photosynthesis compensates for increased transpiration. You can safely increase VPD by 0.2-0.4 kPa when supplementing CO₂, often raising temperatures to 82-86°F while maintaining 50-60% RH.