💧 VPD Calculator
Calculate optimal vapor pressure deficit for cannabis growing based on temperature and humidity
Calculate VPD
Your VPD Results
Optimal VPD Ranges by Stage
- Clone/Seedling: 0.4-0.8 kPa (high humidity, gentle transpiration)
- Vegetative: 0.8-1.2 kPa (moderate transpiration, strong growth)
- Flowering: 1.0-1.5 kPa (balanced transpiration, bud development)
- Late Flowering: 1.2-1.6 kPa (lower humidity, reduce mold risk)
Understanding VPD
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 measured in kilopascals (kPa). VPD helps you understand the relationship between temperature and humidity in your grow space.
Why VPD Matters
Maintaining optimal VPD encourages healthy transpiration, which drives nutrient uptake and CO₂ absorption. Too low VPD reduces transpiration and can cause nutrient deficiencies. Too high VPD stresses plants and can lead to leaf curling and stunted growth.
How This Calculator Works
This calculator uses the Magnus-Tetens formula to calculate saturation vapor pressure (SVP) based on your temperature, then determines actual vapor pressure (AVP) from your humidity reading. VPD is simply SVP minus AVP. The result tells you how much "drying power" the air has.
Adjusting Your Environment
To lower VPD: decrease temperature or increase humidity (humidifier, wet towels, lower ventilation). To raise VPD: increase temperature or decrease humidity (dehumidifier, better ventilation). Small adjustments can make a big difference in plant health.
How to Use This Calculator
- Measure your grow room temperature at canopy level using a reliable thermometer
- Measure relative humidity at canopy level using a hygrometer (avoid placing near fans or water sources)
- Enter your temperature in Fahrenheit and humidity percentage
- Select your current growth stage (clone, vegetative, flowering, or late flowering)
- Click "Calculate VPD" to see your vapor pressure deficit
- Review whether your VPD is in the optimal range for your stage
- If needed, adjust temperature or humidity based on the recommendations provided
Frequently Asked Questions
How accurate is the VPD calculator?
This tool uses the Magnus-Tetens formula, a scientifically validated equation for calculating saturation vapor pressure. It provides accurate VPD calculations for typical growing conditions (50-95°F, 20-90% RH).
Is this tool free to use?
Yes! This tool is completely free with no hidden costs, subscriptions, or limitations. Use it as often as you need.
Do I need to create an account?
No account needed. The tool works entirely in your browser without requiring signup or personal information.
Does this work on mobile devices?
Yes, this tool is fully responsive and works on smartphones, tablets, and all desktop browsers.
Is my data private?
Absolutely. All processing happens locally in your browser. Your data never leaves your device and is not stored on any server.
What is the optimal VPD range for cannabis?
Optimal VPD varies by growth stage: Clone/Seedling (0.4-0.8 kPa), Vegetative (0.8-1.2 kPa), Flowering (1.0-1.5 kPa), and Late Flowering (1.2-1.6 kPa). These ranges encourage healthy transpiration and growth.