Vapor Pressure Deficit (VPD) Calculator

Vapor Pressure Deficit (VPD) Calculator

Enter your temperature and relative humidity to calculate VPD (in kPa). Ideal VPD ranges vary by plant growth stage, but typically fall between 0.8 and 1.4 kPa for many indoor grows.

Calculated VPD: 0.00 kPa

*Remember, this is an estimate. Actual VPD can be influenced by leaf temperature, airflow, and other factors. Leaf temperature is often ~1–2°C cooler than air temperature in a well-ventilated environment.


What is Vapor Pressure Deficit (VPD)?

VPD refers to the difference between the actual moisture in the air and the maximum amount of moisture the air can hold at a given temperature. It directly affects plant transpiration—how plants release water through their leaves. A balanced VPD helps plants efficiently uptake nutrients and grow more vigorously.

Why is VPD important for cannabis growth?

Maintaining an optimal VPD ensures your plants can transpire and absorb nutrients efficiently. If VPD is too low, excess humidity can lead to mold, mildew, and stunted growth. If VPD is too high, plants may transpire too quickly, causing stress or nutrient burn.

What’s an ideal VPD range?

Many growers aim for a VPD between 0.8 and 1.4 kPa, though the exact target can vary by growth stage:
Seedlings/Clones: Lower VPD (~0.8–1.0 kPa) to avoid excessive moisture loss.
Vegetative Stage: Around 1.0–1.2 kPa for balanced growth.
Flowering Stage: Around 1.2–1.4 kPa to reduce mold risk and drive healthy bud development.

How does this calculator work?

It uses the Tetens formula to determine the saturation vapor pressure (SVP) based on your input temperature (converted to °C if necessary). Then it calculates VPD by factoring in relative humidity (RH). The result is displayed in kPa with two decimal precision.

Why is there an option for Celsius or Fahrenheit?

Growers worldwide use different temperature scales. The calculator converts Fahrenheit to Celsius automatically if you select °F, because the Tetens formula requires °C. This ensures accurate VPD results no matter where you grow.

Should I measure leaf temperature or air temperature?

Many growers enter air temperature for simplicity. However, leaf temperature can be slightly cooler (by 1–2 °C) than the surrounding air. For maximum accuracy, measure leaf temperature using an infrared thermometer or temperature gun, then input that reading to approximate leaf VPD.

My room’s temperature or humidity changes quickly. Will this calculator still help?

Yes! You should periodically recheck your environment as conditions fluctuate. Real-time or frequent readings help you make timely adjustments (e.g., turning on a dehumidifier or adjusting ventilation). A stable environment typically benefits your plants the most.

Does VPD replace the need to monitor relative humidity alone?

Not exactly—VPD is essentially a more accurate way of combining temperature and RH into a single metric. Monitoring VPD alongside RH and temperature gives you a more complete picture of your grow environment.

What if I’m adding CO₂ to my grow room?

COâ‚‚ supplementation can allow plants to thrive at slightly higher temperatures, which could shift your optimal VPD range. You might find your ideal VPD is on the higher side to accommodate increased transpiration and metabolism under COâ‚‚-rich conditions.

Is VPD the same during lights off?

No. When the lights are off, temperatures often drop—along with VPD. Some growers maintain a slightly higher temperature at night to avoid excessive humidity that can lead to mold. Monitoring VPD during lights off can be just as important as during the photoperiod.

How do I control VPD if my reading is too high or too low?

High VPD (too dry): Decrease temperature, increase humidity, or reduce airflow to slow evaporation.
Low VPD (too humid): Increase temperature, improve ventilation, add a dehumidifier, or adjust exhaust fans.

Can the calculator tell me how to fix my VPD if it’s off?

This tool only calculates your VPD. Adjusting your grow environment requires changing temperature or humidity. By monitoring and recalculating often, you can see how tweaks affect VPD and move toward your target range.

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