Grow tools
Reviewed by Jeff Zorn, founder of Cannabis Training University (since 2009, 50,000+ students) · LinkedIn
Jeff Zorn is the founder of Cannabis Training University (CTU), an online cannabis school with 50,000+ students since 2009. CTU is an IACET Accredited Provider, and its courses cover cannabis cultivation, business, medical cannabis, budtending, cooking, extraction and industry regulations.
What VPD should I run?
Common grower targets: 0.4–0.8 kPa for clones, 0.8–1.2 kPa in veg and early flower, 1.2–1.6 kPa in mid to late flower.
VPD (vapor pressure deficit) is how much more water vapor the air could hold at the leaf's temperature. Higher VPD pulls more water through the plant; lower VPD slows it down.
Checked October 2026 · Sources below
Measure leaf temperature with an infrared thermometer if you can. Under LED lights leaves often run a little cooler than the air; under HPS they can run warmer (enter 0).
Leaf VPD in kPa for each air temperature and humidity. Find your row, read across to your humidity.
| Air °F | 40% | 45% | 50% | 55% | 60% | 65% | 70% | 75% | 80% |
|---|---|---|---|---|---|---|---|---|---|
| 64°F | 1.08 | 0.98 | 0.88 | 0.78 | 0.68 | 0.57 | 0.47 | 0.37 | 0.27 |
| 66°F | 1.16 | 1.05 | 0.94 | 0.84 | 0.73 | 0.62 | 0.51 | 0.40 | 0.29 |
| 68°F | 1.25 | 1.13 | 1.01 | 0.90 | 0.78 | 0.66 | 0.55 | 0.43 | 0.31 |
| 70°F | 1.34 | 1.21 | 1.09 | 0.96 | 0.84 | 0.71 | 0.59 | 0.46 | 0.34 |
| 72°F | 1.43 | 1.30 | 1.16 | 1.03 | 0.90 | 0.76 | 0.63 | 0.49 | 0.36 |
| 74°F | 1.53 | 1.39 | 1.25 | 1.10 | 0.96 | 0.82 | 0.67 | 0.53 | 0.39 |
| 76°F | 1.64 | 1.49 | 1.33 | 1.18 | 1.03 | 0.87 | 0.72 | 0.57 | 0.42 |
| 78°F | 1.75 | 1.59 | 1.43 | 1.26 | 1.10 | 0.94 | 0.77 | 0.61 | 0.45 |
| 80°F | 1.88 | 1.70 | 1.53 | 1.35 | 1.18 | 1.00 | 0.83 | 0.65 | 0.48 |
| 82°F | 2.00 | 1.82 | 1.63 | 1.44 | 1.26 | 1.07 | 0.88 | 0.70 | 0.51 |
| 84°F | 2.14 | 1.94 | 1.74 | 1.54 | 1.34 | 1.14 | 0.95 | 0.75 | 0.55 |
| 86°F | 2.28 | 2.07 | 1.86 | 1.65 | 1.43 | 1.22 | 1.01 | 0.80 | 0.59 |
Sources: saturation vapor pressure e°(T) = 0.6108 × exp(17.27 T ÷ (T + 237.3)) kPa, T in °C — FAO Irrigation and Drainage Paper 56 (Allen, Pereira, Raes and Smith, 1998), the Tetens form. Leaf VPD = e°(leaf temperature) − e°(air temperature) × RH ÷ 100. Stage targets are common grower targets, not research limits: Quest Climate, AROYA VPD chart; general explainer: Michigan State University Extension, VPD vs RH. Checked October 2026.
Dial in your grow room
The Indoor & Outdoor Growing course covers temperature, humidity, airflow and light, stage by stage, with a certificate at the end.
IACET Accredited Provider · online cannabis education
Since 2009 • 50,000+ students trained worldwide
Copyright Cannabis Training University, a subsidiary of Online Training Providers. © 2026. All Rights Reserved.
Medical Disclaimer: The information on this website is for informational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment.
Always seek the advice of a physician or other qualified health provider with any questions you may have regarding a medical condition.
Legal Compliance: Cannabis remains federally illegal. Products and information are intended only for jurisdictions where cannabis use and possession is legal.
By using this site, you affirm that you are of legal age (21+) and responsible for understanding your local laws.
