VPD calculator and chart for cannabis
Enter the temperature and humidity in your grow room to see the leaf VPD, whether it suits your plants' stage, and which humidity would bring it into range.
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How the VPD is calculated
VPD = saturation vapor pressure at leaf temperature − actual vapor pressure of the air.
Saturation vapor pressure comes from the Tetens equation used in the FAO-56 guidelines. The actual vapor pressure is the saturation pressure at air temperature multiplied by the relative humidity.
The ranges we use per stage:
- Seedlings and cuttings: 0.4 to 0.8 kPa
- Vegetative growth: 0.8 to 1.2 kPa
- Flowering, first weeks: 1 to 1.5 kPa
- Late flowering: 1.2 to 1.6 kPa
Formula source: FAO Irrigation and Drainage Paper 56.
Frequently asked questions
What is VPD?
Vapor pressure deficit is the gap between how much moisture the air could hold and how much it holds now. The bigger the gap, the harder the air pulls water out of the leaves. It tells you more than humidity alone because it also takes temperature into account.
Why use leaf temperature instead of air temperature?
The plant loses water through its leaves, so the pressure at the leaf surface is what counts. Leaves under LED lights are usually cooler than the air, which lowers the real VPD. With HPS lamps the leaf can be warmer, so enter a negative offset.
Which VPD should I aim for in each stage?
Seedlings and cuttings have few roots, so keep VPD low, at 0.4 to 0.8 kPa. In vegetative growth, 0.8 to 1.2 kPa works well. In the first weeks of flowering, 1.0 to 1.5 kPa suits most plants, and in late flowering a drier 1.2 to 1.6 kPa helps keep bud rot away.
What do I do when VPD is too high?
Raise humidity with a humidifier, or lower the temperature. Turning the extraction fan down also keeps more moisture in the tent. Check again after an hour, because the room needs time to settle.
What do I do when VPD is too low?
Lower humidity with a dehumidifier or more air exchange, or raise the temperature a little. Low VPD in late flower is a common cause of bud rot, so act on it quickly.
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