VPD Chart for Cannabis: Vapor Pressure Deficit Explained
A cannabis VPD chart shows the vapor pressure deficit for each pairing of air temperature and relative humidity. MSU Extension describes VPD as the gap between the water vapor saturated air could hold and the vapor the air really holds, usually given in kilopascals (kPa). Find your room temperature and RH, and the cell gives the VPD.
MSU explains why growers use it: the water air can hold rises with temperature, so RH alone is not a consistent measure of how fast plants lose water. The sources here are greenhouse guidance rather than cannabis trials. Kind Seed Co gives you the chart, the method behind it and what each band means.
What Is Vapor Pressure Deficit?
VPD is the drying pull of the air. MSU Extension defines it as the gap between saturated and actual water vapor pressure, and calls it a more accurate guide to transpiration and water loss than RH.
When air is saturated, MSU says VPD is zero and nothing pulls water out of the leaves. As the water in the air falls, or the temperature rises, VPD climbs and plants lose water faster through their leaves. That loss happens through tiny leaf pores, explained in our page on what stomata are.
How Do You Calculate VPD?
You need air temperature and RH. MSU notes that VPD can be worked out from canopy temperature, or from air temperature as a stand in, plus RH.
The chart below uses the Tetens equation. Saturation vapor pressure in kPa is es = 0.6108 × e^(17.27T / (T + 237.3)), where T is the air temperature in C. Multiply es by the dry share of the air, 1 minus RH as a decimal, to get VPD. Worked example: at 77 F (25 C), es comes to about 3.17 kPa, and at 55% RH the VPD works out near 1.43 kPa.
| Air temperature | 80% RH | 70% RH | 60% RH | 50% RH | 40% RH |
|---|---|---|---|---|---|
| 18.3 C (65 F) | 0.42 | 0.63 | 0.84 | 1.05 | 1.26 |
| 21.1 C (70 F) | 0.50 | 0.75 | 1.00 | 1.25 | 1.50 |
| 23.9 C (75 F) | 0.59 | 0.89 | 1.19 | 1.48 | 1.78 |
| 26.7 C (80 F) | 0.70 | 1.05 | 1.40 | 1.75 | 2.10 |
| 29.4 C (85 F) | 0.82 | 1.23 | 1.64 | 2.05 | 2.47 |
Method: every cell is calculated, in kPa, rounded to two decimals, with the Tetens equation above. Leaf temperature offset: 0 F. Leaf temperature is taken as equal to air temperature, the stand in MSU allows. If an infrared thermometer shows the leaves running below air temperature, the true leaf VPD is lower than the cell shows.
As a cross check, MSU’s own table gives 0.90 kPa at 75 F and 70% RH. The formula above gives 0.89.
What the VPD Bands Mean
MSU sorts VPD into four bands for plants grown in controlled environments. It calls them subjective classifications, so read them as greenhouse guidance, not cannabis stage rules.
| VPD band | MSU description |
|---|---|
| Very low, below 0.3 kPa | Air at or near saturation. Leaves stay wet and the mix dries slowly, which MSU links to stronger attacks from many plant pathogens |
| Low, 0.3 to 0.7 kPa | The lowest band MSU rates as desirable for most plants. Water loss is moderately slow |
| Moderate, 0.7 to 1.5 kPa | MSU’s target range for growth in most plants. Air neither too dry nor too wet |
| High, above 1.5 kPa | Dry air. Water loss from plants and media is relatively rapid |
MSU adds that VPD usually only becomes a production issue at extremes. Below 0.7 kPa, outside propagation, some pathogens can be more problematic. Above 1.5 kPa, water loss is high and photosynthesis in some crops can fall.
How Do Temperature and Humidity Change VPD?
At a fixed RH, VPD climbs as the air warms, and at any temperature more humidity brings it down. MSU’s table shows a steady 70% RH giving 0.55 kPa at 60 F, 0.90 at 75 F and 1.45 at 90 F.
Look down the 50% RH column above: VPD runs from 1.05 kPa at 65 F up to 2.05 kPa at 85 F. UMass Extension explains the cause: each 20 F rise doubles how much water air can hold. Read more about climate for indoor cannabis before you set targets.
How Do Growers Use a VPD Chart?
Read your current cell, compare it to the MSU band you want, then move one setting. The chart only does the arithmetic, so it gives no stage targets of its own.
- Measure at canopy height. Read air temperature and RH where the leaves are, not on the wall.
- Find the cell. Pick the row nearest your temperature and the column nearest your RH.
- Compare to the MSU band. Moderate is MSU’s general target. Low suits young plants with new roots, MSU notes.
- Change one thing. Raise or lower temperature, or raise or lower RH, not both at once.
- Recheck after the change settles. Give the room time to level out before you read again.
- Log lights on and lights off. VPD shifts when the lamps switch, so record both periods.
The humidity side of the setup has its own page on cannabis humidity control.
Troubleshooting
When the chart and the plants disagree, check the reading first. These cases come up most.
VPD is high and top leaves curl. The air is drying the canopy fast. Raise RH or lower the temperature to move toward the moderate band.
VPD is low and the pot stays wet. MSU links low VPD to slow drying. Add air exchange, or run a dehumidifier, covered in our dehumidifier guide.
Readings swing at lights off. Cooling air raises RH, UMass notes. Run the exhaust as the lamps go out.
The chart says fine but buds feel damp. The canopy may be wetter than the room reading. Move the sensor into the canopy and improve airflow.
Two sensors disagree. Place them side by side at canopy height for a while and compare before you trust either.
Sources
- Michigan State University Extension, Runkle, Water Vapor-Pressure Deficit: https://www.canr.msu.edu/floriculture/uploads/files/Water%20VPD.pdf
- Michigan State University Extension, Wollaeger and Runkle, VPD vs. Relative Humidity: https://www.canr.msu.edu/uploads/resources/pdfs/vpd-vs-rh.pdf
- UMass Amherst Center for Agriculture, Food, and the Environment, Reducing Humidity in the Greenhouse: https://www.umass.edu/agriculture-food-environment/greenhouse-floriculture/fact-sheets/reducing-humidity-in-greenhouse
FAQ
Is VPD the same as relative humidity?
No. Relative humidity is the share of water the air holds compared with its maximum at that temperature. MSU Extension calls VPD the gap between saturated and actual vapor pressure in kPa, and says it predicts plant water loss more accurately than RH because it accounts for temperature.
Why is air temperature used for the leaves in this chart?
Leaf temperature is harder to measure. MSU Extension says VPD can be worked out from canopy temperature, or from air temperature as a stand in. This chart uses a leaf offset of 0 F. If an infrared thermometer shows leaves below air temperature, the true leaf VPD is lower than the cell shows.
What units is VPD measured in?
Kilopascals, written kPa. MSU Extension notes that pressures can be given in other units, but for plant work kPa is the usual unit. Every cell in this chart is in kPa, rounded to two decimals, so you can compare it directly with MSU's bands from below 0.3 to above 1.5.
How do you read the VPD chart?
Find the row closest to your air temperature, then read across to the column closest to your RH. The cell is the VPD in kPa. Check it against the MSU bands under the chart, then raise or lower temperature or humidity and recheck once the room settles.
What VPD range does a cannabis VPD chart show as moderate?
MSU Extension calls 0.7 to 1.5 kPa moderate and names it a target range for growth in most plants. That is general greenhouse guidance, not a cannabis stage rule. In this chart, 70 F at 50 to 70% RH and 80 F at 60 to 70% RH fall in that band.
