VPD Calculator for Growers
Enter your air temperature and humidity in Fahrenheit or Celsius and get the leaf VPD, the dew point and the humidity your growth stage needs. It checks your room against the temperature, humidity and VPD targets for each growth stage, warns you when the humidity is a mold risk even if the VPD looks fine, and tells you what lights-off will do to the numbers.
Leaf VPD 1.22 kPa
Checks for this stage
To land in the middle of the band
At lights-off
Drying a harvest? Do not use VPD for that. Dead tissue has no stomata to protect, so a drying room runs on a humidity schedule instead.
Your VPD chart
Rows are air temperature, columns are relative humidity, and each cell is leaf VPD in kPa.
VPD, temperature and humidity targets by stage
| Stage | Air temperature | Relative humidity | Leaf VPD |
|---|---|---|---|
| Seedlings and fresh clones | 72 to 78°F (22 to 26°C) | 70 to 80% | 0.4 to 0.8 kPa |
| Vegetative | 71 to 82°F (22 to 28°C) | 55 to 65% | 0.8 to 1.2 kPa |
| Flower, weeks 1 to 3 | 70 to 80°F (21 to 27°C) | 55 to 65% | 0.9 to 1.1 kPa |
| Flower, weeks 4 to 6 | 70 to 80°F (21 to 27°C) | 50 to 60% | 1.1 to 1.3 kPa |
| Late flower, week 7 to harvest | 65 to 75°F (18 to 24°C) | 45 to 55% | 1.1 to 1.3 kPa |
If you only remember one band, remember 1.0 to 1.2 kPa. It overlaps the band for veg and for every week of flower. Each VPD band is what that stage's temperature and humidity work out to.
How the calculator works
VPD is how much more water vapor the air could hold than it does, measured at the leaf. It is worked out in three steps, all with the saturation vapor pressure equation from the United Nations Food and Agriculture Organization's Irrigation and Drainage Paper 561: es = 0.6108 x exp[17.27 x T / (T + 237.3)], with T in Celsius and es in kPa.
| What | How it is worked out |
|---|---|
| Actual vapor pressure of the air | es at the air temperature x relative humidity / 100 |
| Leaf VPD | es at the leaf temperature, minus the actual vapor pressure of the air |
| Air VPD | es at the air temperature, minus the actual vapor pressure of the air |
| Dew point | The temperature where es equals the actual vapor pressure of the air |
| Absolute humidity | 2,167 x actual vapor pressure in kPa / (air temperature in Celsius + 273.15), in grams per cubic meter |
| Lights-off humidity | The same actual vapor pressure, divided by es at the lights-off temperature, assuming no water is added or removed |
The one guess in all of this is the leaf temperature. A healthy, well-watered leaf usually sits within about 2°C (3.6°F) of the air, under LED, HPS or the sun2, so we assume it runs 2°F (about 1°C) cooler, the same assumption most VPD charts make. A leaf that is short of water cannot cool itself and can run far hotter than the air. That is why the most useful thing you can do with this calculator is pick "I measured it" and enter the reading from an infrared thermometer pointed at a healthy fan leaf in the middle of the canopy, lights on. At 77°F (25°C) and 55% humidity, a leaf 4°F (2°C) cooler than the air reads 1.03 kPa and one 4°F (2°C) warmer reads 1.87 kPa, in the same room.
When the humidity matters more than the VPD
Gray mold (Botrytis) and powdery mildew respond to relative humidity and free water, not to VPD. A tent at 80°F (27°C) and 62% humidity and one at 70°F (21°C) and 50% read almost the same VPD, and they are not the same room to a mold spore inside a dense flower cluster. So in late flower, when the humidity band and the VPD band disagree, humidity wins: hold the humidity band, take the cool end of the temperature range, and let the VPD read high. The calculator flags that case for you.
The same air holds more humidity as it cools, which is why the lights-off check is there. Air at 77°F (25°C) and 50% humidity that cools to 67°F (19°C) reads about 70% with no water added. Run your exhaust and dehumidifier on a humidity setpoint through lights-off, never on a timer.
Frequently asked questions
What is a good VPD for veg and flower?
Between 0.8 and 1.3 kPa covers veg and every week of flower. If you want one number to hold from the start of veg to the day before harvest, use 1.0 to 1.2 kPa, which overlaps every stage's band. Seedlings and fresh clones want a gentler 0.4 to 0.8 kPa, which is what a propagation dome gives you.
What is the difference between leaf VPD and air VPD?
Air VPD treats the leaf as if it were the same temperature as the air, which is what most hygrometers and controllers show. Leaf VPD uses the leaf's own temperature, which is what the plant actually feels. With a leaf 2°F (about 1°C) cooler than the air, leaf VPD reads about 0.2 kPa lower than air VPD at normal room temperatures.
Should I use Fahrenheit or Celsius?
Either. The calculator converts everything to Celsius for the math and gives the same VPD in both. Switching units also converts the numbers you have already entered.
What leaf temperature should I enter?
The real one, if you can. Point an infrared thermometer at a healthy fan leaf in the middle of the canopy with the lights on. If you do not have one, keep our default of 2°F (about 1°C) below the air. If the leaf reads several degrees above the air, stop adjusting humidity and check the roots and the watering first: a hot leaf usually means the plant is not drinking.
Is 1.6 kPa too high?
For a plant in veg or flower, it is higher than you want. Most charts treat about 1.6 kPa as the point where a plant starts closing its stomata to save water, though that line is a chart convention rather than a measured threshold. Before you fix the room, check the leaf: a reading that high with a hot leaf is usually a root zone problem.
- Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration: guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56, Chapter 3. Food and Agriculture Organization of the United Nations. https://www.fao.org/4/x0490e/x0490e07.htm
- Nelson, J. A., & Bugbee, B. (2015). Analysis of environmental effects on leaf temperature under sunlight, high pressure sodium and light emitting diodes. PLoS ONE, 10(10), e0138930. https://doi.org/10.1371/journal.pone.0138930