VPD Calculator for Cannabis

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 bands from our VPD chart guide, 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.

Print this chart or save it as a PDF

Our VPD chart guide also has a ready-made chart in Fahrenheit and Celsius with the stage bands drawn in.

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. The reasons behind each number are in our VPD chart guide, and the humidity bands come from our indoor growing guide and our flowering stage guide.

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 our printed chart makes. 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. Our guide covers why the leaf matters more than the lamp.

When the humidity matters more than the VPD

Bud rot and powdery mildew respond to relative humidity and free water, not to VPD³. 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 cola. 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. Our bud rot guide and powdery mildew guide cover the reasoning, and our grow tent humidity guide covers how to bring humidity down.

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. Our grow tent temperature guide covers the lights-off drop.