• Adrienne Gould-Choquette, P.E.

How to Use the Vapor Pressure Deficit (VPD) Calculator

Updated: Dec 1, 2021

This article and video provide a demonstration of the Vapor Pressure Deficit (VPD) Calculator.

You can access all our calculators on the HVAC Calculators page or by the drop-down menu. The calculators are listed alphabetically by category.

This Vapor Pressure Deficit (VPD) calculator is used to determine temperature and humidity for indoor grow rooms. I am not a grower, so I cannot advise you what your VPD should be, but here is an excellent article From Cannabis Science and Technology describing VPD at different stages of growth.

So let us get right into it. You can choose either imperial or metric units. For this demonstration, I will use imperial units. Begin by entering room conditions of 78 degrees Fahrenheit and 60% relative humidity. When you hover over the Leaf Temperature input box, a note explains that as the leaf grows, more evaporative cooling takes place, so the leaf temperature drops. If you do not know your leaf temperature, a rule of thumb is that you could assume a temperature drop of about 2 degrees Fahrenheit. Following that rule of thumb, we will use a leaf temperature of 76 degrees Fahrenheit. These inputs give a room VPD of 1.31kPa and a leaf VPD of 1.1kPa.

The clear benefit of targeting a VPD is healthy, robust plants. Another benefit of targeting VPD is energy savings. Let us try one more scenario. Let us say you are in a hot, humid climate. You can allow the temperature to be set to 82 degrees Fahrenheit and 65% relative humidity and still achieve the same leaf and room VPD values of 1.31kPa and 1.1KPa, respectively.

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