How to Calculate VPD – the OTHER VPD


The term VPD has become common in the cannabis cultivation community for good reason- it is one of the keys to controlling plant health and vigor. In order to maximize the genetic potential and overall yield of cannabis, one must first optimize a plants water uptake. Several variables determine the flow of water and nutrients through marijuana plants, but arguably the most important is VPD. Not Vapor Pressure Deficit, but the other VPD: Vapor Pressure Difference. For the rest of this article, the D in VPD will stand for “Difference”. In this post, we’ll explain the distinction and provide some resources showing how to calculate VPD – with precision.

The D in VPD

Most of the commonly used Vapor Pressure Deficit charts have a significant flaw: they assume leaf temperature is equal to air temperature. While this removes a step from the equation, it seldom is accurate in practice. When leaf surface temperature changes,  VPD changes. This is why we want to ensure the actual Difference is calculated. VPD indicates stomatal conductance: how open the stomata are on the plant leaves, and how readily they allow water to move through the plant.Cannabis has a narrow range of acceptable VPD levels (~0.4 kPa – ~1.6kPa). When leaf temperature varies, even by a couple degrees, the VPD can fluctuate significantly, steering generative phases towards vegetative or vice-versa.

Low VPD, Cause and Effect

Low VPD means low pressure and a low flow rate of water (and nutrients) from the substrate to the roots, through the cannabis plant, and out through the stomata (a process called evapotranspiration which deserves a post to itself). During propagation of cuttings this is the goal: water levels within the plant are preserved until roots develop. However, once plants are established, a low VPD lessens calcium uptake, weakening stomatal growth and subsequent plant vigor. A low VPD can also indicate that the leaf/plant temperature is approaching or below the dew point. This can cause condensation to form on  leaves and flowers, leading to fungal pathogens such at powdery mildew and botrytis.

High VPD, Cause and Effect

High VPD causes stomata to close. This slows the plants self-regulate route of water uptake while pressure builds to the point where water is forced through the closed stomata. This leads to less available water within the plant and an increase in drought stress. Water moves through the plant too quickly, causing an EC spike in the medium as salts precipitate, and leaf wilting as the water stored in the leaves is forced out. The resilience of cannabis plants to a high VPD is, of course, determined by genetics. A decreased tolerance to high VPD levels also is the result of suboptimal Calcium levels within the plant tissue. As mentioned earlier, this is a result of too high a VPD during initial growth / stretch, but can also point to an inadequate fertigation program.

How to Calculate VPD?

To put it as simply as possible, VPD = VP(leaf) – VP(air). 

The calculation for VPD only requires three variables, leaf surface temperature, air temperature, and relative humidity. The calculator we recommend is It is an excellent, unbiased resource where the calculation steps are explained in greater detail. Please note Riococo has no affiliation with, we’re just a fan of their work! For those of you who want to dig into the physics behind it all, check out a white paper titled On the Computation of Saturation Vapor Pressure and also the textbook Principles of Environmental Physics. Both are available for free by following the links.

Proper VPD is an integral element of any optimized cannabis production facility. Another is the consistent properties of a high quality substrate. Learn more about how Riococo helps cultivators in the cannabis industry dial in their grow rooms and greenhouses by dropping us a line or by clicking any of the social links below.