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How to calculate VPD: achieving optimal growing conditions

Lucy Starley
December 7, 2023
11 min read
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Are you ready to dive into the fascinating world of VPD? Well, you’re in the right place!  We’re going to unravel the mysteries of calculating VPD and how it can work wonders for your precious cannabis plants. Now, you might be wondering, “What on earth is VPD?” Don’t worry, my friend, we’ve got you covered!

VPD, short for Vapour Pressure Deficit, is a key factor in the success of your cannabis cultivation journey. It’s all about creating the perfect environment for your plants to thrive and reach their full potential. Imagine being able to fine-tune the temperature and humidity in your grow room to provide the ideal conditions for growth. That’s what VPD is all about!

But don’t worry, we won’t drown you in scientific jargon or complex equations. We’ll break it down into easy-to-understand steps. Even if you’re a beginner, you’ll be able to master VPD calculations like a pro. And trust me, once you grasp the concept, you’ll wonder how you ever grew cannabis without considering VPD.

Disclaimer: Any information given on this site is for educational purposes only. Please ensure if you’re growing cannabis you’re doing so in accordance with the law and subject to appropriate permissions and licences of the applicable country.

Temperature and humidity reading of cannabis plants

What does VPD mean?

In a nutshell, VPD is a measurement that helps us understand the relationship between temperature, humidity, and the water vapour present in the air. It’s a delicate balancing act. Getting it right can make all the difference in your cannabis plants’ growth and overall health.

You see, plants, including our beloved cannabis, have tiny pores on their leaves called stomata. These little openings play a crucial role in transpiration, which is the process by which plants lose water vapour through their leaves. As well as cooling the plants, transpiration also helps in the absorption of essential nutrients from the soil.

Now, here comes the interesting part: the transpiration rate depends on the difference between the water vapour pressure inside the leaf and the water vapour pressure in the air surrounding the plant. And this difference is what we refer to as the Vapour Pressure Deficit or VPD.

So, why is VPD important for your cannabis plants? Well, my friend, maintaining the right VPD range ensures that your plants can efficiently transpire and take in nutrients. It’s providing them with the perfect climate-controlled environment, where they can thrive and flourish.

Think of it this way: if the VPD is too low, the air is too humid. In these conditions, your plants may struggle to release excess moisture. This can lead to a whole host of issues like mould, mildew, or even stunted growth.

On the flip side, if the VPD is too high, indicating low humidity, your plants might lose water too quickly. This can lead to dehydration and nutrient deficiencies.

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The factors influencing VPD

Now that we have an understanding of VPD, it’s time to explore the factors that influence this critical measurement. By understanding these factors, you’ll be better equipped to create the optimal VPD conditions for your cannabis plants to thrive.


The first key factor in determining VPD is temperature. Temperature plays a significant role in the rate of evaporation and transpiration. Higher temperatures generally lead to larger rates of evaporation and transpiration, while cooler temperatures slow down these processes.

To accurately calculate VPD, it’s crucial to obtain precise temperature measurements in your grow room. Use a reliable thermometer and place it at plant canopy height. This is where your plants experience the most direct influence of temperature.


Humidity, the amount of water vapour present in the air, is another crucial factor affecting VPD. The relationship between humidity and VPD is inversely proportional. In simpler terms, as humidity increases, VPD decreases, and vice versa.

Maintaining the optimal humidity range for cannabis plants is essential for proper VPD management. Humidity that is too high can impede transpiration, leading to issues such as poor nutrient uptake and an increased risk of fungal diseases. On the other hand, low humidity can cause excessive water loss and hinder growth.

Vapour pressure:

Vapour pressure is the pressure exerted by water vapour in the air. It directly contributes to VPD calculations. To accurately measure vapour pressure, you can use a specialised device called a hygrometer. Alternatively, you can consult online weather sources for local atmospheric pressure information.

Understanding and monitoring these factors—temperature, humidity, and vapour pressure—will help you gain a better grasp of VPD. Remember, the goal is to create the perfect balance to optimise growth, yield, and overall plant health.

How to calculate VPD

Man recording cannabis plant data

Alright, it’s time to put on our thinking caps and get those calculators ready! Calculating VPD may sound intimidating, but fear not, my friend. We’ll break it down into simple steps so that you can master it with ease.

Determine the temperature and humidity values:

Before we dive into the calculations, you need to gather two essential pieces of information:

  • The temperature in your grow room.
  • The humidity in your grow room.

Make sure you have accurate measurements to ensure precise VPD calculations.

Use a digital thermometer to measure the temperature at canopy height, where your plants are most affected by it. Additionally, use a hygrometer to determine the relative humidity in your grow room. Place the hygrometer at plant level to get an accurate reading of the humidity surrounding your plants.

Introduce the formula for VPD calculation:

Now that you have your temperature and relative humidity values, it’s time to plug them into the VPD formula. Don’t worry, it’s not as complex as it sounds! Here’s the formula:

VPD = (Saturation Vapour Pressure – Actual Vapour Pressure)

Let’s break it down:

Saturation vapor pressure:

This represents the maximum amount of moisture the air can hold at a given temperature. You can find tables or online resources that provide saturation vapour pressure values corresponding to different temperatures.

Actual vapour pressure:

This is the actual amount of water vapor present in the air, determined by the relative humidity reading from your hygrometer.

By subtracting the actual vapour pressure from the saturation vapour pressure, you’ll obtain the VPD value. This will indicate the difference between the water vapour pressures inside the plant and in the surrounding air.

For example:

  • You measure a temperature of 25°C in your grow room
  • Your hygrometer shows a relative humidity of 60%.

Now, using the VPD formula, let’s calculate the VPD value step by step:

Step 1: Find the saturation vapour pressure for 25°C from a reference table or online resource. Let’s say it’s 2.3 kPa.

Step 2: Calculate the actual vapour pressure by multiplying the saturation vapour pressure (2.3 kPa) by the relative humidity (60%). Then divide that by 100. In this case, the actual vapour pressure would be 1.38 kPa.

Step 3: Subtract the actual vapour pressure (1.38 kPa) from the saturation vapour pressure (2.3 kPa):

VPD = 2.3 kPa – 1.38 kPa = 0.92 kPa

So, in this example, the VPD value would be 0.92 kPa.

By following these steps, you can calculate the VPD value specific to your grow room conditions.

What is the ideal VPD range for my plants?

Vegetative cannabis plants

Now that you have a solid grasp of how to calculate VPD, let’s dive into the fascinating world of VPD ranges. Different growth stages of cannabis plants require specific VPD levels to ensure optimal development and yield. Let’s explore these ranges and the effects they have on your green beauties!

Vegetative stage:

During the vegetative stage, when your plants are focused on leaf and stem growth, maintaining the right VPD range is crucial. The target VPD for veg is a range of 0.4 to 0.8 kPa. This range provides an ideal balance, allowing your plants to transpire efficiently and consume nutrients effectively. It promotes robust vegetative growth, and healthy root development, and prepares your plants for a fruitful flowering stage.

Flowering stage:

As your cannabis plants transition into the flowering stage, their focus shifts towards bud development and resin production. To support this critical phase, you’ll want to adjust the VPD range accordingly. Aim for a slightly lower range of 0.8 to 1.2 kPa. This range helps maintain the right level of transpiration while encouraging dense bud formation, resin production, and overall flowering success.

Late flowering stage:

Towards the end of the flowering stage, your plants enter the final push of maturation and ripening. During this late flowering stage, it’s essential to fine-tune the VPD. This maximises resin production and ensures a high-quality harvest.

Aim for a VPD range of 1.2 to 1.6 kPa. This slightly higher range encourages nutrient uptake and terpene development. It also helps to achieve the desired potency and aroma in your final product.

By understanding and applying the appropriate VPD range for each stage of growth, you provide your cannabis plants with the ideal environmental conditions. This gives your plants the best chance to reach their full potential. Remember, these ranges serve as guidelines. Slight variations may be necessary depending on your specific cultivar, environmental factors, and personal preferences.

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How to manage VPD in your grow room

You’re now equipped with the knowledge of VPD ranges and their importance. But how do you put this knowledge into action?

Fear not, for we are here to guide you. We’ll take you through practical strategies to manage VPD like a pro in your grow room. Let’s dive in!

Adjusting temperature and humidity accordingly:

To maintain the optimal VPD range, you’ll need to fine-tune the air temperature and humidity in your grow environment. Here’s what you can do:

Temperature: Use heating or cooling systems to regulate the temperature in your grow room. Keep in mind the VPD ranges discussed earlier for each growth stage and make adjustments accordingly. A digital thermostat with temperature controls will be your best friend in achieving and maintaining the desired temperature range.

Humidity: To control humidity levels, you can utilise dehumidifiers or humidifiers, depending on whether you need to increase or decrease humidity. Monitor your relative humidity using a reliable hygrometer and adjust it within the recommended range for your specific growth stage.

Utilising ventilation and airflow systems:

Proper ventilation and airflow play a crucial role in VPD management. Here’s how you can optimise them:

Ventilation: Install exhaust fans and intake fans to create a continuous flow of fresh air in your grow room. This helps remove excess humidity, replenish carbon dioxide levels, and maintain a healthy airflow for your plants.

Air circulation: Use oscillating fans strategically placed to ensure even distribution of air throughout your grow room. This prevents stagnant air pockets and aids in temperature and humidity distribution, promoting uniform VPD conditions for your plants.

Using VPD charts and calculators for guidance:

Harness the power of technology and readily available resources to simplify VPD management:

VPD charts:

Refer to a VPD chart specific to cannabis cultivation that provide recommended temperature and humidity ranges for different growth stages. These charts serve as handy visual aids to guide you in achieving optimal VPD conditions. You can use an overall chart or a more specific one, such as :

  • VPD chart for veg
  • VPD chart for flowering

VPD calculators:

Make use of online VPD calculators or mobile apps that automate the VPD calculations for you. Simply input the temperature and humidity values, and these tools will provide you with the corresponding VPD value. It takes the guesswork out and ensures accuracy in your VPD management.

By implementing these practical strategies, you’ll be able to fine-tune your grow room. This means you’ll create an optimum environment and maintain the ideal VPD conditions for your cannabis plants. Remember, VPD management is an ongoing process that requires careful monitoring and adjustments as your plants progress through different growth stages.

Growers recording cannabis data

Tools and resources for VPD calculations

Having the right tools and resources for VPD measuring can make your life as a grower much easier. Let’s explore some handy tools and resources that will assist you in your VPD journey:

Online VPD calculators and mobile apps:

In this digital age, we’re fortunate to have online VPD calculators and mobile apps readily available. These tools take the guesswork out of VPD calculations by automating the process.

Simply input the temperature and humidity values, and they’ll provide you with the corresponding VPD value in a matter of seconds. Some even offer additional features like VPD charts, growth stage recommendations, and optimal VPD ranges.

VPD charts and tables:

VPD charts and tables are invaluable resources for growers. These visual aids provide recommended temperature and humidity ranges for different growth stages. They offer a quick reference guide, allowing you to easily determine the target VPD values for your plants.

You can find VPD charts and tables in cannabis cultivation books, online forums, grower communities, or from reputable sources. For example, our VPD Calculator and Environmental timeline.

Hygrometers and thermometers:

Accurate measurements of temperature and humidity are vital for VPD calculations. Invest in reliable hygrometers and thermometers to ensure precise readings in your grow room. Look for models with digital displays, built-in sensors, and a wide measurement range.

Place the hygrometer at the plant canopy level for accurate humidity readings. Position the thermometer at the same height to obtain the temperature values relevant to your plants.

Grow room environmental control systems:

For advanced growers or those with larger grow operations, consider investing in grow room environmental control systems. These systems allow you to monitor and regulate various aspects of your grow room environment, including temperature, humidity, ventilation, and more. They offer precise control and automation. This makes it easier to maintain optimal VPD conditions throughout the different growth stages of your cannabis plants.

By utilising these tools and resources, you’ll have the support you need to confidently calculate VPD. You’ll also be set up to troubleshoot issues, and fine-tune your grow room environment. Remember, knowledge is power, and these tools will empower you to become a VPD master in no time!

Troubleshooting VPD issues

As a grower, it’s not uncommon to encounter VPD-related challenges in your cannabis cultivation journey. In this section, we’ll address common VPD issues and provide practical solutions to help you overcome them. Let’s dive in and troubleshoot like pros!

High VPD:

Where the VPD value exceeds the recommended range, consider the following solutions:

Increase humidity:

To lower VPD, you can increase the humidity in your grow room. Use humidifiers or implement water sources like trays or humidification systems to release moisture into the air. This will help bring the VPD into the desired range and prevent excessive water loss from your plants.

Adjust temperature:

Another approach is to slightly lower the temperature in your grow room. This helps decrease the saturation vapour pressure, which in turn reduces the VPD value. Ensure that you maintain the temperature within the appropriate range for your specific growth stage.

Low VPD:

Conversely, if you’re facing a low VPD scenario where the VPD value falls below the recommended range, here are some solutions:

Decrease humidity:

Lowering the humidity in your grow room will help increase the VPD. Use dehumidifiers or implement proper ventilation to remove excess moisture from the air. This allows your plants to transpire more effectively, maintaining a healthy VPD range.

Adjust temperature:

Raising the temperature slightly can also help increase the VPD value. This leads to higher saturation vapour pressure, balancing the water vapour pressure inside and outside the plant. Be mindful of maintaining the appropriate temperature range for your specific growth stage.

Remember, VPD management is a delicate balance. By making targeted adjustments to humidity and temperature, you can steer your VPD back into the optimal range. This helps to ensure your plants have the best possible growing conditions.


Congratulations, my fellow cannabis cultivator! You’ve journeyed through the ins and outs of VPD calculation and management like a true pro. You’ve learned how to calculate VPD, understand its significance, and troubleshoot any issues that may arise.

You’re now equipped with the knowledge and tools to create the optimal VPD conditions for your beloved cannabis plants. Give yourself a pat on the back!

By mastering VPD, you’ve unlocked the key to unlocking your plants’ potential. You’re no longer simply a grower; you’re a VPD maestro. You are orchestrating the perfect harmony of temperature, humidity, and vapour pressure for optimal growth and success. Your plants will thank you with vigorous growth, robust yields, and exquisite cannabinoid profiles.

Remember, VPD is not just a fancy acronym—it represents a fundamental principle in cannabis cultivation. It’s about creating an environment where your plants can thrive, transpire, and reach their full potential. By maintaining the right VPD ranges throughout each growth stage, you’re providing the best possible conditions for your plants to flourish.

So, go forth with enthusiasm and confidence. Apply your VPD knowledge, fine-tune your grow room environment, and watch your cannabis plants reach new heights. Stay curious, keep learning, and continue to connect with the vibrant community of cannabis growers who share your passion.

Thank you for joining me on this VPD adventure. May your cannabis cultivation journey be filled with bountiful harvests, remarkable flavours, and the joy of nurturing nature’s finest. Happy growing, and until we meet again, my friend!

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Lucy Starley
December 7, 2023
11 min read

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