How to calculate CFM for a Grow Room Fan Size

Last Updated on July 16, 2020 by Gary Stephen

Over the years, you may have seen farming estates having small enclosed areas. You may be wondering what these closed spaces in the middle of farmlands are doing. You are not the only one who gets perplexed by the growing number of grow rooms. Grow rooms where plants get cultivated in a controlled environment. People use grow rooms for several reasons. It is different than the other when it comes to size, functions, and techniques. There was a time when scientists and botanists used these small enclosures. Botanists needed a controlled setting to conduct their experiments.

They would grow exotic or sensitive plants in these enclosures. Some of the best experiments performed in a grow room are aeroponics and hydroponics. They were also used by criminals for growing banned plants like cannabis and poppy.

But, with time, grow rooms are being used by common people. There are reasons why common people are adopting these enclosed spaces as their own. One is the knowledge about making and maintaining grow rooms with a fan have is now accessible to them.

Why Grow Rooms Need A Fan?

Grow rooms are often equipped with machinery that emits heat of its own. This machinery includes temperature control tools like heaters and air conditioners. Other than that, grow rooms have sources of light that emit heat. These sources of light can be artificial, such as lamps and bulbs, or one can have open glass panels for sunlight. Often the sources of light are a combination of sunlight and artificial bulbs.

Most grow rooms by design are in closed spaces. They lack natural ventilation, and this, in turn, causes the grow rooms to get heated up very fast. When plants precipitate or emit gases like nitrogen and CO2, the place becomes hot. Hence, it is important to put effective ventilation in place. A well-ventilated place also makes it easier for the plant grower to function.

Without ventilation, it’s impossible to grow plants in an overheated and humid atmosphere. To install a fan, one needs to know things like the importance of CFM for a fan. Without a good fan, the plants in the grow room will have stunted growth, or they will wilt due to overheating. Thus, it is important to have a fan in one’s grow room.

How To Calculate Fan Size For Grow Room

The whole process of getting familiar with the calculation process may be hard to get used to. It can also be intimidating for those who are not into mathematics or calculation of volumes. Once you know the formula to calculate the fan size, it will be easy for you to pick what fan size is most suitable for you.

First, let us understand what CFM stands for and what it means. The acronym CFM stands for cubic feet per minute. When we talk about cubic feet per meter, then it means the capacity of a fan to move the volume of air in one minute. The faster the CFM means, the faster the fan works. Thus, we elaborate on the factors of how to calculate CFM for a grow room further.

Too much CFM can damage your plants by uprooting them. Also, too low CFM will not circulate air and cause overheating. Both situations will be harmful to your plants. One needs a moderate CFM in its fans for the best results. It’s suggested that one should get a fan where the speed and the CFM are adjustable.

To decide what fan size is most suitable for your glasshouse or grow room:

The formula

Determine the room volume in cubic feet. To do so, calculate the length, breadth, and height to multiply them. Now, it depends on your needs whether you want a fan that exhausts all the air within a minute, two, three, five, or what. So we will divide the total volume of the room with those minutes.

For example, a 10x10x5 ft grow room will have 500 CFM by default (considering 1 minute for air exhaustion). But, if you need to exhaust air in 5 minutes, then it will be 500 / 5 = 100 CFM fan. After that, the main calculation will begin.

The Factors That Contribute To CFM Calculation

We now know why a grow room fan is very important for the successful cultivation of plants and other crops. Additionally, we are also aware of what factors to look for in a good fan. But, there is another important thing that we should keep in mind while picking a fan for our green room. That is the size of the fan.

The size of your grow room determines the size of your fan. But, before we learn how to do the calculation, it is important to know what other facts one should keep in mind. Some of these factors are:

Read Also: Learn How to Efficiently Cool a Grow Room

Room Volume: Room volume is the first thing that one needs to consider while installing. Not only the fan, but any equipment in a grow room. The room volume refers to all the space that is inside the grow room. You need to know the volume of air that needs replacement before deciding the size of a fan.

The fan has to remove the air inside the room and replace it with the outside fresh air. Other than this, the fan must also circulate the air within the grow room. The volume will decide how large the fan will be. The larger the grow room, the bigger the fan and vice versa.

Ducting Style: The ducting style is the second important factor that decides the fan size. Ducting refers to the construction vents used for air conditioning and ventilation purposes. Almost all grow rooms have ducting structures that are necessary for the transfer of air.

Ductwork makes sure how and at what capacity the air will flow across an enclosed structure. The ducting design affects the capacity of a greenhouse fan. It won’t matter how strong your fan is, or how efficient it’s aligned with the room volume. If the fan doesn’t have the right fit with the duct design, then the transfer of air will not be successful.

Some of the popular duct designs are:

  • Long ducts: They have close to 5 to 6 bends, and their length is around 20 to 30 feet.
  • Medium ducts: They have close to 2 to 3 bends, and their length is around 5 to 10 feet.

There is an addition of a 5 percent increase in CFM whenever there is an increase of 5 feet in the duct design. Another addition of 20 percent in CFM whenever the duct design includes a 90-degree turn.

Some grow rooms do not have any ducts. They’re made of open space and have a roof. You can attach your fan on the roof and enable faster ventilation. Whether or there are ducts or not, the design of the grow room is necessary to consider while selecting the fan.

Insulation. Exposure to natural sunlight is a critical factor that determines fan size and ideal CFM. This means that you have to consider where your to grow room gets situated. Whether it is in the basement, attic, balcony, or in an open space.

Insulation determines the ideal CFM to a large extent. If your grow room is warm, then the CFM will increase by 20 percent. At the same time, if the temperature is low, the CFM is set to decrease by 15 percent. For small spaces, insulation might not seem like a big issue. But, even with the difference of 10 percent in CFM, it still impacts the performance of a fan.

Carbon Filter Presence. The carbon filter can reduce the productivity of a fan by a quarter. This 25 percent loss in productivity can impact the growth of plants. It also affects the cultivation quality of your grow room. When the air goes through a carbon filter, its speed gets a significant reduction.

So, from the above-determining factors, we will conduct calculations like:

With an insulation factor, you will multiply it with + or – 25%. Then, use the carbon filter factor + or – 25%, ducting factor (add 10% for every 10 feet), and light factor (add 10% every 1000 watt). The result will be your CFM.

So for the above-given example. With 500 Volume of room and + 25 percent of Carbon Filter, you will have 625, and +25 percent. More so, it will be around 800 CMF, then for ducting factor’s 10 percent for everything 10 feet, you will have 880 CMF. If it’s a 1000 watt light, an extra 10%, and your result will close to 1,000.

Thus, you will need a fan with CMF of around 1,000 to exhaust air within one minute in a 10 x 10 x 5 feet grow room.

Important notes for calculation:

carbon filtersIt is important to note that one can not apply this formula straight away in all situations. One needs to tweak it according to the existing factors in play. For example, if you are not using any carbon filters, then it will get excluded from the formula. If the design of your grow room does not have any duct style, the ducting factor will not be a part of any calculations. One needs to adopt this formula with the characteristics and the features available.

Another factor while doing these calculations is the standard of conversion. In European markets, the standard format for the measurement of grow rooms is Cubic Meters Per Hour (M3/H).

While in North American markets, the extractor fans get measured in Cubic Feet Per Minute. You should keep these things in mind while you do your calculations or ask for them.

Features of a Good Grow Room Fan 

There are many options for a grow room fan available in the market these days. It can be hard to pick a fan that is suitable for a plant cultivation room. To help you with the decision making in picking the right fan for grow room, here are some of the factors:

Lightweight and Durable. The first sign of a good ventilating fan is in its construction material. You should buy a fan that is not only lightweight but, also very durable. This is a rare combination. A lightweight fan will run smooth and does not make much noise. It is easy to assemble and repair.

At the same time, most lightweight fans are not well built and do not last long. So to find a fan that is lightweight and yet durable is a big deal. One must buy a fan that is water-resistant and rust-free. These two features, in turn, make it a very low maintenance fan.

Reliable Motor and Energy Efficient. Another sign of a fan that works well in grow rooms is the one with controllable motor settings. A good fan should have a motor energy consumption of 400 watts to 120 watts.

Since you will be using a fan a lot in a greenhouse, it is cost-effective to have a fan that does not consume much energy. This way you will be saving a lot of money on your electricity bill. The fan’s motor should have features like thermal overload safety and overheating auto-cut.

Warranty. Having a warranty is a sure shot sign that the fan is of high-quality. This ensures the buyer that the company will take care of all manufacturing errors. It also protects you from damages and in-built errors that may result in a major loss of money in repairs.

A good warranty period for a grow room fan should be at least one year. Also, it must cover manufacturing defects under it. The longer and wider the warranty coverage, the better your fan is. Additionally, it is also desirable that the product has a 30 days return and refund option. This way, you can figure out if the fan aligns with your needs or not.

Have Safety Certification. A good grow room fan must come with safety certification. Every brand claims that their product is the best in the market. But, without authorized certification, their claims mean nothing. So, you must buy equipment like a fan that comes with verified certification. Your fan manufacturer should be the Air Movement and Control Association International Certified.

Grow room fans are not like your common table fans. They run on high electricity and take a lot of power. Thus, they should have industry-grade clearances for the safety of your grow room. A faulty fan can lead to accidents, fire hazards, and might cause harm. It is worth spending money on a fan that comes with all the necessary certifications. More worthy than buying a fan that does not have any safety or quality checks.

Noise Reduction. A good fan must have low noise reduction quality. It is not easy to work in a noisy, grow room for a long time. A sign of a good fan lies in how smooth it works without making any noise. If a fan makes a lot of noise, then its a sign that its construction is not smooth enough.

Some of the best fans are the ones that have quiet operations. This feature may not get advertised well. You can ask the manufacturers how much sound the product makes during its operations. Low humming is normal in every grow room fan, but buzzing or squeaks are not.

A noisy fan is not a good option, and you should definitely go for the product that has silent operations. You might have a grow room in an apartment building or terrace. Then the noisy fan might cause annoyance to your neighbors or flatmates. While a silent fan will not cause any nuisance and you can work in your grow room without any disturbances.

Easy to Install. A reliable grow room fan is easy to install. You should not be dependent on anyone to assemble and install your fans. In this DIY world, you should opt for a fan that you can put together and install on your own. For industrial-grade fans, it makes sense to have a professional team do the assembling of the fan for you. But, for a small or medium size grow room, you should be able to do it yourself.

A DIY fan helps you save money that you will otherwise have to pay to technicians. Also, when you install equipment by yourself, you will understand its structure well. This will later help you in maintaining the same equipment.

Why People Are Moving To Grow Room Farming

Before, growing plants meant cultivating them in an open environment. They get direct exposure from sunlight, air, and other elements. Over time, there have been many factors that prevented people from open-air cultivation. There are many reasons for the decline in open-air farming. Many are now opting for laboratory-controlled farming. Some of these reasons are:

Climate Change. One of the main reasons for the popularity of grow rooms got attributed to climate change. Nowadays, there are unpredictable weather patterns and the growing frequency of bad weather. That is why people are cultivating their produce in controlled environments.

Climate change has also made the natural atmosphere inhospitable for growing healthy crops. The rains, heavy sunlight, and hasty winds often ruin crops before the harvest. This has led to a major loss. So, it makes more sense for farmers and growers to grow plants in a controlled environment. Their plants will get protected against harsh natural elements.

Lack of Land. Another reason is the shrinking availability of open space. With a growing population, the availability of cultivable land has reduced. Other than that, many people in cities have started urban gardens and plantations. Well, it is not possible to own farmland in cities. So, people opt for grow rooms that they can have in their terraces or balconies.

Grow rooms are easy to make on open terraces and help in growing everything from flowers to herbs. Some people have interests in urban gardening but, live in cities with limited spaces. They use portable garden beds that they fit in makeshift glasshouses.

Ease of cultivation. Unlike growing plants in an open farm, growing plants in a grow room is very easy. One does not have to worry about the plants getting ruined or uprooted from animals or elements. It is very easy to grow even the complicated plants in controlled settings. Also, you don’t have to be an experienced farmer to run a grow room.

It is important to have knowledge about things like CFM’s role in Fan Size. Thus, it is very easy to operate in nurseries and greenhouses. This is one of the major reasons why people are opting for artificial cultivation more.


In this article, we can say that the process of deciding the grow room fan size based on CFMs is too complicated. It requires more than one factor to determine the apt fan size. Even the process of determining the factors, such as room volume and duct design, is very hard to figure out.

Knowing the correct fan size in relation to CFM is important. Only if you want to have a healthy and functional grow room. We hope this article turned out to be helpful on how to calculate CFM for a grow room. We are hoping you will not have to rely on any company to dictate which fan you should pick for your grow room.


Gary Stephen
Gary Stephen

Hi, I am Gary Stephen. I have been gardening for a long time. Gardening gives us much benefits and pleasure. Many of us desire to have a garden, but it cannot fulfill the desire for the proper guide or instruction of gardening. So, I am eager to help them. For this purpose, I have developed the website to make a garden with the proper guide. So, you will get me beside you if you want to know anything about gardening.