Calculating Your Grow Room Fan Size

How to Calculate Your Grow Room / tent's Fan Size

Calculating the fan size needed for your grow room or grow tent is a relatively straight-forward process. However before we get into those calculations let’s take a brief look at the concepts that make up the need for having a correctly-sized extraction and intake fan in the first place.

First of all having the correct fan size is something that beginning growers tend to overlook. This can lead to an insufficient ventilation issue which causes problems during the entire growing and flowering cycle. 

Here's why. 

As you may know plants survive on CO2 (carbon dioxide) which they extract from the air around them. When they’re first beginning to develop, the smaller-sized plants obviously don’t need all that much CO2 since they’re not supporting a large number of leaves/flowers. But as the plants grow the oxygen requirement increase drastically. 

Consider for example, the total area of your plant’s leaf surface. This is where your plant’s photosynthesis and transpiration takes place in order to provide energy to the plant. Your young plants may only have a total leaf surface area of, let’s say, 50cm2. But in a short period of time – maybe 30 days - your plants total leaf surface may get up to 10,000cm2 or greater! And this is just a conservative estimate. So in essence, when your plants are in the early stages of their growing cycle, taking good care of them is a relatively easy task in regards to providing enough CO2. But...

In a matter of time the plants hit a certain size where optimal plant growth and development is hindered by not having enough CO2.

This is why you’ll hear all too often that newbie growers report that their growing efforts went just fine until things suddenly changed at a particular point. At that point, their plants stopped producing as well as they did in their earlier stages of growth. This is where the indoor growers use two methods in order to fix this problem.

Better ventilation – Venting the old CO2 - heavy air out of the growing area and replacing it with fresh air.

Cooling and Supplementing - Room cooling with A.C. and adding more CO2 (with a burner or tanks of CO2).  

Performing the calculations


Ensure you work out the correct calculations whether your working from European or American measurements. 

Step 2

Perform the calculations found below. 

Step 3

Look at the fan's specs before buying and ensure the CFM is enough for your grow room or grow tent. 

To begin,  firstly you’ll need to do the math that’s involved with the calculation of the amount of air in your growing area. The first step is to measure your growing room floor space (the actual plant occupation area) and then multiplying that floor space by the height of the ceiling. 

First, a quick note regarding measurement standards. There are two types, European and American. Below are the workings for both:

  • Europeans use the standards Cubic Meters per Hour (M3/H) when measuring their extractor fan size.
  • In North America, extractor fan size is measured in Cubic Feet per Minute.

So you start by calculating the volume of your growing area by simply multiplying the length of the growing area times (X) the width of the growing area times (X) the height of the growing area.

For example if your grow room is 2 meters X 2 meters with a ceiling height of 2.1 meters you’re calculations should look like this:

European: 2X2X2.1 = 8.4

American: *1 meter = 6.56 feet ; therefore: 6.56X6.56X6.88 = 296.07

Then, the extractor fan size for Europeans (M3/H) is measured by: (the active growing space as worked out above in meters) 8.4 X 60 (minutes) x 1.33 = 607.32

And for North America, the extractor fan size (CFM) is measured by: (the active growing space as worked out above in feet) 296.07 x 1.33 = 393.77

The importance of these calculations

To understand the importance of these calculations, consider this. To keep your plants at optimal growing levels you’ll need to replenish all of the air in the growing room each minute! This is basically the standard for all grow rooms that don’t have the benefit of supplementary CO2 or A.C.

As you can see, the calculations are relatively simple with just a slight difference between the calculation method for European measurements and North American measurements. But what about that times 1.33?

Well the times 1.33 is included to allow for the fact that there is a carbon filter that is attached to your extractor fan. Having a carbon filter attached to the fan drops the extractor fan’s efficiency by about 25%. Of course, this is an approximation and doesn’t take into consideration such as: the filter’s age, the fan’s manufacturer, etc. Therefore, that drop in efficiency is compensated by multiplying by 1.33.

A Calculation Example

Here is a simple example of an indoor garden fan’s calculation. We’ll use the North American standard for this one as that’s more commonly used:

The Formula: Needed Fan size (CFM) = (The Volume of the Active Growing Space) x 1.33

Numbers involved: length of floor X width of floor X height to ceiling X 1.33

Example for a 10 meter X 10 meter  X 9.5 meters grow room: 10 x 10 x 9.5 x 1.33 = 1263.5 CFM

CFM (Cubic Feet per Minute)

Once you've worked out your CFM it represents the minimum size of the fan extractor that you’ll need to obtain for this particular growing area. This is especially true if you’re using a growing area that is well-insulated (for example, a basement area or grow tent). For areas that are exposed to more sunlight you will also generally require a fan that is about 25% stronger.

Of course, you will want to match the size of your calculations to the nearest available size fan. It’s usually a better idea to measure slightly up, rather than down.

Air that comes in

Since we’ve explained the need to have air circulated in your growing area, we should also touch upon getting new, fresh air into your growing area. There are two common ways to look at this task.

An active air intake consists of a smaller fan that will draw in more fresh air into your garden area. This will assist you in placing less strain on your extraction fan. It should be noted that your will want to connect a filter to the fan, in order to prevent insects from entering your growing area.

A passive air intake will cause the growing area room to have a drop in air pressure. The result is that fresh air is drawn in from any crack and crevice. The greater the number and size of these gaps the greater the amount of new air that is drawn in. The benefit of a passive air intake solution is that it merely consists of holes in your growing room that lead to the outside, therefore there’s no cost involved. However, you’ll want to have a bug mesh that covers each intake hole.

Active air intakes are typically smaller inline extraction fans that blow fresh air into the garden, rather than relying on air pressure. This is preferred by many growers because it puts less strain on the extraction fan and actually helps it to work more efficiently. These are some ways to tackle the necessary task of ensuring that your growing area remains at an optimal level regarding CO2 content. When this is accomplished, your garden will produce far better and your plants will remain healthier.

Calculating Carbon Filter Size

Once you've worked out the size of your fans you can then simply find the same sized carbon filter. For example if your grow room needs a 6 inch fan you just need to buy a 6 inch carbon filter. Then simply connect the carbon filter to the extraction fan within the grow room. This way the air that is being taken out of the grow room passes through the carbon filter before it's final journey down the ducting. The air then reaches outside clean, fresh and odor free.  

Still Confused? 

Indoor Grow Room Ventilation Systems


Here at the Hippie House we have dedicated indoor growing experts who have years of experience in hydroponics and the horticultural industry. We stock over 5000 indoor gardening products including a large range of ventilation fans, carbon filters, ventilation kits, humidifiers, aluminum and insulated ducting and a range of other environmental control equipment

Our team are always here to help so if you have any questions or are confused in which fan size to buy - or any other environmental control issues click below to get in touch. 


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