One of the tricky questions growers have to answer nowadays is what is the right fan size for grow tents. If you are one of such growers, you are at the right place. Let’s go through the process of finding what your grow tent fan requirements are and how to choose the best fan that meets such requirements.

Calculate The Base Fan Requirements For Your Grow Tent

Every grow tent has its peculiarities, particularly the airflow or volume. You can only choose the fan size for grow tents if you know how much air your tent needs to ensure the optimal growth of your cannabis plants. This makes the airflow, which is also the total volume of your grow room, the most important piece of the puzzle. 

Calculating The BASIC Airflow Of Your Grow Tent Is Easy

Every grow tent has its peculiarities, particularly the airflow or volume. You can only choose the fan size for grow tents if you know how much air your tent needs to ensure the optimal growth of your cannabis plants. This makes the airflow, which is also the total volume of your grow room, the most important piece of the puzzle. 

Contrary to popular belief, it is pretty straightforward to calculate the basic airflow of your grow tent. But before we get to these simple calculations, it is essential to mention that the airflow of a grow tent is calculated in Cubic Feet per Minute (cfm). This is due to two reasons. 

First, the grow tent fans you will find in the United States are rated in cubic feet per minute, and second, the volume of a grow tent is the same as the amount of wind that needs to be replaced in the tent in a minute. 

For the calculations, all you have to do is multiply the width by the length and by the height. Mathematically, we have;

Volume (airflow) of grow tent = width x length x height

So, for example, if your grow tent has a width of 4 ft, a length of 4 ft, and a height of 6.5 ft, the total volume would be 104 ft³. Now, let’s show the workings using the formula above;

Volume of grow tent = 4 (width) x 4 (length) x 6.5 (height) = 104 ft³

This means you would need grow tent fans rated 104 cfm to replace all the wind/air in your grow tent in a minute. While that is ideal, you can schedule the air replacement once every two or three minutes if the heat is not serious. 

We have now calculated the basic airflow, but is that all? No, there is more.

mars hydro fan

Factors Affecting The Effectiveness Of Grow Tent Fans

It doesn’t end with calculating and knowing the airflow of your grow tent fans. All we have now is the base fan capacity you need. Unfortunately, we cannot work with this because your grow tent setup often comprises additional components that limit the ability of the fan to circulate air effectively. So, if we get a fan rated 104 cfm, it will struggle to serve our 4 x 4 x 6.5 grow tent. 

So, what do we do? First, we factor in these other grow tent requirements or components that limit the fan’s performance by increasing the capacity we work with. However, the exact increment depends on a few factors, including:

Lights: Lights will reduce how effective your grow fan is. To avoid this, we add 10% to our estimated airflow for each grow light in the setup. 

Filters: If you have filters in your grow tent set up, it is recommended to add 25% to the total estimated airflow per filter. 

Ducting: Ducting is another limiting factor – it widens the distance the air from the fan has to travel through to reach its destination. To correct this, we increase our fan’s cfm by 1% for every 1 foot of ducting. 

Bend in ducting: If the ducting has bends, we add 30% for every 90-degree bend or 15% for every 45-degree bend.  

External heat: External heat comes into play if your tent is situated in warmer areas like the attic or similar places where there is no insulation from the external environment. A 10% increase will correct this. But remember, increase the airflow by 10% or not depending on the temperature.

For our hypothetical grow tent of 4 ft x 4 ft x 6.5 ft, we are making the following assumptions;

  • there is one grow light – 10%
  • there is one filter – 25% 
  • there is 5 feet of ducting (5%) with one 90-degree bend (30%) – 35%

Based on these assumptions, the total percentage increase would be:

10% + 25% + 35% = 70%.

Then we convert the percentage to decimal and add it to 1.0 (a constant): 

70%/100 = 0.7 + 1.0 (constant) = 1.7

Multiply 1.7 by the fan base capacity we calculated earlier (104 cfm) to get the eventual capacity required, which would be 176.8 cfm in our own case. 
We can express this as: 

Fan Capacity Needed = Base Fan Capacity X Multiplier


Therefore, to calculate the air volume required for your tent, you need to follow these steps:

Step 1. Calculate the volume of the tent

Step 2. Select the factors that will affect the efficiency of the fan and add their values, converting them to decimals and adding 1.

Step 3. Multiply the volume of the tent obtained in the first step by the value obtained in the second step.

Step 4. 1 plus 25% (extra air volume), converted to decimals and multiplied by value in step 3

Step 5. Finally, according to the value you get choose the appropriate size fan. Less than 205 choose a four-inch fan, between 205-402 choose a six-inch fan.

Factors Affecting The Effectiveness Of Grow Tent Fans

Buy Fans for Grow Tents With More Capacity Than Required

Now you are wondering why we have to go beyond the estimated fan capacity needed after our Here is why: getting a grow tent fan rated 104 cfm for our 4 ft x 4 ft x 6.5 ft grow house means we have to run the fan at full capacity to ensure optimal airflow. Unfortunately, this will strain the fan and increase the operational noise. 

Conversely, if the fan runs below its maximum speed, it runs quietly with fewer strains. You can also increase capacity if you need to up your cooling requirements later. We recommend adding 25% to the estimated fan capacity required to arrive at the ideal fan capacity. 
So, the estimated fan capacity multiplied by 1.25 (25% of estimated fan capacity) gives us the ideal fan capacity. For our hypothetical grow tent, the fan requirement would be:

176.8 cfm x 1.25 = 221 cfm

According to the result we get, you can choose the size of six-inch fan, which has an airflow of 402 CFM. However, the airflow you really need will be selected according to the influencing factors.


Fan Requirement Table For Common Grow Tent Sizes

Perhaps you do not have time to go through the calculation process above? Then, you can consult the table below to know the exact fan requirement for the common grow tent sizes around


  • For grow tents smaller than 5x5, we have set up ONE light and ONE filter.
  • For 4x8 grow tents and 5x10 grow tents, there are TWO grow lights and ONE filter prepared.
  • For 8x8 grow tents, there are FOUR grow lights and TWO filters.
  • 25% extra capacity has been added to make sure the values above are the ideal fan capacity.

Tent Size

(Width X Length X Height)

Add 5 Feet Duct

Add 5 Feet Duct & 90° Bending


Add External Heat


Recommended Fan Size

2x2x32125.5274'' Fan
2x2x4.531.538.340.54'' Fan
2.3x2.3x5.34959.5634'' Fan
2x4x5.982.6100.3106.24'' Fan
3.3x3.3x5.9112.5136.6144.74'' Fan
4x4x6.51822212346'' Fan
5x5x6.5284.4345.3365.66'' Fan
4x8x6.54686246502*4'' Fan
5x10x6.5731.39751015.62*6'' Fan
8x8x6.51040135214044*6'' Fan


Conclusion: For airflow below 205 CFM, we recommend Mars Hydro 4" fans. For airflow below 402 CFM, we recommend Mars Hydro 6" fans. For those above 402 CFM, we recommend using multiple fans in your tent.

Buy The Good Fan For Your Grow Tent

Many fans on the market dazzle you, making it difficult to make a purchase decision. Be careful, or you will buy a poor quality fan, such as a noisy one with low airflow. If you want to buy a fan at a price that is proportional to its quality, Mars Hydro fans are your best choice.
Mars Hydro fans are loved by many people. They may not be the best, but they can make you feel like you are getting more than your money's worth. Mars Hydro fans come in two sizes:

Now you just need to make a strategic estimate for choosing the right fan for your grow tent, as shown in this article. Except you enjoy the math, we advise you to check through the included table – you will most likely find your tent size. This will save you minutes of calculation.

Mars Hydro inline fan