Grow Room Fan Size Calculator with Size-wise CFM

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Different sizes of Grow tent will require different sizes of fans. But that’s not all. There are other critical factors that you need to consider when calculating the CFM of your grow tent.

In this article, we will share with a simple procedure to calculate the size of your fan under every circumstance.

Shall we begin?

Factor to Consider When Calculating Grow Room Fan Size

To get the calculation right for you need to consider some critical factors that would determine the need for your tent ventilation. You need to think about factors that contribute to the needs of airflow in your grow tent.

Understand that for every Grow area; you need to get a fan with a CFM that can accommodate and sustain the conditions of your tent. That’s why it’s important to consider all the conditions that will give your plants the best condition for their growth.

The following are major factors that you need to consider in your calculations.

I. Grow Room Volume

Grow Room VolumeWithout any doubt, this is the first factor to consider. It the total space and volume that your fan has to deal with. It’s all space inside your grow room.

When you buy any fan, it needs to replace all air inside the Grow room. At the same time, it needs to replace it with fresh air from outside. And most importantly, your fan will be responsible for even circulation of air within your Grow area.

So, to get the best out of it, you will need to have a fan that can adequately circulate this air. Its CFM shouldn’t be too low in a way that it can’t circulate or supply air that covers the entire Grow room.

At the same time, the CFM of your grow fan shouldn’t be more as compared to the total volume of your Grow area. If this is the case, it will negatively affect your plants.

Calculating CFM by Volume

To get the CFM of your fan, use the dimensions of the Grow area. Multiply the height, width and length of the tent to get the total volume.

That is:

CFM = Height x Width x Length

The total volume of the tent should be the same as the CFM of your fan.

Note that most fans are measured in Cubic Feet Per Minute (CFM). For that reason, its advisable to measure the dimensions of your tent using ‘Feet’ measurements instead of using ‘meters.’


Peter, after measuring his Grow area he found out that the height was length was 5 feet, width 5 feet, and height 7 feet. The following were the calculations of the total volume for his fan CFM.

CFM = 5 Feet x 5 Feet x 7 Feet

Total = 175 CFM

Use the same formula to calculate the CFM of your grow space.

However, that shouldn’t be the only factor to consider. In the following points, we will discuss other factors that need to be considered when choosing a CFM for a grow tent.

II. Ducting Design and Length

Ducting Design and LengthThis should be the second factor to consider after the volume. Ductwork will determine the volume of air that can pass through them at any particular time.

Ducting curves and length will affect your fan capacity. It’s, therefore, important to consider them before you pick any fan.

Generally, you will find the following Ducting designs in most Grow tents.

Long ducting runs: These have a length of between 10 to 20 feet and around 3 to 6 bends.

Medium Ducting: You can expect a medium length of between 5 to 10 feet with 2 to 3 bends.

Minimal Design: They don’t have any ductwork hence no bends over them. It’s just an opening where you place your fan directly to ventilate your Grow tent.

Calculation of CFM by Ductwork

You need to note that for every 1 foot of ducting length, you will need an additional 1% of the fan CFM. When you think about the curves, you consider the angle. For every 90 degree angles of curves, you will need an additional 20% of fan CFM.

In short, if you have a tent with ductworks of 5 feet and curves of 90 degrees, you will need a total of 25% additional CFM to your fan.


Jones has a Grow tent with 200 CFM volume. It has 3 ducting beds of 90 degrees and a total length of 15 feet.

Here are the calculations of the total CFM he requires for his tent.

By Ducting;

Total CFM = ( 200 CFM + (200×15%) + (200 x 60%)) = 350 CFM

Note: 15% comes from the 15 feet. The 60% comes from the three curves where every curve adds 20% of total CFM.

You can use the same formula to calculate the additional volume you need in your tent. If you are not sure about the CFM needed for your tent volume, use the formula we discussed above to calculate it.

III. Air Requirement

Before you settle on any CFM, you need first to understand the amount of air you would be required to supply to your Grow area per minute. As we earlier mentioned, CFM is the volume that your fan will supply your Grow area within a minute.

These would be a matter of understanding how many minutes you would want to replace the entire air from your Grow tent.

Calculating CFM by Air Requirement

To calculate the CFM of your fan based on the air requirement, you need to get the total volume of the air in your area and divide it with the number of minutes you would wish to replace it with.

For example:

Peter had calculated his total volume and got 175 CFM. If he wishes to replace the entire air withing 5 Minutes or 1 minute, he can calculate the CFM easily.

5 Minutes

Total CFM = 175 cubic feet / 5 minutes

Total = 35 CFM

1 Minute

175 cubic feet / 1 minute

Total = 175 CFM

Use the same formula to calculate the CFM you will need for your fan depending on the volume of your tent and divide it with the minutes you would wish to replace the entire air in your Grow tent.

IV. Grow Room Insulation

This is a critical factor to consider if your Grow room is in areas exposed to sun or less-insulated. These include growing rooms places upstairs, basement, attic, or rooms facing south.

However, if your Grow is is well insulated, you don’t have to consider this factor. Rooms that don’t experience much heat exchange from its surroundings are fine.

The idea is to get a CFM that would maintain an optimum temperature in your Grow room.

Calculating CFM By Room Insulation

If your Grow room is warm than required, get a fan with 20% extra CFM. But for the case where the temperatures are low, get a fan with 15% less than the total CFM.

Example if your CFM is 200, then;

For warm rooms = (200 + (200×20%)) = 240 CFM

For Cold rooms = (200 – (200×15%)) = 170 CFM

Use the same formula to calculate CFM for different volumes. The main idea is to get a fan with a CFM that would give your grow tent the best volume.

V. Carbon Air Filter

A carbon filter drops the efficiency of fan in extracting air from a grow tent. When air is passed through a carbon filter, its speed reduces to a certain level.

The role of carbon filter in a grow room is to purify the air and remove any impurities before they are passed to the air.

To learn and understand why you need a carbon filter, read this article on Carbon filters for a Grow Tent.

Calculating CFM by Carbon Air Filter

Carbon filter reduces the level of air extraction from your Grow tent by 25%. So, if you want to get the best from your fan, add 25% to the total CFM.

For example:

Duke has a Grow size volume of 400 CFM. He will, therefore, need to buy a fan of 500 CFM.

That is;

Total CFM = (400 CFM + (400 x 25%))

Total CFM = 500 CFM

VI. Heat Sources

Electrical appliances in your grow tent a common source of excess heat in your grow tent. Some of these appliances include;

  • lights,
  • humidifiers, and

If you don’t have coolers in your grow tent, then you would need to factor the heat produced in your Grow tent by these appliances. You have to estimate the amount of heat you are experiencing from your Grow tent.

Lights are the major culprit when it comes to heat production. The best way to deal with it is to have lighting systems that have correct wattage for your Grow tent.

To learn how to calculate Wattage for your Grow Tent, read our article on how to calculate Wattage for Grow tent.

Calculating CFM by the Heat Sources

Its recommended that for every heat source in your Grow tent to add 10% of CFM. This will help in removing the excess heat that you will experience in your Grow room.

If you have lights of 1000W and are not cool, in a 200 CFM Grow area, calculate as follows.

200 CFM + (200×10%) = 220 CFM

Also, remember to multiply the percentage with the number of lights you have. That will help you get the right CFM for your Grow tent.

Grow Room Fan Size

Procedure for Calculating Total CFM for Your Grow Tent

We have already discussed different things that you need to consider when calculating the CFM for your Grow tent. It’s important to ensure that you add the CFM of every factor that affects your Grow tent.

Don’t add all the factors. Only add that matters in your case.

For instance, if you don’t have air filters in your Grow room, there is no need to add the CFM of a Carbon Air Filter when calculating the total CFM for your Grow tent.

The CFM should be calculated for Extraction Fan and Intake Fan.

Extraction Fan

These are fans responsible for removing air from the Grow tent. Follow the following simple steps to calculate the volume for an 8 feet by 8 feet by 7 feet.

Step 1: Calculate Volume of Grow Room

We already mentioned that the following formula calculates the volume.

Width x Length x Height

In this case, the Volume will be:

Volume = 8 feet x 8 feet x 7 feet = 448 Cubic Feet

Note this is the total volume occupied by the air in your Grow room.

Step 2: Divide Volume with Air Replacement Time

The formula for getting CFM is taking the total volume then divide it with the number of minutes you desire to remove the air from the Grow room. If you want to remove the entire air within one minute, the following will be a solution for your tent.

CFM = 448 Cubit feet ÷ 1 minute

Total = 448 CFM (Cubic Feet per Minute)

Step 3: Add Carbon Filter Requirement

We mentioned that a carbon filter would reduce the efficiency of your fan by 25%. To get the best operation rate you need to add 25% to the total CFM of your tent.


Fan CFM = 448 CFM + (448×25%)

Fan CFM = 560 CFM

Step 4: Add or Minus the Insulation Effect

When there is excess heat, add 20% of the CFM to the fan. And when the room is cooler remove 15% from the fan CFM.


Cooler Room

Fan Size = 560 CFM – (560 x15) = 476 CFM

Warmer Room

Fan Size = 560 CFM + (560 x 20%) = 672

Then from here, take the volume that is appropriate to your Grow tent. To avoid confusion, we will proceed with the volume of a cooler room.

Step 5: Add the Volume of Ductwork Effect

Most 8 by 8 grow tents have a 10 feet ducting length with 2 bends of 90 degrees. Every foot adds 1% of volume while the bends add 20% of the total CFM.


Fan size = 476 CFM + (476 x 10%) + (476 x 40%) = 714 CFM

Step 6: Heat Source

Every 1000 watts is supposed to raise the volume by 10%. However, if you have cooled appliances, this step won’t be applicable (our recommendation).

But if you don’t have cooled lights, add 10% of CFM.

Fan Size: 714 CFM + (714 x 10%) = 785.40 CFM

In the end, if all the above factors apply to your Grow area, you will need a Fan that can deliver 785.40 CFM.

The formula for Calculating Fan Size for an Intake Fan

The intake fan should be 15% less powerful than the extraction fan. So, to get the fan size, minus 15% from the total volume of your Grow Fan.

Best Fans For Your Grow Tents

The bottom line is the fan you buy at the end of the day. You might calculate all the sizes well, but if you pick a fan that doesn’t give you the best performance, it will be useless. This is why it’s important after you have calculated the size of your fan, you consider buying a superior fan.

After research, we have found the following Grow tent fans that will give you an amazing performance. Go ahead and buy either of these funs, and you will notice the difference.

AC Infinity Cloudline S6 (Our Top Pick)

The fan is perfect for a mid-sized grow tent. It achieves an airflow of 351 CFM.

Key Features

  • Ideal strength for a mid-size room
  • Extremely quiet
  • The variable setting for efficient power consumption

The fan is available on Amazon. Click here to check the latest price and buy it today.

TJERNLUND M-67 Inline Duct Booster Fan

The fan attains a power output of 460 CFM. This means it can run for a long time without changing its performance.

Key Features

  • Made with strong and high-end materials
  • Efficient blade design to blog a huge volume of air
  • Super quiet fan

The fan is available on Amazon. Click here to check the latest price and buy it from Amazon.

Vortex 347 CFM S Line S-600 Fan 6”

With it, you will plant everything you would wish to plant in your tent or indoors.

Key Features

  • Mounts easily
  • Convenient with different speed controllers
  • A super quietest tent grow fan even at the highest speed

The fan is available on Amazon. Click here to check the latest price and buy the fan from Amazon today.

Hurricane Inline Fan

This makes it a perfect choice for anyone with a small grow area.

Key Features

  • Durable construction
  • Super Quiet Fan
  • Ease of installation

The fan is available on Amazon. Click here to check the latest price and buy it from Amazon today.

Terrabloom 10” Inline Duct Fan

It delivers a high airflow of up to 1065 CFM. At the same time, it’s cost-effective.

Key Features

  • Runs optimally for a longer period
  • Ultra-quiet fan
  • A powerful CFM coverage with a low energy consumption

The fan is available in the market. Click here to check the latest price and buy the fan today.

Parting Shot

It’s that simple. Nothing complicated. Now you can use our easy-to-follow procedure to calculate the size of your fan. But most importantly, buy either of the fans we have recommended in this article.

It will be useless for you to spend much time calculating the size of your fan only to buy a fan that can’t give you superior performance.