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Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions

Ventilation for grow tent starts with choosing grow room fans and making CFM calculations. Learn how to do it like a pro
07 June 2022
16 min read
Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions

  • 1. Cannabis ventilation guide
  • 1. a. Does cannabis need ventilation?
  • 1. b. Cannabis ventilation at night
  • 2. Ventilation for grow tent: how does it work?
  • 3. How much ventilation does a grow tent need?
  • 3. a. Cfm grow tent calculator
  • 3. b. Cfm for some standard tents
  • 3. c. When fan flow rate is specified in metric units (not cfm)
  • 3. d. Want to skip cfm altogether and use only metric units?
  • 3. e. A calculation shortcut for hps users
  • 4. Grow room ventilation setup
  • 5. Components for ventilating a grow tent
  • 5. a. Exhaust fans / extractor fans
  • 5. b. Vent ducts/pipes
  • 5. c. Oscillating fans
  • 5. d. Carbon filter
  • 5. e. Intake fan
  • 6. If you use an hps with a cool tube or a cool hood
  • 7. Taking grow tent equipment into consideration
  • 7. a. Factoring in grow equipment
  • 7. b. Managing grow space noise
  • 8. In conclusion

Ventilating a grow room is one of those things that separate success from failure. Ventilation may be not as crucial as lights, nutrients, or genetics, but still a very important aspect of growing weed plants!

A simple ventilation for grow tent or any other grow room consists of an exhaust fan, a duct, and a carbon filter. Online stores often sell complete grow tent ventilation kits. But you can also build a homemade ventilation system to save some money. We’ll teach you how.

1. Cannabis Ventilation Guide

In this guide, we’ll share with you some tips on how to build a proper grow tent ventilation and make sure it serves its many purposes. And, in case you’re wondering, growing weed without fans is hardly a good idea.

Does Cannabis Need Ventilation?

Ventilating a grow tent is definitely important, and for many reasons.

  • Photosynthesis

First of all, cannabis plants need a constant supply of fresh air for photosynthesis. Photosynthesis is a process in which green plants take CO2 and water, apply energy that they receive from the sun or lights, and convert all this to plant matter. Thus, CO2 is actually food for plants. Your goal as a grower is to provide plenty of food every day. And grow tent ventilation is one way to do it.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Fan grow room

Large operations equipped with HID lights require very serious ventilation.

  • Keeping temperature down

Second, indoor grow lights generate a lot of heat. Marijuana doesn’t like too hot environments. It thrives with temperatures of up to 85°F (30°C), but then shows the symptoms of heat stress and basically stops developing. Apart from installing a grow room air conditioner, venting a grow tent is the only way to keep the temperature down.

  • Humidity control

Grow room ventilation also keeps the relative humidity in check. This is especially important during the flowering stage when buds begin to fill out and become susceptible to mold. Novice growers seldom realize it, but mold and bud rot are very common problems.


Remember that a lot of water evaporates from your soil, or coco, or other medium. Even more, humidity is released into the air by plants themselves. This process is called transpiration. An exhaust fan for grow room takes care of all that excess moisture.

  • Pests prevention

Hot and humid environments also attract various pests. Spider mites, for example, thrive and multiply in great numbers when the air is hot, humid, and stale. If you don’t want to lose your whole crop, set up a good ventilation system.

Cannabis Ventilation At Night

The grow room fan schedule can be the same as your light cycle because ventilating a grow room is really necessary only during the day. At night, the photosynthesis stops, and lights are off and generate no heat. So, no ventilation is necessary. As for the oscillating fans, you can leave them on. They will move the air around and thus decrease the risk of mold or other infestations.

2. Ventilation For Grow Tent: How Does It Work?

Some beginner growers choose to cultivate only a plant or two in a closet or a small tent, don’t use powerful lights, and rely on passive ventilation. A passive ventilation system makes use of the fact that the hot air always rises. So you only need one vent in the bottom part of your grow space and another — in the upper part. When the air gets heated by the lights, it goes out the tent through the upper hole. This creates the negative pressure, and the cool air from outside enters the tent through the bottom hole. In the process, the fresh air not only cools down the tent from the inside, but also passes through the canopy and thus provides the plants with CO2.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Natural (passive) ventilation in grow room

Grow tent ventilation diagram.

And this is basically how ANY air ventilation works. In more sophisticated setups, various types of fans for grow rooms simply boost and speed up the natural exchange of air.

3. How Much Ventilation Does A Grow Tent Need?

This section contains a lot of numbers and a few tables, but don’t be put off by them. The basic formula is very simple. When venting a grow tent, the first thing to consider is the amount of CO2 your weed plants need. It’s best to think of ventilation like this: how often to replace the whole volume of air in your grow space? If you browse through cannabis forums, you may come across numbers like every 3-5 minutes. It could be enough, especially with weaker lights, but cheap ventilation may lead to smaller yields. So for our grow room ventilation calculator, we’ll use a more serious approach.

Most cannabis professionals agree that it’s best to push this number to at least once every minute.

This will be our baseline number and will give your plants enough CO2 for photosynthesis. However, we should also think about the other function of ventilation in a grow room: the removal of excess heat and humidity.

Besides, most growers use carbon filters and ducts of various lengths. They will greatly decrease the efficiency of your weed fans. So, to account for all this, multiply the baseline number by 3 or 4. Just to be on the safe side.

And now let’s calculate.

CFM Grow Tent Calculator

To calculate the fan ratio for weed grow, we first take the measurements of the tent—length, width, and height—and multiply them to get the volume. We do the calculation in feet to eventually get the fan ratio in cubic feet per minute, or CFM.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation grow room calculator

Grow room ventilation calculator: convert dimensions (in feet) to CFM.

All acceptable fans should provide at least that much, so that there’s no shortage of CO2 for your plants. Then think whether you’ll be using a carbon filter (we recommend that you do) and a long duct pipe and whether your lights will run very hot or not. All that will affect both the needed capacity of your exhaust fan and its efficiency. As we said, multiply the baseline figure by a factor of 3 or 4, and you’ll be alright. Of course, there’s always a tradeoff. Let’s compare the advantages and drawbacks of choosing more powerful vs less powerful exhaust fans:


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation fans for grow rooms pros and cons

Should you choose a more powerful exhaust fan for ventilation?

We have looked at some commercially available solutions complete with grow tent ventilation kits to see what extractor fans the manufacturer has chosen.

Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - 4 inch grow tent ventilation kits fan CFM

Fan CFMs in some commercially available tents.

Please note that these setups use relatively cool-running LED lights. Imagine how much more heat you’ll get using the old-fashioned HID lights (HPS or MH).

So don’t be afraid to go a bit overboard with venting grow tent indoors.

CFM For Some Standard Tents

For your convenience, we have compiled a table showing the minimum CFM for some tent sizes that are available for sale online.


Size, inchesMin. CFM

Size, feet


Size, cm

Min. CFM


Table 1. Baseline CFM for some of the popular tent sizes.

Of course, grow tents can come in many more shapes and sizes, so you may find useful the following table that converts some popular dimensions in inches to feet.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Inches to feet conversion

Table 2. Some numbers in inches converted to feet.

Sometimes, the dimensions are specified in centimeters. It’s quite easy to convert them to feet, too:


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Centimeters to feet conversion

Table 3. Some dimensions in centimeters converted to feet.

Please don’t forget: When you’re done calculating the minimum CFM for your weed grow, multiply it by 3 or 4 to get a real-life figure.

When Fan Flow Rate Is Specified In Metric Units (Not CFM)

Depending on where your particular fan was manufactured (or sold), its flow rate may be given in metric units. So you can start with calculating how much CFM you need and then convert it to metric units using the following formula:

1 cfm = 0.0283 m3/min = 1.699 m3/h

Example: Your grow tent ventilation kit should provide 300 CFM. You’re looking at a 680 m/h3 grow tent extractor fan. Is it powerful enough? Multiply 300 by 1.699. You get 509.7 m3/h. So 680 m/h3 is more than enough.

Or, vice versa, you can start with the flow rate in metric units of a particular fan for grow room, then convert it to CFM, and see if it covers your ventilation needs. In this case, use the following formula:

1 m3/h = 0.589 cfm

Example: You’re looking at a 280 m/h3 grow tent extractor fan. How much is it in CFM? Multiply 280 by 0.589. You get 164.92 CFM. Compare this number to what you need.

Want To Skip CFM Altogether And Use Only Metric Units?

You don’t really have to convert everything to feet and cubic feet per minute. You can start with dimensions in meters and end up with m3/h just as easily. The formula is:

Length x Width x Height x 60min x 3 (or 4)

Example: You have a 100x100x200 tent. In meters, it’s 1x1x2. The volume is 2 m3. Multiply it by 60 minutes and then by a factor of 3 or 4. You get 360-480 m3/h.

A Calculation Shortcut For HPS Users

When using the old-fashioned HPS or MH lights, the most important function of grow room exhaust fans is to extract heat. Because you’re sure to get a lot of it. In this case, the fan flow rate should be equal to the wattage of the light.

So, if you use a 600W HPS, buy at least a 600 m3/h exhaust fan. In cubic feet per minute, it’s 600 x 0.589 = 353 CFM.

4. Grow Room Ventilation Setup

Using fans for grow room doesn’t change the general principle of ventilating grow room. Again, we should make sure that the air flows naturally from the bottom, through the canopy, toward the ceiling, and finally outside. Any fans simply facilitate this natural ventilation.

Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Grow room setup ventilation

Ventilation: Grow tent side view.

5. Components For Ventilating A Grow Tent

When drawing your ventilation grow room plans, don’t forget the following components (some are optional, see the diagram).


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation in grow room

Grow room ventilation diagram (top view)

Exhaust Fans / Extractor Fans

Let’s start with the simplest solutions, namely computer case fans. They are surprisingly efficient and can be used for different micro grows, like space buckets, PC case grows, and small closets with CFL lights. The drawback of case fans is that it’s difficult to use them with carbon filters (unless you make a DIY filter yourself). On the other hand, micro grows with just one small plant don’t really stink that much. Nothing a window ventilation can’t take care of.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilating grow room with case fans

The use of a case fan in a space bucket.

Case fans have different diameters: 80mm, 120mm, 140mm, 200mm. They use any common 12V power supply (odds are you have several of them at home). When choosing a high CFM case fan, pay attention to their noise level (DBA). The higher this number, the noisier the fan.



Table 4. Examples of PC case fans (manufactured by Delta).

As you can see from the table, bigger case fans are more efficient and less noisy. If you have a small grow box with rigid walls, rather than a grow tent, you can install a wall fan. They are also called bath fans or bathroom fans. This is a good option for those who want to buy their cheap ventilation kit locally and don’t have a hydro shop nearby.

Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Exhaust fan grow room, low CFM bath fans

Low-CFM wall fans are acceptable for small setups with weak lights.

But, of course, the most widely used grow room exhaust fan is an inline duct fan. This is what most people picture when they think of fans for growing weed.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilating grow room with a duct fan

Inline fans — best ventilation for grow room.

A very important specification of this type of fans is their diameter. Depending on the size of your setup, it can be 4, 6, or 8 inches (100, 125, 200mm). Make sure that every other piece of ventilation equipment that you’re going to use (carbon filter, ducts, cool tubes, cool hoods) has the same diameter. This will allow you to easily assemble any configuration you need.

Vent Ducts/Pipes

Sometimes, you will use ducts to connect grow tent fans, hoods, and filters to each other. However, in smaller setups and when venting your grow tent into the same room, you can do without ducts and connect all parts directly to each other. But you can’t avoid using them if you’re venting your grow tent out a window, a dryer vent, into the attic, or outside. There are various types of vent pipes that can be used in a ventilation setup for grow room. We recommend avoiding plastic or vinyl ducts because they are fire hazards.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation grow room, vent types

Ventilation: Grow tent ducting types.

The most common type is aluminum foil ducts. They are so flexible that you can easily bend them into any shape. Or you may choose more sturdy rigid or semi-rigid metal ducts. Don’t forget about the diameter. It should be the same across the whole range of your ventilation equipment. So, if you use a 6-inch fan, both the carbon filter and ducts must be the same 6 inches (125mm) in diameter. You can connect and hold them in place easily with jubilee clips or other types of clamps.

Attention! Ventilation ducts should never exceed the maximum allowed length of 8ft (~2.5m). And don’t forget that they will accumulate lint and other debris over time which may cause fire. So clean them regularly with a vacuum cleaner.

Oscillating Fans

Oscillating fans are used inside the tent and create an air movement above the canopy level. This allows to cool down the plant tops which are the closest to the lights and suffer the most from heat. These fans serve another important function: they make branches sway a bit, and this makes them stronger and better suited to support heavy colas in the final weeks before harvest. Finally, giving the air a nice stir helps prevent mold and bud rot. In micro grows, fans don’t really have to oscillate. So you can use clip-on fans, box fans, or case fans in their place. 

Just remember: whatever fans you choose, they shouldn’t blow directly on the plants, only above the canopy. Otherwise, cannabis can get stressed or even suffer from windburn.

Carbon Filter

Carbon filters are the only sure way to successfully combat strong smells associated with cannabis cultivation. All other methods are hardly effective. You may dispense with a carbon filter if you raise one small plant with a duct going from your grow box directly outside. In all other cases, install a carbon filter of the appropriate capacity. Choose a filter whose capacity is somewhat bigger than that of the fan. Because if the fan is more powerful, it will pull the air through the filter at a too high rate. As a result, the filter won’t keep up scrubbing all the odors. We’ve looked at some ventilation kits available for sale to see what filters they use with what grow tent fans.


Fan CapacityFilter Capacity

Table 5. A ventilation kit for grow tent should use a slightly bigger carbon filter if possible.

Multiple numbers divided by a slash mean that a fan has two or three speed levels. Again, make sure that both your fan and your filter are the same size: either 4, 6, or 8 inches (100, 125, 200mm). Many novice growers ask the question of whether a fan should pull or push the air through a carbon filter. In most setups, the filter comes first, and then comes the fan pulling the air outside. This way, not only aromatic molecules but also dust and other impurities get filtered and won’t damage the fan or accumulate in the duct.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation grow room, push or pull air through carbon filter

Installation of a carbon filter inside the tent is the most common.

Having said that, some setups are too small and crowded to provide a space for a bulky carbon filter. No worries! You can mount the fan on the wall of the tent and make it push the air through the filter placed outside. This is not ideal, but acceptable. And, of course, it’s possible to install BOTH the fan and the filter outside the grow box if it’s too small for anything but a light bulb.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation for grow room setup, external version

While Option 1 is the most common, you can use 2 and 3, too—if otherwise, things get too crowded in your setup.

Intake Fan

An intake fan isn’t really necessary in most small-scale grows because your regular exhaust fan creates negative pressure inside the tent. (The negative pressure becomes obvious when you turn on the fan and the soft walls of the tent become concave.) The negative pressure forces the fresh air to be sucked inside through a vent in the bottom part of the tent. Just make sure that this vent isn’t too small for your ventilation system, and you’ll be okay.

However, in larger operations, intake fans may prove useful. At the very least, they can prevent cannabis odors from leaking outside through the bottom vent. You can also install a HEPA filter here, so that no mold spores and bugs get inside your grow room. The intake fan should be always less powerful than the exhaust fan, so that, instead of the negative, you don’t get the positive pressure and the tent doesn’t get filled like a balloon, sending weed odors everywhere.

6. If You Use An HPS With A Cool Tube Or A Cool Hood

Cool tubes and cool hoods are used to guard plants from the excess heat generated by lights.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Cool tube or cool hood as part of ventilation system for grow room

Vents at both sides of cool tubes/hoods allow you to connect ducts and fans for heat extraction.

If you use something like a 400W, 600W, or 1000W HPS, a cool tube (or a cool hood) sounds like a good idea. It gets connected on both ends to other components of your ventilation systems: a fan and a carbon filter, either directly or via a duct. At least in simpler, cost-effective setups.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilating grow room, the cool tube option

How to make a simple and cheap ventilation system with a cool hood or cool tube.

If you have a large operation, you may use two circuits instead: one — for providing the plants with CO2 and scrubbing odors from the air, the other — for cooling the light(s). In this case, no carbon filter is necessary for the cool tube circuit: the fresh air will be taken directly from outside the tent and pulled through the cool tube to the other side, never mixing with the air inside the tent.


Ventilation For Grow Tent or Grow Room: Cheap And DIY Solutions - Ventilation for grow room, grow tent ventilation setup with two circuits

Grow tent ventilation diagram with a separate circuit for cooling the light.

Obviously, you can also connect two or more cool hoods into one cooling circuit (if the extractor fan is powerful enough).

7. Taking Grow Tent Equipment Into Consideration

As mentioned, the various sizes of grow spaces require different amounts of airflow for proper air exchange. Exchanging stale air inside your tent is vital for optimal plant growth so it’s highly recommended you calculate cubic feet per minute (CFM) in order to provide enough fresh air for your plants. So in order to find the right amount of airflow needed for your grow tent, you need to calculate its volume. All you have to do is convert the dimensions from inches to feet or, if you’re calculating in meters, convert them to centimeters then multiply the length x width x height. The result will be your grow tent’s volume, which is the CFM you should aim for. To make it easier, here’s the formula:

Length x Width x Height = CFM

So for example, 48 x 36 x 72 inch grow tent converted to feet would be 4 x 3 x 6 feet, which when multiplied the dimensions together equals 72 cubic feet. This means that if you have a 4 x 3 x 6 feet grow tent you would have to move at least 72 cubic feet per minute, meaning that you would need a 72 CFM extractor. But, when growing indoors there is other equipment that you should take into consideration such as ducting, filters, etc… so let’s understand how to factor in equipment to our CFM calculation.

Factoring in Grow Equipment

Now that you know the required CFM for your grow tent, it’s time to factor in the equipment and accessories in the tent because components like ducting and filters, for example, will reduce fan performance, decreasing CFM. So when talking about ducting, the length will affect airflow because the ait needs to travel further, so the shorter and straighter the ducting, the better. Keeping this in mind, you can calculate CFM + 20% for straight ducting and CFM + 60% for long, long ducting.

Also, if you’re growing under LED lighting you have to account for the heat output, so it’s recommended to increase CFM by 50%. Then, remember that you need a high-performance extractor in order to move the calculated CFM so make sure to take the following formula into consideration in order to find the right extractor for your grow tent.


(CFM x Equipment CFM percentage) x (Grow Light Heat Percentage) = Total CFM


So let’s explain it thoroughly with the 4 x 3 x 6 grow tent example mentioned before. You’ve calculated the CFM for this size grow tent before, which is 72 CFM, then you have to multiply this CFM by the percentage for each component, that would be:

  • Ducting: + 20%
  • Carbon filter: + 60%
  • Silencer: + 20%
  • Grow lights: +50%

These are the percentages you would have to increase to your base CFM, so if the base is 72 CFM and you’re planning on using ducting (+20%), a carbon filter (+60%), and a silencer (+20%), the formula would be the following:


72 CFM + 60% + 20% + 20% = 164 CFM


After adding the grow equipment, you can go ahead and add the grow light fixture (+50%):


164 CFM + 50% = 246 CFM


Keep in mind that this is an average and can vary depending on several factors, but it’s a good place to start. So now that you know you need 246 CFM, it’s time to buy your equipment. When shopping for grow tent extractors, it’s highly recommended to go for equipment with speed controls and if possible, go for + 25% CFM than the value reached with the formula so that your extractor can run quieter at low speeds and also for you to be prepared in case you need to upgrade your equipment. This takes us to the next important thing.

Managing Grow Space Noise

When using all of these grow tent equipment you will definitely hear a noise so it’s important you take care of the noise as you don’t want to attract unwanted attention. As mentioned above, apart from increasing CFM by 20% it’s a good idea to buy equipment that doesn’t make much noise. On top of that, buying equipment with speed control and going for 25% higher CFM will allow you to run your extractor at low speed without affecting performance. You can also soundproof your grow room to reduce noises even further, attach a silencer to the fan and also go for insulated ducting which will help reduce the noise even more.

8. In Conclusion

A grow tent ventilation setup could be a most basic contraption or a very sophisticated system. It all depends on the scale of your grow-op. In any case, setting up ventilation systems for grow rooms is extremely easy if you use a few formulas and numbers that we have provided in this article.

And should you have any questions, the comments section below is at your disposal. Don’t be afraid to ask!



  1. Growing medicine: Small-scale cannabis cultivation for medical purposes in six different countries. Monica Barratt, et al. 2014
  2. The case for small-scale domestic cannabis cultivation, International Journal of Drug Policy. 2010
07 June 2022