Exhaust Fan Calculator
for Indoor Grow Rooms

When controlling temperatures manually, digital timers can be set to control the exhaust fans and CO2  (particularly during 'lights out')

Use the exhaust fan calculator below to figure out the appropriate type and size exhaust fan for you. The first thing to consider is the size of the garden space you are planning to grow in. The general rule is to have an exhaust fan that will completely exhaust the garden area in 5 minutes or less. This is how...

Exhaust Fan Calculator:

First, figure out the cubic feet of your garden area. Measure the floor of the area (example 3 feet by 4 feet), and multiply this by how tall the area is (example 8 feet). In this example, you would have 3 times 4 times 8, which equals 96 cubic feet.

Naturally, if you buy a 96 cubic foot per minute (CFM) fan, you would be able to exhaust the whole room in one minute.

If you divide this number by 5, you get the smallest CFM (cubic foot per minute) that you would want to go with. In this example, 96 divided by 5 = 18.24 CFM. An 18.24 cubic foot per minute fan is the smallest fan you would want to consider.


Now, to get a better idea of your fan needs, you need to look at your whole exhaust setup. Your light, your fan, and the way you move the air through the garden area are known together as your exhaust setup.


Other Things to Consider

In reality, there are a number of things to consider that will affect the actual cubic feet that your garden exhaust fan will move each minute. These things fall into three categories...

  • your air exhaust duct runs

  • your light reflector options

  • the addition of a carbon filter


Placing the Carbon filter high allows you to exhaust the hottest air from the room first

The exhaust fan is designed to move air FROM your garden area TO another area. In order to do that, you normally need to move the air through air ducts.


6 inch ducting is absolutely the minimum size to consider using, in my opinion

The minute you connect an air duct to your exhaust fan, the amount of air you can move (your CFM) drops. This is because it takes more energy to blow air through ten feet of air ducting compared to just blowing air straight out of the fan. The longer the air duct run the more your CFM will drop, especially if there are bends and turns. Try to keep it under 20 feet.

Your grow light reflector can block your airflow and reduce your CFM. This is critical information for anyone who wants to get started in the right direction. These detailed considerations can be found in my about your reflector section. I highly recommend you read this section.

The above are all good reasons to get an over-sized fan (one larger than indicated by the exhaust fan calculator).

This 273 CFM squirrel cage fan costs about the same as some much stronger centrifugal fans

As I discussed in the garden exhaust fans page, there are only two types of fan that are appropriate to use as exhaust fans... they are squirrel cage fans (pictured here) and centrifugal fans. Everything you have read up to this point can be accomplished with a squirrel cage fan.


Exhaust Fan Options
When Using a Carbon Filter

Centrifugal fans are the most efficient air movers. Recently, they have also become more affordable than squirrel cage fans. Pictured here is a 465 CFM fan

If you decide to add a carbon filter, than a squirrel cage fan is not quite powerful enough to do the job well. You need the kind of air pressure only one kind of fan can deliver. You need a centrifugal fan (pictured here). Your centrifugal fan will both filter your air, cool you lights, AND exhaust your garden area.

The Carbon filter here is the highest rated on Amazon. Carbon filters are the most effective way to control unwanted odors

In addition, a centrifugal fan will maintain it's CFM in many situations where a squirrel cage fan will not, so it is not necessary to get an "over-sized" fan. Simply follow whatever CFM recommendation the exhaust fan calculator indicates.


The Exhaust Fan Setup

By now you have a good idea of where your garden will be, what size it will be, what light you will use, and what fan to use. Now it's time to put it all together. It is critical to control the high temperatures in your grow room....that means you must spend a little time and money running your exhaust setup properly.

Leave the exhaust fan calculator page and
Visit the Exhaust Setup Page


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