Beginning growers always want to skip the fans.

I was the same. I figured I could just use whatever fan I already had lying around and stick it in my grow tent.

And it can work. But it's usually not enough.

All but the smallest grows need both an inline fan and an oscillating fan. At a minimum.

The good news is that even the best fans for your grow tent are not overly expensive. You just have to know which models represent good value.

And more importantly which ones you should avoid at all costs (hint: most of them).

Why is air circulation needed for marijuana plants

Clip-on fans are a great way to provide air circulation above the canopy in your weed grow tent.

Marijuana grows best when exposed to a constant supply of fresh air with lots of carbon dioxide (CO2).

Plants need a constant supply of fresh C02. For outdoor grows this is provided by the wind. To replicate this indoors, it's important to keep a constant stream of air movement to replenish the CO2 around your plants.

Your tent's exhaust system will provide the intake of new air. You'll need fans inside the tent for air circulation so this fresh air reaches all parts of your pants. Proper air movement will also prevent problems with plants, such as mold, mildew, and pests.

Plants respond to the wind by increasing the strength of their branches. Lots of air movement encourages a strong, healthy branch structure which will be needed to keep top-heavy buds standing up close to the light.

ECO Farm 6 Inch Clip Oscillating Fan Fit for Grow Tent with 3-Speed...

Features:

The 6 inch ECO Farm Fan is portable comfort that easily mounts wherever it's needed to provide quality airflow with reliable performance, ideal for homes, greenhouses, and workshops and more. The versatile mounting position puts the fan wherever it's needed; the fan features an adjustable tilt mechanism that changes the fan's positioning and direction. The quiet, powerful motor allows you to adjust for optimal airflow and comfort in 3 choices of speeds. This grow tent fan features a metal fan grill and strong steel neck support.

Secret Jardin Monkey Fan 16W 7.5'

Features；

Secret Jardin Monkey Fan - 16w MF16 New Monkey clip-on Fan by Secret Jardin, will fit 0.63" - 0.83" inch poles, the monkey fan hook allows you to attach it quickly to virtually every type of grow tent pole. With its unique coupling, the fan is attached to the metal structure/frame in your grow tent. Monkey Fans are the perfect product to keep air circulating throughout your crops.

What Size Fan Do I Need For My Grow Tent?

Fan size is not what's important. It is fan capacity.

First, you need to figure out the capacity you need. And the capacity then determines the fan size. For most home growers this will end up being a 4 or a 6-inch fan.

How do we figure out the fan capacity you need for your grow tent fan?

By following the calculation we'll cover next.

Calculating Base Fan Requirement For Your Grow Tent

The first piece of information you need is the total volume of your grow tent. To get that, simply multiply the width by the length by the height.

Volume of grow tent = width x length x height

(Note that we are using feet for our calculations because fan capacities in the US are given in cubic feet per minute. The calculation is still the same if you use meters).

Let's say you have a 5 by 5 foot grow tent with a height of 78 inches, which is 6.5 feet. The total volume would be 162.5 ft³.
Total volume = 5 x 5 x 6.5 = 162.5 ft³

In order to replace all the air in that tent in one minute, you'd need a fan with a capacity of 162.5 cfm. Fan capacity is measured in cfm, which stands for cubic feet per minute. Makes sense, right?

Generally, we recommend you aim to replace the air in the tent once per minute. However, if you are not dealing with major heat issues, you could get by replacing it once every two, or even three, minutes.

In that case, divide the total grow tent volume by 2, or 3, to get the base fan capacity you need.

Now that we have the fan capacity, we're done, right?

Not so fast. You didn't think it would be that easy, did you?
What we have is the base fan capacity needed, if you only have the fan removing air and nothing else. But your setup is likely to have several other components that reduce the fan's ability to move air. Plus factors that increase the capacity needs
So we need to make some adjustments.

Adjusting For Ducting, Filters, Lights, Etc.

Components like ducting and filters all reduce the capacity of your fan. You have to account for this in your calculation and get a fan that is more powerful, to make up for this reduction.
Other components like lights increase your cooling needs. You'll want to add some capacity to account for those. If your tent is in a hot location, you should also add some capacity.
Here are common components and factors you should account for and the effect of the additional capacity you'll want to add, in terms of a percentage increase.

lights: 10% increase for each grow light

filters: 25% per filter

ducting: 1% for every foot of ducting

bend inducting: 30% per 90° bend (15% per 45° bend)

external heat: 10% increase if your grow tent is in a warmer location (e.g. the attic or a sunny room) and/or is not well insulated from the external environment

Add all of the increases up to get a total percentage increase. For example, let's say we have a single grow light, one filter, and 6 feet of ducting with one 90 degrees bend. But the tent is not in an especially hot location.

Total percentage increase = 10% (grow lights) + 25% (filter) + 36% (ducting and bend) = 71%

Then you want to take this percentage as a decimal and add it to 1.0.

For example, the 71% from above would become 1.71.

An increase of 44% would become 1.44.

An increase above 100% is treated the same (added to 1). Thus, a 125% increase would become 2.25

Take note that an increase under ten has a zero in front of it. Thus, 5% would be 1.05 (and not 1.5, which is a 50% increase).
Multiply this new number by the original base capacity we calculated above. In our example, that was 162.5 cfm.

Capacity required = base capacity + multiplier

In our example, the capacity needed = 162.5 cfm x 1.71 = 277.875 cfm

That means we would want a fan with a capacity of at least 277.875 cfm. However, it is generally a good idea to buy a fan that has around 25% more capacity than you need.

Get A Fan With More Capacity Than Required

We highly recommend getting a fan with more capacity than you need. Running fans at 100% capacity puts more strain on them, so they won't last as long. It also makes them noisier.

If you run a fan below its maximum speed, it lasts longer and runs more quietly. It also allows you to increase capacity should your cooling requirements increase in the future.

You never know, there could be a heatwave, for example. Or you might start by venting the grow tent directly into the same room, but decide later that you want to add ducting to vent it outside.

Adding 25% to our example above works the same as before: multiply the capacity by 1.25. That gives us an ideal capacity of 347.34375 cfm.

Just round up to the nearest capacity fan of the type you want to get.

Conclusion

Air is equally essential to humans and plants since we both are living beings. Proper ventilation allows you to take complete control of the growing conditions all year round.

Whether you opt for which brands of a fan, the end goal is the same - to keep the plants fresh and increase production in the grow tent.