What Does CFM Mean for Fans? Meaning of CFM in Air Flow Defined

Looking to gain a deep understanding of the meaning of CFM for air flow in fans? Your search ends here! 

Whether you're preparing your indoor growing space or looking to improve the climate control capabilities in your home or office space, you need an in-depth understanding of the meaning of CFM in air flow. Otherwise, you run the risk of undersizing or oversizing your inline fan - and in either case, problems will arise.

Fortunately, if you've been wondering - what does CFM mean for fans? - you've come to the right place. At TerraBloom, we've earned the reputation as the most trusted source for all things ventilation components. And today, we're going to unveil the CFM meaning for fans so that you can size and shop for your equipment with confidence. Not only will we define the meaning of CFM in air flow applications - but you'll also learn how to calculate your unique CFM requirements - no matter what you're using your fan for. We don't want to waste any time - let's get right into the topic at hand. What does CFM mean in fans?

What Does CFM Mean in Fans? The Meaning of CFM in Air Flow Defined

In the simplest terms, CFM stands for cubic feet per minute. It's a unit of measurement that tells you the volume of air that a given fan can move in one minute's time. When it comes to inline fans, the CFM rating is going to be one of your primary concerns because this will ultimately tell you what size room or grow space your fan can effectively ventilate. If you purchase an inline fan with too low of a CFM rating, it won't be able to keep up with the air changes required in your space - and if you buy one with too high of a CFM rating, it will cost more to operate than necessary while also creating more noise than necessary.

That's why we always recommend that our customers take the time to calculate their CFM requirements before shopping for an inline fan. We'll talk more about how to do that in just a minute - but first, let's discuss the importance of truly understanding the meaning of CFM.

Why the True CFM Meaning for Fans Can Easily Become Convoluted

You might be wondering - what's wrong with just using the CFM rating on the fan itself as a guide? After all, if a company is selling a 400 CFM inline fan, shouldn't that be enough to effectively ventilate a 400-cubic-foot grow room?

Unfortunately, it's not quite that simple. The fact is, there are two different ways that companies can test and advertise the CFM rating of their inline fans. The first way is what's known as "free air delivery" or FAD. This is a true measure of the volume of air that a given fan can move. It tells you exactly how much movement the fan produces at zero static pressure.

The second way to test CFM is known as "total pressure" or TP. This method takes into account not only the volume of air that a fan can move, but also the static pressure within your ducting system. As you might imagine, this tends to inflate the CFM rating - sometimes by as much as 50%!

Now, here’s where things can get convoluted for anyone shopping for fans. The vast majority of brands that sell online will report incorrect CFM figures. We’ve previously tested many of our biggest competitors' fans ourselves and confirmed that their CFM levels are overstated by 20-25% on average. We paid all of the fees for testing/certification with the HVI because our goal is to provide accurate information about our products. We know how important it is for you to have accurate information to make the correct business decisions.

Unfortunately, there is no real way to enforce these brands overreporting their fans’ performance - so they get away with it. The only way can feel confident when shopping is to buy your fan from a company that has paid for an accurate CFM test done through an HVAC organization. It’s important that the specific testing has been done through a company such as Home Ventilating Institute (HVI) or the ASHRAE.

Now - with all this said, it’s important to note that duct fans are typically certified at 0.2” w.g. static pressure, because the testing orgs assume that the application which these fans will be used in will have an average of 0.2” static pressure built into it due to the turns in the ducting, drag on the ducting surface and so on. So our fans when certified with the HVI are certified to produce a certain amount of CFM at 0.2” static pressure, while their output in free flow scenario without ducting (at 0 static pressure) will be higher. 

Calculating Your CFM Requirements: A Step by Step Guide

The first step in calculating your CFM requirements is to determine the size of the space that you need to ventilate. To do this, simply multiply the length (L) times the width (W) of the room or grow space. Then, you need to factor in the height to turn that square footage into cubage footage. From there, consider how frequently you want to replace the air in that space. We’ll walk you through the calculation below.

For example, let's say you have a grow room or grow tent that is 4 feet wide and 8 feet long feet wide. To calculate the square footage, you would multiply 4 x 8 to get 32 square feet. Then, assuming a height of 8’, you would get a cubic footage of 256’.

Now that you know the size of your space, you need to determine how many times per hour you want to complete an air change. This will depend on what type of plants you're growing and what stage of growth they're in. A general rule of thumb is four air changes per hour for vegging plants and eight air changes per hour for flowering plants.

Using our previous example, if you were growing flowering plants in a 256-cubic-foot grow room, you would want to complete eight air changes per hour. To calculate the number of cubic feet per minute (CFM) that you need, simply multiply your square footage by the number of air changes required per hour.

In our example, 256 cubic feet x eight air changes per hour = 2,058 cubic feet per hour. Since there are 60 minutes in an hour, you would then divide 2,058 by 60 to get 34 CFM.

You can use this same formula to calculate the CFM requirements for any size room or grow space - just be sure to use the correct number of air changes per hour for your particular application. And later on, we’ll talk about factoring in the drag/static pressure from ducting, carbon filter, etc.

The Relationship Between CFM and the Static Pressure in the Air Line (Pressure Curve)

The above section will provide you with most of what you need to know to correctly size your fan. However, we want to briefly talk about the pressure curve - which shows how much CFM the fan will produce at different pressure levels. This is typically measured in inches of the water gauge or in Pascals. Really all that you need to know is that the higher the static pressure in the air line, the lower CFM you’ll actually get. The fan has to overpower that static pressure in order to push air in a certain direction. Static pressure is by far the most important factor often overlooked by growers because it is not spoken enough about. It is common for a carbon filter to introduce an equivalent of 0.5-0.8” w.g. pressure, which will cause the fan to lose 30-50% output when the fan has to blow or pull through that filter. 

In other words, the higher the static pressure the bigger the loss of CFM from the number written on the fan to the actual CFM you’ll be getting in that particular application. Manufacturers that have done granular lab tests on their fans will have pressure curves available so that the customer can determine the more precise CFM output they will get in their particular application with the applicable static pressure factored in. To see what this looks like, view the image below:

This cut sheet is from one of our fan models which has the pressure curve on the right hand side of the page. For example, at 0” w.g. static pressure the output of the fan is 1662, while at 1” static pressure level the fan’s output drops to 1000 CFM. The speed with which the fan will lose CFM in relation to an increase in static pressure is not always the same, though. Some fans are better than others at handling various pressure because of the fan blade geometry and electronics which feed more power to the motor at higher pressure loads. Other fans, can lose upwards of 50% of output as soon as you attach a carbon filter to them. That’s because their fans have motors with lower RPMs. Thus, the cap on the power fed to the motor does not allow it to boost it with more power when the static pressure increases. This speaks to the importance of getting the best inline duct fan possible - which we cover in our complete guide.

What Does CFM Mean For Fans? Wrapping Things Up

We hope this article has answered the question you came here with today - what does CFM mean for fans? We've defined the meaning of CFM for fans along with the key differences between the FAD and TP ratings when choosing an inline fan. And, we've walked you through sizing your fan based on the CFM needs of your unique space.

At this point, there is just one thing left to do - head over to TerraBloom and explore the best selection of inline exhaust fans online. Our fans are carefully crafted with the best materials and the latest technology - to help you enjoy the best of both worlds in performance and efficiency. Home growers and commercial cultivars around the world trust TerraBloom in their spaces - and you'll even find out inline duct fans in office buildings, commercial warehouses, meat processing facilities, crypto mining operations, and anywhere else dependable ventilation is needed. You can even pair your new fan with one of our industry-leading carbon air filters and an inline fan speed controller for the ultimate ventilation setup.

Still not fully grasping the CFM meaning for fans? Don't stress - we're here to help. Simply reach out and we'll walk you through the CFM meaning in fans and help you make the right selection for your unique needs! And if you want to learn more about ventilation, check out our blog posts on how an inline duct fan works or where to install an inline duct fan.