CFM Calculator: How To Calculate CFM? (Easy Calculation + Charts)

CFM or Cubic Feet per Minute is a unit for airflow we use in HVAC calculation. Most commonly, we need to calculate CFM for a room for fans, air purifiers, air conditioners, and so on.

Example of a question LearnMetrics’s received: We have a 300 square foot standard bedroom. How much CFM should a fan for such a room have if we’re looking to completely change all air 2 times per hour (every 30 minutes)?

Calculation: Airflow has to be strong enough to change the complete volume of 300 sq ft room (with 8 sq ft ceiling height) 2 times per hour. Volume of a room = 300 sq ft x 8 ft = 2,400 ft3. To change it 2 times per hour (ACH = 2), we need to deliver 4,800 ft3 per hour. CFM is a ‘ft3 per minute’ unit. That’s why we need to divide the total volume by 60; hence 4,800/60 = 80 CFM.

Answer: You need an 80 CFM airflow (for 300 sq ft standard room and 2 ACH).

Here’s a neat CFM calculator that calculates CFM based on room area, ceiling height, and the number of air changes per hour (ACH).

Below the calculator, we will demonstrate how the cubic feet per minute calculator works by solving one example using the calculator and CFM formula. You will also find a CFM chart, with airflow in CFM calculated for areas between 100 sq ft and 3,000 sq ft (useful for ductwork as well) further on.

CFM Airflow Calculator

0.00 CFM

Cubic Feet per Minute (CFM)


How To Calculate CFM For A Room? (Solved Example)

Let’s say we have a big 1,000 sq ft room with standard 8 ft high ceiling. We want to calculate the CFM of a fan that will exchange all the air in such a room every 15 minutes (ACH = 4).

We can use calculate fan CFM in two ways:

  1. Use the CFM formula.
  2. Use the fan CFM calculator above.

To demonstrate how to use the CFM calculator to calculate fan airflow, we’ll start by using the calculator. Here are the results:

how to use cfm calculator to estimate cubic feet per minute fan speed

The result is clear. For a 1,000 sq ft room with an 8 ft ceiling and 4 ACH, you need a fan capable of delivering 533 CFM airflow.

Let’s use the CFM formula to see if we get the same number (this is the very formula used in the calculator):

CFM = (Area x Height x ACH) / 60

If we input the figures from our example, we get:

CFM = (1,000 sq ft * 8 ft * 4) / 60 min = 533 ft3/min = 533 CFM

In short, we get the same number.

You can check this list to get an idea of how much CFM the best air purifiers can produce (for reference).

You can freely use the CFM calculator to calculate airflow for any room, and for any ACH. To help you out, we have created a CFM chart where we calculated CFM for the most common room sizes:

CFM Chart For Common Room Sizes

In all these calculations, we predispose 8 ft ceiling height and use 2 ACH. If you want to use other ACH values, you can use the CFM calculator above. For ACH calculation based on CFM, you are free to use the ACH calculaton here.

Room Size: CFM (At 2 ACH)
How many CFM for a 100 sq ft room? 27 CFM
How many CFM for a 200 sq ft room? 53 CFM
How many CFM for a 300 sq ft room? 80 CFM
How many CFM for a 400 sq ft room? 107 CFM
How many CFM for a 500 sq ft room? 133 CFM
How many CFM for a 600 sq ft room? 160 CFM
How many CFM for a 700 sq ft room? 187 CFM
How many CFM for a 800 sq ft room? 213 CFM
How many CFM for a 900 sq ft room? 240 CFM
How many CFM for a 1000 sq ft home? 267 CFM
How many CFM for a 1500 sq ft home? 400 CFM
How many CFM for a 2000 sq ft house? 533 CFM
How many CFM for a 2500 sq ft house? 667 CFM
How many CFM for a 3000 sq ft house? 800 CFM

How Many CFM Per Square Foot

One of the most common questions is how many CFM airflow do we need per sq ft. Obviously, that depends on the ceiling height and ACH. If we predispose 8 ft ceiling height, we can calculate CFM per sq ft for different values of ACH:

  • 0.13 CFM per square foot at ACH = 1.
  • 0.27 CFM per square foot at ACH = 2.
  • 0.40 CFM per square foot at ACH = 3.
  • 0.53 CFM per square foot at ACH = 4.
  • 0.67 CFM per square foot at ACH = 5.

ACH is very important for air purifiers, for example. Allergy-prone people will most benefit from the best H13 HEPA air purifiers for allergies; the coverage area of those units needs to be calculated for 5 ACH. On the other hand, even the best air purifiers for mold will require at most 4 ACH. In practice, the coverage area of these units is calculated at 2 ACH.

If anything is unclear here, you can pose the question in the comments and we’ll help you out.

Duct Diameter CFM Chart

For everybody who needs CFM calculation for ductwork, you will also need the duct diameter to achieve that airflow.

Example: If we need a 300 CFM airflow, we’ll need a 10-inch flex duct diameter.

To correctly size your ductwork, you can reference the CFM sizing chart here:

Flex Duct Diameter: CFM (Airflow)
4-inch 20 CFM
5-inch 50 CFM
6-inch 80 CFM
7-inch 120 CFM
8-inch 170 CFM
9-inch 230 CFM
10-inch 300 CFM
12-inch 500 CFM
14-inch 740 CFM
16-inch 1050 CFM
18-inch 1400 CFM
20-inch 1875 CFM

Using this duct CFM chart, you can properly estimate how big ducts you need to deliver the airflow needed.

Other Airflow Units Like L/Min Or Cubic Meters Per Hour

CFM is an imperial unit, commonly used in the US. If you are using other units, like l/min or m3/h, you can use these unit-to-unit relations to translate other units in CFM.

1 CFM = 1.699 m3/h
1 CFM = 28.317 l/minute

If you have any problems using the calculator, you can use the comments to give us some numbers and we’ll try our best to help you out.

14 thoughts on “CFM Calculator: How To Calculate CFM? (Easy Calculation + Charts)”

  1. Hi there,
    I had calculations done of our Dental practice by a professional and the ACPH for the office is 11. Our office is approx. 2800 square feet. I purchased additional HEPA filters for each operatory and enclosed the rooms. How do I calculate the air exchanges with the added HEPA filters in the rooms? The CFM for the HEPA filters purchased is 60.
    Room 1 is:
    11 feet in length
    8.5 feet wide
    9 feet ceiling height
    Room 2 is:
    10 feet in length
    8 feet wide
    9 feet ceiling height
    Thank you in advance for any insight given.

    • Hello Tammy, HEPA filters increase CADR rating. They do not increase airflow; rather, they can decrease it. You can use the ACH calculator to calculate ACH in Room 1 and Room 2, as well as for the whole office. Hope this helps; please do specify a bit if this doesn’t answer your question.

    • Hi Tammy. My suggestion to you would be to hire a professional company with lots of insurance to handle this for you. That way if you “F” it up and kill a patient in your operating room with too high humidity levels, it isn’t your fault. You know that when the humidity is too high electric charges can pass through the air into an open patient and zap their heart just like a defibrillator can. Not good is all I am saying. Don’t be frugal in this aspect of your project.

  2. I think you made an error in your opening sentence. You said “CFM or Cubic Feet per Meter”. I think you mean Cubic Feet per Minute. CFM is a flow rate (volume over time), not a ratio of volumes in english to metric units. It appears to be right everywhere else.

    • Hello L., gosh, yes. Thank you for the correction; we’ve corrected the error. Quick typing and complex thoughts might have to do something with the misspeling. Thank you for pointing it out and allowing us to correct it.

  3. I have a warehouse that is 1600 (40′ x 40′) with 12′ ceilings. I’m considering adding mini-split heat pumps or central AC/Heat and would like to use exposed spiral rigid ducting. The CFM for the space is 640 according to the calculator. If I use a 3 ton air handles that will deliver 1200 cfm what diameter ducting would you recommend? a 14″ flex delivers 740 cfm (air return rate used was 2 x p/hr). Is a 3 ton too much?
    Do i need to reduce the spiral from 14′ to 12′ to 10′ over the 40′ span? I was going to run one span of single spiral pipe w/3 or 4 diffusers.

    • Hello Courtney, for duck sizing, you can check our article about HVAC flex duct sizing here. In short, 18″ flex ducts can handle 1300 CFM; that would be the most optimum option. A 3-ton unit would is adequately sized for a warehouse. We’re sorry; we don’t have the required know-how to answer the spiral reduction question.

  4. I am still confused. If I use an airscrubber that has 11 air changes/hr and I have a room that has 6 exchanges per hour, how to I calculate how long it takes to clear airborne particles from the room?

    • Hello Jessica, air scrubber has 11 ACH for a room with a specified volume. If you put it in a bigger room, the number of ACH will fall. If you have 6 ACH now, that means that, theoretically, all the air in the room is changed 6 timers per hour. That’s every 10 minutes. ACH doesn’t really tell you anything about airborne particle removal. Essentially, the higher the number of ACH, the quicker the room should be cleared of these particles. But that also depends on what kind of filters the air scrubber uses, what kind of particles are we talking about, and so on.

      • If I have a hepa filtration system rated at 390 CFM how many times will it replace the air with 8 ft ceilings in 900 square feet?
        How many hours? Rate air change in 24 hours?

        Does any body know this answer for sh & giggles?

        • Hello Rome, let’s calculate a bit. The total amount of air in your room is 8 ft * 900 sq ft = 7,200 ft3. The HEPA filtration system provides you with 390 CFM (ft3 per minute). In 1 hour, it can handle 390 CFM * 60 min = 23,400 ft3. So, in 1 hour, the air will be replaced 24,300 ft3 / 7,200 ft3 = 3.375 times.

          That’s 3.375 air changed per hour. In 24 hours, that 24 * 3.375 = 81 air changes per 24 hours. Hope this helps.

  5. I have a unique situation. I’m operating some dedicated servers which consume about 2000 watts. I have built a box to put them in, complete with a 6″ intake and 6″ exhaust. I am trying to ventilate the heat to outside so I have routed the ducts through a filter over my servers then exhausted. I get a 25 degree temperature increase from intake to exhaust, which is to be expected from my BTU calculations. However, it seems with the door on the unit, the fans want to go 100% and the frames and everything gets warm, almost like a oven effect. Overall the heat is exhausted and the systems stay functioning within temperature limits but I suspect increased wear on the components. I am thinking I need a bigger duct, maybe 8″ to more than double my airflow. The calculations would be nice to do as the box is set to hold 4000 watts of computer equipment that needs to have the heat moved away from them on a 24/7 basis, and to take the guess work out. I would like to avoid using a a/c system as the power consumption is too much, but I am afraid I might need to knock my intake temperatures down a few degrees at the peak times of the summer during the days. I would like to accomplish this with pure ventilation of unlimited air to dump the heat. I am considering a 2nd exhaust and intake setup but that would double my fans and my power consumption. The box they are located in is 40 cubit feet with a 6″ rated at 402cfm.

    • Hello Jordan, a 25-degree increase is quite substantial. This is a clear indicator that you would require more airflow. Increasing airflow will help decrease the degree increase as well as cool your servers more adequately. This is not your everyday HVAC problem we try to solve; maybe you could use the duct sizing chart here and duct velocity calculator to do the calculations. It’s a complex system; but it would be smart to solve it, yes, the wear and tear are increased without a doubt.


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