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)?
CFM 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
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:
- Use the CFM formula. This is how to calculate CFM of the room.
- Use the room CFM calculator above.
This comes quite useful when you are trying to figure out which HVAC units to buy. Here is an example of how to select the best ceiling fans based on CFMs.
To demonstrate how to use the CFM calculator to calculate fan airflow, we’ll start by using the calculator. Here are the results:
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 calculation 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). It is also interesting to see how the CADR rating for dust-removing air purifiers depends on airflow.
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.
Another example are bathroom exhaust fans. When sizing a bathroom fan, you need to take into account 8 ACH. You can check how many CFM for the bathroom exhaust fan you need here (both the calculation and application of 8 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.
Note: What is CFM? CFM is cubic feet per minute; it is a unit that measures airflow.
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.
Tammy
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.
Sir I want to learn clarity kindly help me as a master.sir
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.
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.
spiral rigid ducting has less pressure drop than flex,you need manual j calculation on the building first then measure the fatherest supply outlet and the same for the return.then get your available static pressure for the air handler,this will give you the friction rate to size your duct some where around 0.5 – 0.8.your air handler should have the static pressure label on it that it can have to give the cfms. your building may only need 1.5 to 3 tons depending on your geographical location and how well your insulation is.
Hi, this was really helpful. Thank you for the contribution
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.
First of all the filtration system doesn’t have enough capacity for this size room, since it must handle about 400 Cfm per ton of a/c, and a 900 square feet room will need more than 1 ton. In addition a filtration system doesn’t provide air changes, but filters the supply air . Air changes are provided by leaks of building envelope or makeup air supply ducts from outside.
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.
hello , how can i know how many duct meters should i use according to the calculations? my space is 1667 sq ft. with 5 meter ceiling height, and i meed air to change 10 times per hour, i got 2000 cfm so how can i know what the length of the 20 inch duct should be?
Hello there, in short, you would require ductwork that can handle 2000 CFM. The cross-section is important here; not the full length. For example, you can either have a 1-meter or a 10-meter duct, it doesn’t really matter. What matters is that you have sufficiently large cross-section (size of the duct, if you will) that can handle that airflow.
If you consult the duct sizing chart here for flex and round ducts, you can see that for 2000 CFM airflow, you can choose a 20-inch metal round duct or either 36×10-inch or 30×12-inch rectangular duct. Hope this helps.
hi.. how can i achieve better air flow without using duct?
Hello there, the airflow output of an HVAC device is constant. You get the same CFMs with or without ducts.
? I noticed the duct pipe chart calculates for flex pipe. Is there a difference for solid smooth pipe.
Hello Bob, you can consult our duct sizing chart article here. You will find both flex and solid pipes there with the specified CFMs for all sizes.
I just want to know if the difference between 200 CFM and 400 CFM is a lot
Hello Pat, well, you practically double the airflow going from 200 CFM to 400 CFM. The numbers are clear here.
I am storing a car in a garage in Florida for the summer the garage is roughly 450 ft.² I want to know how big a fan how much CFM do I need to move the air during the summer
Hello George, in garages it makes sense to have some ventilation in order to avoid CO and NOx accumulation. The usual ACH (air changes per hour) for storage garage are between 4 and 6. Let’s say that 450 sq ft garage with 8 ft ceiling; that is 3600 cubic feet of air. A fan should ventilate this amount of air 4 to 6 times per hour; that comes to 14,400 to 21,600 cubic feet per hour (CFH).
Now, all fans have the airflow specified in CFM, so you have to divide CFM by 60 to get CFM. This comes from 240 CFM to 360 CFM. This is how big a fan you need to adequately ventilate your garage. Hope this helps.
Hello Sir,
i’m constructing a lab of 27ft length, 26 ft breadth and 8ft height and i want to maintain 25degree centigrade temperature in the lab.
So, i want to know everything about the design that is needed to maintain this temperature, like AHU capacity, AC capacity in TR, inlet and outlet damper numbers and size, duct size, ACH, etc
Hi Sagar, well, maintaining a constant temperature is a heating/cooling/insulation topic. ACH and airflow (CFMs) don’t really impact the temperature if you use adequate heating/cooling.
What is the average Air Change Per Hour in a residence? Great information!
Hi Randy, according to EPA citing ASHRAE, the average air change per hour for homes and houses is 0.35 ACH. Hope this helps.
Hi there,
I want to have a clear calculation solve.
How i can measure how much cfm i need for 1000 eggs?
Room is 100 sqft and ceiling is 10 ft.
Please help me to solve this
Hi Sudip, hatcher rooms are not really something we have a good knowledge of. Doing a bit of Googling, we figured out you would need 15 CFM per egg airflow. Given that you have 1000 eggs, that would be 15,000 CFM. That’s quite a lot for such a small room. If you know how many CFM per egg you need, you can quite easily calculate that, hope this helps a bit.
How can we decide the required ACH in each room? On what basis we cean use 2 ACH, 4ACH, etc.?
Hi Richard, a good question. Namely, specific spaces like offices, classrooms, boilers rooms and so on have a specified ACH recommendation (commercial use). For residential use, we have 8 ACH for things like ceiling fans rule of thumb, and 2 ACH for air purifiers (or 4.8 ACH for allergy-prone people). Engineering Toolbox here has a good ACH list. Hope this helps.