Air purifiers and HVAC devices should achieve 2 to 5 ACH in a room. How many CFM do we need for that? Well, that depend on the ACH we want to have and the volume of air in the room.
To help you out with figuring out how many CFM we need for any ACH (Air Changes Per Hours), we created an online ACH to CFM calculator you can freely use.
Below the calculator, we explain the formula that converts ACH to CFM, and, quite importantly, we feature a 2 charts for how many CFM we need for 100-2,000 sq ft spaces to achieve 2 ACH or 5 ACH, respectively.
Here is the calculator that converts ACH to CFM:
Alright, let’s solve one example to show how this calculator works:
Say you are an allergy-prone person and would, therefore, need an air purifier that can achieve 5 ACH in a 16×20 ft room (standard 8 ft ceiling height). How many CFM should this HVAC device produce?
We just insert ‘5’ in the ACH input field, ’16’ in the width field, ’20’ in the room length field, and slide the ceiling height to ‘8’. Here is the result we get (screenshot):
We can quickly see that, to achieve 5 air changes per hour in the room with these dimensions, we need 213.33 CFM of airflow.
Now, let’s have a look at how we can convert ACH to CFM manually:
ACH To CFM Formula
Alright, we know that the ACH to CFM conversion basically depends on the volume of air an air purifier or HVAC device has to clear. We also have to apply 1 hour = 60 minutes (ACH is per hour, while CFM is per minute). Here is the ACH to CFM formula:
CFM = ACH × Width × Length × Ceiling Height / 60
Namely, to convert ACH to CFM, we need to multiply CFM by air volume (width × length × ceiling height, all in feet), and divide the product by factor 60 (this comes from 1 hour = 60 minutes).
Now, we have an equation we can use to make this conversion for any room or home.
Quick Example: How many CFM do we need if we want to have 5 ACH in an 18×20 ft room (with a standard 8 ft ceiling height)?
We just input the 4 relevant numbers in the equation below (just like we would input them into the calculator above):
CFM = 5 ACH × 18 Ft × 20 Ft × 8 Ft / 60 = 240 CFM
To achieve 5 ACH in a room with these dimensions, we need 240 CFM of airflow. You can use the calculator above to check if it gives you the same result as this formula.
For air purifiers, you we usually looking for a minimum of 2 ACH and a maximum of 5 ACH (for allergy-prone people). That’s why we calculated how much airflow (CFM) we need in 100 sq ft rooms to 2,000 sq ft homes to achieve 2 ACH or 5 ACH.
Here are the CFM results in 2 charts:
2 ACH To CFM Chart For 100-2000 Sq Ft Spaces
2 ACH means that air changes every 30 minutes. In all the calculations, we presume the standard 8 ft ceiling height. These are the CFM requirements for 2 ACH:
| Square Footage (Sq Ft): | Airflow (CFM): |
| 100 Sq Ft | 26.67 CFM |
| 150 Sq Ft | 40 CFM |
| 200 Sq Ft | 53.33 CFM |
| 250 Sq Ft | 66.67 CFM |
| 300 Sq Ft | 80 CFM |
| 350 Sq Ft | 93.33 CFM |
| 400 Sq Ft | 106.67 CFM |
| 450 Sq Ft | 120 CFM |
| 500 Sq Ft | 133.33 CFM |
| 600 Sq Ft | 160 CFM |
| 700 Sq Ft | 186.67 CFM |
| 800 Sq Ft | 213.33 CFM |
| 900 Sq Ft | 240 CFM |
| 1,000 Sq Ft | 266.67 CFM |
| 1,100 Sq Ft | 293.33 CFM |
| 1,200 Sq Ft | 320 CFM |
| 1,300 Sq Ft | 346.67 CFM |
| 1,400 Sq Ft | 373.33 CFM |
| 1,500 Sq Ft | 400 CFM |
| 1,600 Sq Ft | 426.67 CFM |
| 1,700 Sq Ft | 453.33 CFM |
| 1,800 Sq Ft | 480 CFM |
| 1,900 Sq Ft | 506.67 CFM |
| 2,000 Sq Ft | 533.33 CFM |
We can see that we need anywhere from 26.67 CFM to 533.33 CFM to achieve 2 air changes per hour in areas ranging from 100 sq ft to 2,000 sq ft, respectively.
Let’s have a look at how many CFM we need for 5 ACH (obviously, we will need more CFM than for 2 ACH):
5 ACH To CFM Chart For 100-2000 Sq Ft Spaces
At 5 ACH, all the air in the room is exchanged every 12 minutes (since 60 min / 5 ACH = 12 min). Here are the CFM requirements for different sized rooms/homes to achieve 5 ACH:
| Square Footage (Sq Ft): | Airflow (CFM): |
| 100 Sq Ft | 66.67 CFM |
| 150 Sq Ft | 100 CFM |
| 200 Sq Ft | 133.33 CFM |
| 250 Sq Ft | 166.67 CFM |
| 300 Sq Ft | 200 CFM |
| 350 Sq Ft | 233.33 CFM |
| 400 Sq Ft | 266.67 CFM |
| 450 Sq Ft | 300 CFM |
| 500 Sq Ft | 333.33 CFM |
| 600 Sq Ft | 400 CFM |
| 700 Sq Ft | 466.67 CFM |
| 800 Sq Ft | 533.33 CFM |
| 900 Sq Ft | 500 CFM |
| 1,000 Sq Ft | 666.67 CFM |
| 1,100 Sq Ft | 733.33 CFM |
| 1,200 Sq Ft | 800 CFM |
| 1,300 Sq Ft | 866.67 CFM |
| 1,400 Sq Ft | 933.33 CFM |
| 1,500 Sq Ft | 1000 CFM |
| 1,600 Sq Ft | 1,066.67 CFM |
| 1,700 Sq Ft | 1,133.33 CFM |
| 1,800 Sq Ft | 1,200 CFM |
| 1,900 Sq Ft | 1,266.67 CFM |
| 2,000 Sq Ft | 1,333.33 CFM |
We see that for 5 ACH, we need 2.5x more airflow than for 2 ACH in the same room/home size. Namely, to achieve 5 ACH in 100 sq ft to 2,000 sq ft rooms, we need anywhere from 66.67 CFM to 1,333.33 CFM, respectively.
Using the online calculator above, or consulting these 2 charts, you are now well equipped to convert any ACH to required CFMs. If you need a bit of help, you can always reach out to us via the comment section below, give us a few numbers, and we can help you with the conversion.