When you size air purifiers, a lot of times you ask yourself the following: “Alright, this air purifier has 300 CADR CFM; what square footage can it cover?” It’s likely overkill for a 12×12 room, but can it cover 600 sq ft, for example?
Solution (using the calculator): 300 CADR CFM air purifier can cover 450 sq ft (at 5 ACH) or 1125 sq ft (at 2 ACH). We presume the standard 8 ft ceiling height.
To simplify this CFM to sq ft calculation, we created an online CADR CFM to sq ft calculator. The only 3 inputs you need are the following:
- Air Purifier Capacity (expressed in CADR CFM Rating). On the specs sheet, you will notice CADR rating (Clean Air Delivery Rate). It tells you how many CFM of clean air can an air purifier deliver. Air purifiers can produce anywhere from about 30 CADR to over 500 CADR CFM.
- Desired ACH (Air Changes Per Hour). This tells you how many times per hour all the air in the room should be filtered. For air purifiers, it is recommended to shoot for anywhere between 2 ACH and 5 ACH. 5 ACH (all air changed every 12 minutes) produces the cleanest air.
- Ceiling height. We are changing the volume of air. The higher ceilings we have, the more air per sq ft we have, and we have to account for that.
Alright, let’s use the calculator:
In the intro, we had a 300 CADR-rated air purifier example. Here is how we can tell how many sq ft it can effectively cover using this calculator:
We insert ‘300’ into the air purifier CADR CFM rating; let’s say we want the cleanest air, so we slide to ‘5’ ACH and keep the ‘8’ ft ceiling height. Here is the result we get (screenshot):
We see that 300 CADR CFM air purifier can cover 450 sq ft at 5 ACH. If we set the air changes per hour to lower air cleanliness of 2 ACH, the coverage increases to 1125 sq ft.
Let’s look at the math behind this calculation:
CADR To Sq Ft Formula
Now, it is important to note that with air purifiers, we can’t just use the maximum airflow (CFM). We need to use the CADR rating. According to AHAM, CADR rating for an average air purifier is about 2/3 of the max. airflow.
Example: An air purifier with 300 CFM max. airflow has about 200 CADR CFM rating (that’s the 2/3 rule).
We can write CADR to coverage in sq ft formula like this:
Coverage (Sq Ft) = CADR Rating × 60 / (ACH × Ceiling Height)
Let’s solve one example for a big 500 CADR-rated air purifier. We want the cleanest air (5 ACH), and let’s say that we have a standard 8 ft ceiling height. Let’s input all these numbers in the equation above like this:
Coverage (Sq Ft) = 500 CADR Rating × 60 / (5 ACH × 8 Ft Ceiling) = 750 Sq Ft
We see that 500 CADR rated air purifier has 750 sq ft coverage at 5 ACH.
To avoid all these calculations, we created a chart of air purifiers with 50 to 600 CADR rating, and calculated how many sq ft they can cover at 5 ACH (8 ft ceiling height). Here is the chart:
CADR To Sq Ft Chart (At 5 ACH)
| Air Purifier CADR Rating (CFM): | Coverage (Sq Ft): |
| 50 CFM | 75 Sq Ft |
| 100 CFM | 150 Sq Ft |
| 150 CFM | 225 Sq Ft |
| 200 CFM | 300 Sq Ft |
| 250 CFM | 375 Sq Ft |
| 300 CFM | 450 Sq Ft |
| 350 CFM | 525 Sq Ft |
| 400 CFM | 600 Sq Ft |
| 450 CFM | 675 Sq Ft |
| 500 CFM | 750 Sq Ft |
| 550 CFM | 825 Sq Ft |
| 600 Sq Ft | 900 Sq Ft |
From this chart, we see that (at 5 ACH), 1 CADR CFM can effectively cover a 1.5 sq ft area. Example: 500 CADR unit can cover 750 sq ft; that’s 1.5 sq ft per CADR.
If we would use lower 2 ACH (standard clean air), 1 CADR could cover a 3.75 sq ft area. Example: At 2 ACH, 500 CADR can cover 1,875 sq ft; that’s 3.75 sq ft per CADR.
We hope all of this makes sense. With the calculator and this chart, you now have the tools to convert CADR to square footage for air purifiers. You can check out how to adequately size an air purifier here.