Your air purifier should be too small or too big. How big should it be? That’s exactly what we are going to figure out here with the help of the air purifier size calculator and air purifier size charts (2 of them, actually).
There are small air purifiers with below 100 CFM CADR rating, and the big ones with 500+ CFM CADR rating. Here are the 2 key factors that determine what size air purifier you need:
- Room size (volume of air). Smaller rooms need smaller units, bigger rooms need bigger ones. Since air purifiers filter all the air in the room, we need to calculate the volume of air using room length × width × ceiling height. Note: We can’t just go with square footage, ceiling height is important.
- Desired ACH (Air Changes Per Hour). The air purifier should change all the air in the room at least 2 times per hour (we refer to this as 2 ACH). For allergy-prone people, it is recommended they use a unit capable of achieving 5 ACH (all air filtered every 12 minutes), for the cleanest breathing air.
Now, before we use the calculator, and check the size charts, let’s answer this key question:
What is actually the “size” of an air purifier?
Air purifier size is expressed in CADR rating (Clean Air Delivery Rate). This tells you how many CFMs of clean air can an air purifier produce. Thus, when we are calculating the size of air purifier you need, we are practically calculating how high CADR rating the unit should have.
Note: It’s quite common to confuse max. airflow (CFM) with CADR rating (CFM). Example: A unit might have 300 CFM max. airflow, but the CADR rating is only 200 CFM. They are not the same. AHAM uses a simple 2/3 rule: CADR rating is about 2/3 of the max. airflow.
Alright, here is the calculator (we’ll explain the inputs later on), followed by the air purifier size formula and size charts:
Ok, let’s say you need an air purifier for a 12×12 room (with standard 8 ft ceiling height). You want the cleanest air possible, thus you want 5 air changes per hour (ACH). What size air purifier should you get?
In the calculator, you input ’12’ for the room width, ’12’ for the room length’, slide ceiling height to ‘8’, and set the ACH to ‘5’. Here is how you do that, including the result (screenshot):
We see that for a standard 12×12 room with the cleanest air (5 ACH) we would need an air purifier with about 96 CFM CADR rating. This is one of the smaller units (bigger ones can have a 500+ CFM CADR rating).
A good example for a 12×12 room (144 sq ft) would be Medify MA-14 air purifier. If you check the specs, it has a 120 CFM CADR rating. This means it has effective coverage of 180 sq ft at 5 ACH, and up to 450 sq ft at 2 ACH:
Let’s have a look at how to calculate the size of the air purifier needed manually:
Air Purifier Size Formula
The air purifier capacity we need (expressed in CADR rating) depends on room size (width, length, and ceiling height) and on the desired ACH. Using only these factors, we can write the following air purifier size equation:
Air Purifier Size (CADR Rating) = ACH × Width × Length × Ceiling Height / 60
This equation is pretty straightforward. The factor 60 comes from 1 hour = 60 minutes (ACH is per hour, and CFM is per minute).
We can solve this sizing question simply by inputting all the numbers in the equation above like this
Air Purifier Size (CADR Rating) = 5 ACH × 20 Ft × 24 Ft × 8 Ft / 60 = 320 CFM CADR Rating
For this bigger 20×24 room (480 sq ft), we would need an air purifier with about 320 CADR rating (you can check it with the calculator above). These are more powerful units. A perfect, but expensive (since it’s super-quiet for bedrooms) is this Alen BreatheSmart 75i with 350 CFM CADR rating.
With 350 CFM CADR rating, you can achieve:
- 5 ACH (cleanest air) in rooms with up to 525 sq ft (that’s about 22×24 room, or two 16×16 rooms).
- 2 ACH (standard clean air) in rooms with up to 1312.5 square footage.
In order to create air purifier size charts, we calculated the CADR ratings required to clean rooms from 50 sq ft to up to 1,000 sq ft.
Here are two charts; the 1st is for the cleanest air (5 ACH), and the 2nd chart is for the standard clean air (2 ACH):
Air Purifier Size Chart For Cleanest Air (5 ACH)
At 5 ACH, all the air within a room is fully filtered every 12 minutes. Here are the sizes of air purifiers (in CADR rating, CFMs) you will need to achieve 5 ACH (presuming standard 8 ft ceiling height):
| Room Size (Sq Ft): | Room Examples: | Size Of Air Purifier (CFM CADR Rating): |
| 50 Sq Ft | 8×6 Room | 33.33 CFM CADR |
| 100 Sq Ft | 10×10 Room | 66.67 CFM CADR |
| 150 Sq Ft | 12×12 Room | 100 CFM CADR |
| 200 Sq Ft | 14×14 Room | 133.33 CFM CADR |
| 250 Sq Ft | 16×16 Room | 166.67 CFM CADR |
| 300 Sq Ft | 16×18 Room | 200 CFM CADR |
| 350 Sq Ft | 20×18 Room | 233.33 CFM CADR |
| 400 Sq Ft | 20×20 Room | 266.67 CFM CADR |
| 450 Sq Ft | 22×20 Room | 300 CFM CADR |
| 500 Sq Ft | Two 16×16 Rooms | 333.33 CFM CADR |
| 600 Sq Ft | 25×25 Room | 400 CFM CADR |
| 700 Sq Ft | 2 Bigger or 3 Smaller Rooms | 466.67 CFM CADR |
| 800 Sq Ft | 2 Bigger or 4 Smaller Rooms | 533.33 CFM CADR |
| 900 Sq Ft | 3 Bigger or 4 Smaller Rooms | 600 CFM CADR |
| 1000 Sq Ft | 3 Bigger or 5 Smaller Rooms | 666.67 CFM CADR |
You can see that for the cleanest air (5 ACH) you would need anywhere from 33.33 CFM CADR to 666.67 CFM CADR sized air purifiers for 50 sq ft to 1,000 sq ft rooms, respectively.
For multiple rooms (let’s take combined 900 sq ft area, as an example), it is usually better to get more than 1 smaller air purifiers, and put each one in their own room. The CADR ratings from these multiple units should add up to 600 CFM CADR ratings (Example: 4 x 150 CADR units in four 15×15 rooms).
If you are not allergy-prone, and would just need a standard clean air, you can set the ACH to 2 (changes air every 30 minutes). Here is the 2 ACH size chart (notice that you need less powerful units):
Air Purifier Size Chart For Standard Clean Air (2 ACH)
| Room Size (Sq Ft): | Room Examples: | Size Of Air Purifier (CFM CADR Rating): |
| 50 Sq Ft | 8×6 Room | 13.33 CFM CADR |
| 100 Sq Ft | 10×10 Room | 26.67 CFM CADR |
| 150 Sq Ft | 12×12 Room | 40 CFM CADR |
| 200 Sq Ft | 14×14 Room | 53.33 CFM CADR |
| 250 Sq Ft | 16×16 Room | 66.67 CFM CADR |
| 300 Sq Ft | 16×18 Room | 80 CFM CADR |
| 350 Sq Ft | 20×18 Room | 93.33 CFM CADR |
| 400 Sq Ft | 20×20 Room | 106.67 CFM CADR |
| 450 Sq Ft | 22×20 Room | 120 CFM CADR |
| 500 Sq Ft | Two 16×16 Rooms | 133.33 CFM CADR |
| 600 Sq Ft | 25×25 Room | 160 CFM CADR |
| 700 Sq Ft | 2 Bigger or 3 Smaller Rooms | 186.67 CFM CADR |
| 800 Sq Ft | 2 Bigger or 4 Smaller Rooms | 213.33 CFM CADR |
| 900 Sq Ft | 3 Bigger or 4 Smaller Rooms | 200 CFM CADR |
| 1000 Sq Ft | 3 Bigger or 5 Smaller Rooms | 266.67 CFM CADR |
You can see that for 2 ACH, you need 2.5x smaller (less capable) units than for 5 ACH. Thus, you would need anywhere from 13.33 CFM CADR to 266.67 CFM CADR for 50 sq ft to 1,000 sq ft areas, respectively.
Using the air purifier size calculator above or these 2 size charts, you now have a good understanding of how big an air purifier you should get. If you have any questions or would like for us to help with the math, you can use the comment section below to tell us what you are trying to solve, and we can help you out.