What Is a CADR Rating? What It Is and How to Use It

If you’ve been shopping for a new portable air purifier, then you’ve probably come across the term “CADR rating.” CADR stands for Clean Air Delivery Rate.

It is a metric that expresses how quickly air moves through the purifier and how effectively the filter removes particles from that air. This value is assigned by AHAM and is calculated using their certified test.

But most air purifiers don’t just have one CADR score—they have three. And understanding what each of these is really telling you is key to finding the right unit to fit your needs.

Below, we’ll take a more in-depth look at what a CADR rating is, how it’s calculated, its limitations, and how you can use CADR to calculate square footage coverage for portable air purifiers.

What Does CADR Rating Mean?

CADR formula

Clean Air Delivery Rates are assigned by the Association of Home Appliance Manufacturers (AHAM). They calculate these ratings by performing a standardized efficacy test. This test consists of placing the unit in a test chamber filled with a specific amount of specific particles and measuring how much clean air the unit delivers over time.

This test measures two separate things which come together to determine the unit’s CADR.

First, the test looks at how much air the unit moves. This is typically calculated in cubic feet per minute (CFM). Purifiers with large, powerful motors move more air per minute than small units with weaker motors.

The second item measured is the efficiency of the air filter. The higher the percentage of particles that get trapped in the air filter with each pass, the faster the air will get cleaned.

Note: CADR is a combination of these two features. Units that score high in both categories will have the highest CADR ratings.

Since filters have different efficiencies depending on the size of the particles being filtered, each unit receives multiple CADR scores. These scores are measured by testing the unit against small particles (tobacco smoke), medium particles (paper mulberry pollen), and large particles (fine dust). Each of these ratings comes with a different score range:

Particle TypeRating Range
Smoke (particles between 0.9 and 1.0 μm)0 to 400
Pollen (particles between 0.5 and 3.0 μm)0 to 450
Dust (particles between 5.0 and 11.0 μm)0 to 450


CFM is a unit that represents the number of cubic feet of air a unit moves per minute. It is not the same as CADR, which has no units. However, because the rate of air exchange is one of two features measured by CADR, it is an important piece of the puzzle.

CADR is a combination of air exchange (measured in CFM) and filter efficiency, which is given as a percentage.

How is CADR Rating Calculated?

While wrapping your head around what CADR tells us might be a little hard to understand, actually calculating the number is fairly straightforward. CADR is simply the unit’s air exchange rate times the filter’s efficiency.

Air Exchange Rate (CFM) X Filter Efficiency (%) = CADR

For example, if an air purifier has an air exchange rate of 200 CFM and the filter captures 80% of particles that passed through, the CADR would be 160.

If a unit has an air exchange rate of 175 and a filter that captures 100% of particles, then the CADR would be 175. 

As you can see, the slower air purifier is actually more effective because it has a much more efficient filter. That is why the CADR score is helpful. It tells you exactly how well the unit cleans, not just how powerful the motor is or how many particles the filter will capture.

As we mentioned above, each unit is tested for small, medium, and large particles. This gives you the smoke, pollen, and dust CADR ratings.

Note: Often, finding all three ratings on a product is difficult. If only one is listed, it is likely to be the smoke CADR, even if that is not specified. If you are getting an air purifier specifically for allergies, then it is worth hunting down the dust and pollen CADR scores as well.

What CADR Rating Do I Need?

The CADR score of an air purifier can be very helpful in determining what size room the purifier should be used in. But this is not a straightforward conversion.

Why? Because how effectively and quickly an air purifier needs to work changes based on the needs of the person using it. Because of this, there are different ways of calculating room coverage based on CADR ratings.

Note: When calculating room size, always use the smoke CADR, as the smallest particle test gives the most accurate estimations for overall coverage.

Coverage Calculation Option 1: Air Exchanges Per Hour

One way to model the different needs a user might have is by calculating how many full air exchanges per hour a unit will produce in different-sized rooms. The larger the room, the fewer air exchanges will be completed.

In the table below, the CADR score of the unit is given on the left. The optimal square footage for that unit is expressed based on how many air exchanges per hour are desired.

CADR3 Air Changes p/h4 Air Changes p/h5 Air Changes p/h6 Air Changes p/h7 Air Changes p/h
50125 sq ft93.75 sq ft75 sq ft62.5 sq ft53.5 sq ft
100250 sq ft187.5 sq ft150 sq ft125 sq ft107 sq ft
150375 sq ft281.25 sq ft225 sq ft187.5 sq ft160.5 sq ft
200500 sq ft375 sq ft300 sq ft250 sq ft214 sq ft
250625 sq ft468.75 sq ft375 sq ft312.5 sq ft267.5 sq ft
300750 sq ft562.5 sq ft450 sq ft375 sq ft321 sq ft
350875 sq ft656.25 sq ft525 sq ft437.5 sq ft374.5 sq ft
4001000 sq ft750 sq ft600 sq ft500 sq ft428 sq ft

You can utilize the chart above to find the best CADR rating for your needs by choosing how many air changes you want per hour, locating the approximate square footage of the space, and referencing the corresponding CADR score.

Note: In most instances, an air change between 3 and 5 times per hour is optimal. However, if you’re battling allergies, smoke, or mold, 5 air changes per hour should be the minimum target range.

Coverage Calculation Option 2: The 2/3rds Rule

A much simpler way to calculate square footage from CADR is to go by the AHAM recommendation for the suggested room size based on CADR score.

By taking the square footage of your room and dividing it by 1.55 you get the minimum CADR rating needed to provide continuous air filtering to remove 80% of particles.

Room Size (sq ft) / 1.55 = Minimum CADR

Using this calculation, we would find that a 500-square-foot room requires an air purifier with a CADR of 322 or higher.

You can also use this calculation backward to determine the maximum square footage covered by an air purifier using its CADR score:

CADR X 1.55 = Maximum Room Size (sq ft) 

Using this calculation, we estimate that a unit with a CADR of 322 is appropriate for a maximum room size of 500 square feet.

This is also known as the “2/3rds rule” since it stipulates that the CADR should equal about 2/3rds of the square footage of the room the unit is being used in.

Note: Since CADR scores only go up to 400 (or 450, depending on particle size), rooms over 620 square feet will require more than one unit to properly purify, according to this calculation.

The 2/3rds rule aligns well with the metrics given in the table above for 5 air changes per hour. If your purification needs are less intense, you can certainly size down in terms of your target product’s CADR. If they are more intense, be sure to size up.

Is a High CADR Better?

A high CADR score represents an air purifier that exchanges a high volume of air per minute and has a highly effective filter. These kinds of units are optimized for use in large spaces where a high density of pollutants exists.

But in smaller spaces, higher CADRs are not always better. 

As you can see from the table above, a purifier with a CADR rating of 50 is perfectly capable of changing the air 5 times per hour in a 75-square-foot bathroom. In fact, this unit works just as effectively in a small space as a 400 CADR unit does in a 600-square-foot space.

Note: When shopping for your new air purifier, it’s most important to match the CADR to the size of the room you’ll be using it in. While it is true that a more powerful unit will offer more air changes per hour, it will also use much more energy. And, at a certain point, those extra air changes stop offering additional benefits.

CADR Limitations 

CADR scores offer an easy way to compare air purifiers that have different filter types and different-sized motors. But there are also some limitations to this metric.

Limited Particles Tested

woman changing a dirty filter

For one, only three different particles are used to create CADR scores. While these three are meant to span the spectrum of particle sizes air purifiers are meant to combat, they don’t tell us the full story.

CADR tells us nothing about the unit’s ability to combat odors, VOCs, and toxic gasses like carbon monoxide, sulfur oxide, and ozone. Because mold spores (which can vary in size) are not specifically tested for, it can be hard to know which unit is best for dealing with active mold problems.

In today’s environment, it is also worth noting that these tests do not look at bacteria and virus filtering abilities. Again, these are particles that come in a huge variety of sizes, so using current CADR metrics makes it difficult to ascertain how effective a given unit is at capturing all pathogens.

Lastly, the smallest particle tested in the CADR tests is 0.5 microns. A high percentage of the particles in the air are going to be below this size, including viruses. HEPA filter certification gives us some idea of how well a filter can deal with these tiny particles, but the CADR rating system ignores them entirely.

Trapped vs. Neutralized

CADR tests also make no distinction between particles that are trapped in the filter and those that are neutralized. This isn’t a big deal when it comes to dust, pollen, and smoke, but it makes a big difference in terms of mold, VOCs, and pathogens.

When these living and innately harmful particles are trapped but not neutralized, there is a high potential of them reentering the environment later on. This may happen during filter changes or each time the unit powers off.

Particles like mold that aren’t neutralized, can even begin to multiply within the framework of the filter, increasing the likelihood of mold spreading through the room.

Long-Term Efficiency

air purifier with a new filter installed

CADR tests are performed on new air purifiers with new filters. The tests are limited to a given amount of time and do not account for changes in purifier performance over time.

While you can assume a new purifier will perform as well as the CADR scores attest, you can also assume this performance will only weaken with time.

Overall, the CADR rating of an air purifier is highly helpful in comparing units against one another and determining the proper room size. But these ratings have limitations which means other factors of the product should be taken into consideration before you make your final decision.

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Sara Seitz
Sara Seitz is a freelance writer and novelist. She lives with her husband and wildling toddler in Colorado where she spends her days working on their house, gardening, and reconnecting with nature.

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