how to calculate cadr for air purifier
Kristina Davis

Relax! I know the title says, “How to calculate CADR for air purifier” and you probably are thinking this is a math tutorial. But stay calm, I promise you this isn’t.

CADR stands for Clean Air Delivery Rate and it is a very important metric for air purifiers, and honestly, I do advise you to know all about it, or at least how to determine it before purchasing an Air purifier.

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This article would help you get that needed knowledge, but you have to keep reading

On. I’ll tell you all you need to know about the Clean Air Delivery Rate and why it is important to know it before purchasing a purifier.

But first, what is CADR? and how did it come to be?

Using the AHAM recommended 1.55 rule to calculate the CADR. You could get your air purifier CADR by dividing your given room size by 1.55

So, What Is a CADR?

what is cadr

The CADR or Clean Air Delivery Rate is a very important metric that is associated with air purifiers.

By knowing the CADR, you can tell just how quick or effective an air purifier would be when purifying the indoor air.

It could also help in determining the level of power we might need in air purifiers for our apartments, homes, or offices (assuming we know the exact volume).

How Did CADR Come to Be?

Well, back in the 1980s, whoever bought an Air purifier would have to worry about the filter and maximum airflow of the purifier (which was and still is measured in CFM).

how cadr came to be

But sadly, knowing all that was not enough, as no one could tell how effective air purifiers were at removing pollutants like smoke, dust, and mold spores from the indoor air.

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Then AHAM, an acronym for Association of Home Appliances Manufacturers came up with a system to connect end-users with air purifier manufacturers.

This gave birth to the CADR test. And with this CADR test, every user could now figure out just how effective their purifiers truly are. How? Well, I’ll give you an example.

Supposing an Air purifier possesses a maximum or final airflow of let’s say 300 CFM. And it has a CADR rating of let’s say 200 CFM.

That would mean that the air purifier would only be able to process about 300 cubic feet a minute.

But it would only clean 200 cubic feet effectively in its first purifying cycle and this was because of its CADR rating. The other 100 CFM would get cleaned in its second cycle.

Note: The CADR oftentimes doesn't consider the noise level of the air purifier.

So How Does the CADR work?    

how does cadr work

To get an Air purifier’s CADR, three particle sizes are measured. Each of them demonstrates the ability of an air purifier to remove small particles like (smoke) to large particles like (pollen).

  • For smoke, the CADR ranges from 0-09 to 1micron.
  • For dust, the CADR ranges from 0.5 to 3 microns.
  • For pollen, the CADR ranges from 5 to 11 microns.
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Now we have two factors that could influence the CADR rating. These factors are the maximum airflow through the filter and the filter’s efficiency.

If these two factors are taken together, they would give you an estimation of just how much-filtered air you could get from the air purifier.

The CADR works with the fan speed, as it represents the total amount of air coming out of an air purifier based on its fan speed.

To help your understanding, let’s say the air purifier works on a lower fan speed, the CADR would automatically be lower.

Note: The CADR is not influenced by features like UV-C Light. But it could have an impact on the Energy Star as additional power that could be used by the air purifier is added by the UV-C.

The Clean Air Delivery Rate Particle Sizes

clean air delivery rate particle

As we stated earlier, three CADR numbers currently exist. But you would mostly see one CADR rating and, it oftentimes is the average of three ratings.

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Although their names are pollen, smoke, and dust particles, they only represent the particle sizes that are airborne and could be tested.

For instance, dust particles could range from anything smaller than the CADR range scale-like fine dust.

The largest airborne particles are represented by the pollen CADR. It could represent anything like pet hair, pet dander, larger size mold spores, dust mites, dust, and pollen.

If you are searching for a tobacco smoke remover, then it is most likely that you would opt to remove the smoke odor and the smoke particles. Now the smoke CADR would only measure particulate matter.

It never reflects what you expect from odor removal.

However, it is ideal for removing smoke and air pollution, as it shows the smallest particles you could test.

The CADR scale 

a cadr scale

Based on the size of the particle to be measured, a CADR scale could range from as low as 400 to as high as 450.

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As for pollen and smoke CADR, it could measure as high as 450 while the dust CADR could measure up to 400.

The CADR does give a performance indicator and is mainly used in determining the size of the purifier.

Also, the air purifier is expected to possess a dust CADR rating that is as powerful as the maximum fan speed all to achieve the energy star program.

Note:  The CADR rating of the smoke CADR would reflect smaller air particle removal only (such as smoke particles from tobacco). It still gives no evidence that a particular household air purifier is effective in removing cigarette smoke.

The CADR to Room Size

You could also convert the Clean Air Delivery Rate to something as practical as your room size.

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cadr to room size

You could take your CADR and multiply it by 1.55 and that gives you your room size. This is all based on a room with an 8ft tall ceiling height.

But if the reverse is the case and you’d like to know how to calculate CADR for air purifiers, given your room size.

You divide your room size by 1.55 and voila! That’s your CADR.

Tip: It is recommended by AHAM, that you use a smoke CADR when calculating the square footage of the room.

What Is a Good Cadr for an Air Purifier?

Obviously, homeowners who are familiar with the CADR ratings like to compare the CADR ratings of every air purifier.

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Unfortunately, most manufacturers of air purifiers do not perform AHAM CADR tests. For these, the datasheet only shows the maximum airflow (in CFM).

good cadr for an air purifier

But then, How do you calculate CADR rating from just the given maximum airflow? The short answer is you can’t.

The CADR cannot be calculated. But it could be Measured in the test chambers. However, AHAM subsequently adopted a simple 2/3 rule.

This rule helps to estimate the CADR rating based on the given maximum airflow. This could help give you a clue of ​​what would be a good CADR rating for your air purifiers.

If your air purifier’s CADR rating is at least 2/3 of maximum airflow, then you know that you have an air purifier fitted with a good filtration system.

However, if the CADR rating is lower than 2/3 of the maximum airflow, then you most definitely need to search for better air purifiers.


So far, I’ve introduced you to the meaning of CADR (a voluntary program developed by AHAM) and the history of CADR (how it came to be), and how to calculate CADR for air purifiers.

I also introduced you to how the CADR works, and the different particle sizes for the CADR (pollen, dust, and smoke).

But most importantly, I showed you the best way to determine a good CADR for any air purifier.

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So, that’s it, all you need to know about the CADR for air purifiers, I do recommend you check out our previous articles, you might learn something new. For now, stay safe.