Calculator : CADR and ACH of Air Purifiers

CADR and ACH are two terms that you will come across often when you plan to buy an air purifier. Understanding the meaning and application of these terms are vital for buying the best suitable air purifier for your requirement. That is why we have put forth this short article explaining them in detail  in layman terms.

The basic principle of an air purifier’s working is sucking in air, filtering it and releasing the clean air back in to the environment.

The most important component of an air purifier is its filter. A HEPA (high efficiency particulate air) filter is considered as a must-have as it helps in capturing up to 99.97% particles greater than 0.3 micron in size, which include pollen, pet dander, dust mites, tobacco smoke, and many of the common bacteria and virus found in the environment.

The next important feature is the CADR, which affects the coverage area of the air purifier, which in turn affects the air change per hour( ACH) rating . Lets delve in to both in detail.

Read our buyer's guide to understand the important factors to consider while buying air purifier.

CADR ACH Air Purifier


CADR is the abbreviation of Clean Air Delivery Rate. It is either measured as cubic meter of air delivered in hour or cubic foot of air delivered in a minute. Essentially, it is the indicator of how much air the purifier cleans in a given time. Obviously, higher the better. The Association of Home Appliance Manufacturers suggest having three CADR numbers mentioned on each purifier, namely CADR for tobacco smoke, dust and pollen. 

So, if you see a label on a HEPA air purifier mentioning smoke CADR 300 m3/hr, it means that the unit removes smoke particles from 300 cubic meter of air in an hour. However, typically on Indian air purifiers, you usually see just one CADR figure, which usually indicates the smoke CADR and it is given in cubic meter per hour.

The AHAM guidelines recommend 2/3rd rule, which means that air filters for a room should have a smoke CADR greater than 2/3rd of the room area in sq. ft, provided the room has a maximum height of 2.4 metre. Of course larger rooms can be effectively cleaned with an air purifier having lower CADR, provided there is no air coming from outside and there is no source of particulate matter inside the room.


ACH simply is the number of times an air purifier filters all of the air in a room in one hour. So, an ACH rating of 4 means that the purifier filters the air in the room four times an hour. As you would have already figured out by now, it is one of the most important factor that signifies the effectiveness of the air purifier.

So, what is a good ACH rating? The American Society of Heating, Refrigerating and Air-conditioning Engineers a minimum ACH rating of 4 for people with respiratory ailments like asthmatics and allergy.
Most of the manufacturers don’t mention the ACH of air purifiers. Hence, we may have to calculate ACH ratings ourselves  to ascertain whether the air purifier is right for us.

ACH Calculation

In order to calculate the ACH rating, you need to know the air purifier’s CADR and the sq. ft area where the air purifier will be used.

The CADR mentioned on air purifiers in India is usually given in m3/hr. We have to first convert it to cubic feet per minute ( CFM)by multiplying it with 0.588 (1 m3/hr=0.588ft3/min)

The coverage area is usually given in sq. ft. If it is in m2, you can convert it to sq. ft. by multiplying it with 10.76

Now, let us calculate how much cubic feet of air the air purifier delivers in an hour by multiplying the CADR in CFM by 60.

Next, we find the room volume by multiplying the room area in sq. ft by the room’s height. In India, most of the building floors have a height of 9 ft; hence we have assumed this height for finding the ACH rating of all air purifiers mentioned in this website.
Now, we can obtain the ACH by dividing the volume of air delivered by air purifier in an hour by the room volume.

Lets work out an example to help you understand better.

Let us calculate the ACH rating of an air purifier with CADR of 300 m3/hr is used in a room of 250 sq. ft.

CADR = 300 m3/hr
CADR in cubic feet per minute= 300 x 0.588= 176.4 cfm

Air changed in an hour = 176.4 x 60= 10,584 ( CADR in CFM x number of minutes in an hour)

Room height is assumed to be 9 ft.

Hence, room volume= 2250 (250 x 9)

Therefore, ACH rating = Air changed in an hour / Room Volume =10,584/2250=4.70

How To Use ACH rating to decide which air purifier to buy?

There are two major complaints seen in online marketplaces about air purifiers. The first being—the air purifier they bought is not effective enough. The second is about the price. On the one hand, if you buy an air purifier that has a lower CADR​ compared to your requirement, it won't purify the air well enough. On the other hand, if you buy an air purifier that has a higher CADR compared to your requirement, you are splurging money on something that you don't need. So, lets help you choose an air purifier that exactly matches with your need by helping you calculate the CADR required for your room.

First, measure your room size in sq. ft and height in ft. Then, decide what ACH rating do you want your air purifier to have. Ideally, it should be at least 4. So, now that you have both the figures, you can calculate what CADR your air purifier ought to have. This way, you do not have to spend a lot on an air purifier with a high CADR, when a lower CADR air purifier is sufficient for your requirement.

In order to help you understand better, let us explain with an example.

Being allergic to dust, you need an air purifier that has ACH rating of 4. You plan to use it in your bedroom that is of the size 10 x 12 ft and height of 9 ft.

Room volume= 10 x 12 x 9= 1080 cubic feet


Hence, air to be changed in an hour = 1080 x 4= 4320 cubic feet per hour

Now, lets convert cubic feet per hour to cubic meter per hour ( 1 ft3/hr=0.028 cbm/hr)

Air changed per hour= 4320 x .028 =120 m3/hr

Hence, you need an air purifier of CADR 120m3/hr for a room of size 120 sq. ft.

Overwhelmed by the calculation? Here is a calculator to help you find the CADR of air purifier required for a given room size (assuming room height to be 9 ft)

With that, we hope you are able to identify the right CADR that you need and hence buy an appropriate air purifier. You could also read our top picks of air purifiers which we have short-listed after thorough review and analysis of its features and functions.


  1. Sam February 8, 2018
    • Admin February 8, 2018
      • Sam February 8, 2018

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