What is ACH in Air Purifiers?

Air Changes per Hour (ACH or ACPH) is the number of times an air purifier filters all the air in a room within one hour. A purifier with an ACH rating of 5 filters air five times in one hour (every 12 minutes). It is among the factors that determine the effectiveness of air purifiers. The higher the ACH rating, the more air the purifier can clean.

An air purifier is said to have a good ACH rating if it has a rating of 4 and above. You see, air purified more frequently will have fewer contaminants building up, ensuring high-quality air.

How to calculate ACH

If the manufacturer has not indicated the ACH, you can calculate it before taking the appliance home. To calculate ACH yourself, you need:

  • Clean Air Delivery Rate (CADR) of the air purifier
  • The volume of the room

Clean Air Delivery Rate (CADR)

CADR is the measure of how much clean air comes out of an air purifier. It is a measure of how fast a purifier can clean air within a particular room. CADR is taken in cubic feet per minute (CFM). For example, a purifier with a CADR of 400 CFM cleans a 400 sq. ft. room faster than a purifier with a 300 CFM CADR.

Since air has different particles, CADR gives you the numeric value of how many specific particles the air purifier eliminates.

It is measured using pollen, dust, and smoke particles because they are the most common contaminants in indoor air. For example, we could have an air purifier that removes 200 CFM of smoke, dust 191 CFM, and pollen 201 CFM.

Smoke particles represent ultra-fine air particles measuring between 0.1 to 0.3 microns while dust represents common particles measuring between 0.5 to 3 microns. On the other hand, pollen stands for larger air particles measuring 5 to 11 microns.

What is ACH in Air Purifiers - GermGuardian AC4100 CADR
GermGuardian AC4100 CADR

Air purifiers with high CADR rates show that their fan is powerful. Note that CADR is a measure of cleaning capacity, not capability. While it is important to look at the CADR ratings, it should not be the only factor in deciding the purchase of an air purifier.

The volume of the room

To get your room’s total volume, multiply the Length by Width by the room’s Height (LxWxH). However, if your rooms’ design is not the standard shape, then you may need a more complex formula.

Calculating ACH

ACH = CADR/Room Volume

Let us assume that an air purifier with a CADR of 300 CFM is used in a 300 sq. ft. room whose height is 10 feet.

CADR in one hour equals 300 x 60 (minutes) = 18,000

Volume = 300 x 10 = 3,000

Therefore ACH = 18,000/3,000

ACH rating is 6.

Calculating CADR using ACH

If you buy an air purifier with a lower CADR than what is ideal for a room, it will not be effective. On the other hand, if you buy one with a higher CADR than necessary, you will be spending more money on an appliance you do not need. Check out our reviews of the best air purifiers in Canada for more information.

Here’s how to calculate the CADR of a specific room:

Minimum CADR = (ACH x Room Volume)/60

(6 x 3,000)/60 = 300 CFM

From the figures above, you need an air purifier with a CADR of 300 CFM for such a 300 sq. ft. room.


Air purifiers with an ACH of 4 and above are more effective in removing air contaminants. So, if you have asthma and allergic complications, it is advisable to purchase an air purifier with a high ACH.

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