One of the main features to keep an eye out for when looking for an air purifier is the ACH. It stands for Air Changes per Hour and it’s an aspect that many people get confused by. 

Below, you can discover everything you need to know about what ACH ratings mean, how they’re calculated, and how they affect people with allergies and asthma. 

What ACH Ratings Tell Us 

ACH ratings provide you with information on the number of times that air purifiers are able to filter the room each hour. 

For instance, an air purifier with an ACH rating of 3 means that it will purify the air throughout the entire room up to 3 times every hour. 

Therefore, ACH ratings are directly linked with the overall performance quality of an air purifier. You can use ACH ratings as a way to compare different air purifiers and their effectiveness. 

The higher the ACH rating, the more the air purifier is able to clean the air in the room each hour. 

How to Calculate ACH?

When it comes to calculating the ACH rating of your air purifier, initially, it can seem like a complicated task. However, once you have the rating, it can help you make the best decision about which air purifier is the right one for your space. 

Prior to making any calculations, you’ll need to find out the CFM (Cubic Feet per Minute) of the machine. 

CFM ratings are easily found on the technical details of an air purifier. You can also learn more about the effectiveness of the air purifier by looking at the CADR rating.

CADR ratings measure the air purifiers ability when it comes to cleaning allergenic particles, such as pollen, pet dander, dust, and mold. You can learn more about it here. 

When you’re calculating ACH ratings, the value you’re given is based on the fan in the air purifier being at the highest speed. 

So, you’ll want to keep this in mind when using the air purifier at low or medium settings. The ACH ratings at these decreased fan speeds will be a little lower than what you’ve calculated. 

Steps on Calculating ACH Ratings

We recommend that you have an ACH rating of at least 4. However, higher ACH ratings mean that the air purifier is more effective.

  • Multiply the CFM rating by 60 minutes. This provides you with the amount of air that the device can clean in an hour.
  • You’ll then want to take a measurement of the height of the room you’re planning to use the air purifier in. Divide the value of CFM x 60 by the ceiling height (in feet). 
  • These calculations are in cubic feet, but you want to end up knowing the values in square feet. 
  • Next, take the value of the CFM x 60 divided by the ceiling height, and divide it by your desired ACH rating.
  • That provides you with the amount of coverage that the air purifier is capable of cleaning in square feet. 

Here’s an example of a formula below that you can follow when making calculations. The numbers are made up but you can follow the same steps with different numbers depending on the height of your ceiling, CFM rating of the air purifier, and your desired ACH rating. 

For the calculation below, the air purifier is going to have a CFM of 300. The height of the ceiling will be 8 inches, and our desired ACH rating is 6. 

  1. 300 CFM x 60 (minutes) = 18,000 
  2. 18,000 / 8 feet (average ceiling height) = 2,250 
  3. 2,250 / 6 ACH = 375 square foot of coverage


Air purifiers with an ACH rating that’s lower than 4 can still be effective at cleaning dust, pollen, pet dander, and mold particles. However, the ones that are rated higher will simply perform better and help people with asthma and allergies be more comfortable.

Hopefully, you can use the information found in this post to help you calculate the ACH rating that’s best for your room and needs.