The ACH rating is a commonly misunderstood metric on air cleaning devices.

So in this post, we’ll explain what is ACH rating and how does ACH apply to air purifiers.

We’ll also share a simple method for how to calculate ACH for those who are interested as well as share what the minimum ACH rating is that you should look for in a good air purifier unit.

What is ACH Rating?

What is ACH rating

ACH rating stands for Air Changes per Hour. This measurement tells you how many times an air purifier filters all of the room air in one hour. An ACH rating for air purifiers ranges between 2 to 6 air changes per hour.

When you’re comparing two different air purifiers, the ACH rating is important because it indicates which unit is better in regards to fresh air exchange. It also helps you determine how big of an air purifier do I need.

Here are the most common ACH ratings for air purifiers:

  • 2 ACH = Two air changes per hour
  • 3 ACH = Three air changes per hour
  • 4 ACH = Four air changes per hour
  • 5 ACH = Five air changes per hour
  • 6 ACH = Six air changes per hour

Most air purifiers list the ACH rating on the technical specifications, but not all will. If an air purifier does not list the ACH rating, then it’s generally safe to assume that it measures at 2 ACH (or two air changes per hour).

The ACH rating will apply to the total square footage coverage of the device. For example, an air purifier could list a 4 ACH rating for 700 square feet of coverage. That means you’ll get four air changes per hour (or every 15 minutes) if the unit is placed in a room that measures 700 square feet.

Take a look at our top air purifiers page to see the best units with good ACH ratings.

>> See all of Amazon's Best Selling Air Purifiers <<

How Do You Calculate ACH?

How do you calculate ACH

To calculate ACH, find the CFM of your device and multiply that number by 60, then divide that total by the ceiling height. Next, divide that number by the square feet of the room to get the ACH calculation.

You can figure out the total square feet for a room by multiplying the width by the length in feet. For example, 22 x 20 = 550 square feet.

Here’s an example for how to calculate ACH:

  1. 300 CFM x 60 (minutes) = 18,000
  2. 18,000 / 8 (ceiling height) = 2,250
  3. 2,250 / 550 (square feet) = 4 ACH

Now, let’s say you wanted to increase the air changes per hour for this device; you want to go from 4 ACH to 6 ACH. You can easily do that by switching around the last part of the equation.

To perform this ACH calculation, all you have to do swap the two numbers in step three: square feet and ACH. Then replace the ACH number with the target ACH rating you’re trying to achieve. This new equation will tell you the maximum area that the air purifier can cover for the target ACH.

Here’s an example for how to calculate a higher ACH rating that targets 6 air changes per hour:

  • 2,250 / 6 (target ACH rating) = 375 square feet

This ACH calculation means that you can increase the number of air changes per hour for an air purifier as long as you use it in a room that doesn’t exceed 375 square feet.

The opposite is also true if you want to lower the ACH rating for a device in order to get more total square footage coverage out of the unit.

Here’s an example of lowering the air changes per hour to 2 ACH:

  • 2,250 / 2 (target ACH rating) = 1,125 square feet

By doing a little bit of math, you can effectively determine the target ACH rating you want for an air purifier regardless of what the air purifier company lists on the technical specifications. As you learned, it’s all related to the total square footage of the room you’re going to use the unit in.

ACH Ratings and Air Purifiers

The higher the ACH rating, the more efficient an air purifier is at cleaning the air in a room of a certain size.

Therefore, a 3 ACH rating is worse than a 4 ACH rating, while a 6 ACH rating is better than a 5 ACH rating.

The more times an air purifier can exchange the air within the room, the cleaner and fresher it will stay.

That’s why we recommend that you never buy a product that has less than a 4 ACH rating for the total square footage coverage you need. If you do, then the unit may not be efficient for your needs.

In fact, a 4 ACH rating is the minimum industry standard for anyone who wants the best room air purifier for allergies or the best air purifier for asthmatics. That’s because there are numerous airborne allergens that can trigger both allergies and asthma to occur. Therefore, an air purifier that can clean the full volume of air in a room up to 4 times per hour is essential for getting the most relief.

Now, an air purifier with a 2 ACH rating can still be good for general air cleaning purposes and it’s also a fine unit for anyone who doesn’t have respiratory or chronic health issues.

Additionally, it’s often the best ACH rating you can find for large rooms over 800 square feet. So if you have a large space to purify, then you likely won’t find anything better.

What CADR Rating Do I Need?

The CADR rating is different than the ACH rating. CADR relates to the efficiency of an air purifier to remove three types of particles from indoor air: smoke, dust, and pollen. Visit our CADR rating scale page to learn more about this metric and how it applies to your needs.

ACH Rating Summary

We hope you enjoyed this guide on answering the question, “What is ACH rating?”

As you discovered, the ACH rating on air purifier tells you how many air changes per hour the unit can perform in a given size room. And you can increase or decrease this measurement to meet your air quality needs by using a simple formula for how to calculate ACH.

Hopefully, this article answered all of your questions about ACH ratings and has helped you feel well equipped to compare air purifiers when shopping for a new unit.

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About David Morrison

David is an Air Quality & Comfort Technician. He has expert knowledge on the technology and design of air purification, air conditioning, and heating systems. His main role is to write content that helps people get the most value out of their air purifiers, air conditioners, and heating units. (See Full Bio)