Open apartment layout showing a living room and kitchen, with a beige color scheme. The room looks clean.

How to Understand Purifier Air Change Rates

One great way to maintain your indoor air quality is by using an air purifier. Air purifiers work by removing contaminants such as dust, pollen, smoke, and other airborne particles and can operate for hours on end. One measurement of their effectiveness is the ACH – the number of air changes (ACH) the machine can perform per hour. While air change rates for purifiers can be confusing, ACH makes it easy to understand. 

What Are Air Changes Per Hour for Air Purifiers?

ACH measures how frequently the air purifier can replace all the air within a room in one hour. Understanding ACH is vital for selecting the right air purifier for your space and ensuring it operates efficiently. This article explains how to calculate ACH based on room size, offers recommendations on optimal hourly change rates, and discusses why the benefits of having an air purifier for your home.

Open apartment layout showing a living room and kitchen, with a beige color scheme. The room looks clean.

Calculating Air Change Rates Per Hour (ACH)

ACH is calculated by a specific formula, with the variables being the Clean Air Delivery Rate (CADR) and the volume of your room. It’s simple and can easily be calculated! Here’s how we solve for ACH: 

Step 1: Determine the Volume of the Room

First, calculate the volume of the room by multiplying its length by width by height (L x W x H). 

Measure in feet for consistency! It makes things easier. For example, a room 20 feet long, 15 feet wide, and 8 feet tall has a volume of 2,400 cubic feet (20 x 15 x 8). If you need it, there are plenty of conversion calculators online. 

Step 2: Know Your Air Purifier’s CADR

The CADR indicates the volume of filtered air an air purifier delivers per minute. The manufacturer typically provides this information, which can be found in the product specifications.

The CADR is measured in cubic feet per minute (CFM). This is why it’s essential to keep your units in feet! 

Step 3: Calculate ACH

To calculate the ACH, you need to convert the CADR from cubic feet per minute to cubic feet per hour by multiplying it by 60 (since there are 60 minutes in an hour). Then, divide this number by the volume of the room.

            (CADR (in CFM)×60)

ACH= ———————————————

         Volume of the Room (in cubic feet)

           (CADR (in CFM)×60)

ACH= ———————————————

         Volume of the Room (in cubic feet)

For instance, if an air purifier has a CADR of 100 CFM for the room mentioned above:


——————————————— = 2.5 ACH



——————————————— = 2.5 ACH


This result means the air purifier can replace the air in the room 2.5 times per hour.

Recommendations on Air Change Rates Per Hour

The recommended ACH depends on the specific needs of the room’s occupants and the activities performed within the space. General guidelines suggest:

  • An ACH of 2 to 4 for residential spaces is typically sufficient for maintaining good indoor air quality.
  • A higher ACH of 4 to 6 can help allergy sufferers by frequently removing allergens from the air.
  • In spaces where pollutants are regularly introduced, such as kitchens or smoking areas, an ACH of 5 to 6 may be necessary.

Why ACH is Important

The ACH rate of an air purifier is essential for several reasons:

  • Health and Comfort: Higher ACH rates can more effectively remove pollutants, improving respiratory health and overall comfort.
  • Efficiency: Understanding the ACH helps ensure that you select an air purifier that is appropriately sized for your space, avoiding over or under-purification.
  • Cost-Effectiveness: Properly sizing an air purifier for the intended space can also be more energy-efficient, potentially saving electricity costs over time.

In conclusion, the number of air changes per hour is critical to consider when selecting and using an air purifier. By calculating the appropriate ACH for your space and understanding the recommended rates for various conditions, you can enhance the indoor air quality of your environment, promoting health, comfort, and efficiency.

Lead Editor (IAQ), Clinical BacteriologistJessica Gunoskey
Jessie is the lead editor for Air Koality, overseeing all IAQ guides to ensure accuracy and quality. She holds a degree in Molecular & Infectious Disease Biology from Washington College in addition to working for multiple top-rated universities, such as UConn and UPenn. Not only does she write and edit, but she also is a STEM tutor and microbiologist!

Expertises: microbiology, infectious disease, biology, air quality

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