Air Change Calculator

What Does Air Changes Per Hour (ACH) Mean in Air Filtration? 

Air changes per hour, which is often abbreviated as ACH, is a calculation of how many times per hour the entire volume of air in a given space is replaced with supply and/or recirculated air.  It is also sometimes referred to as “air change rate” or “air exchange rate.” 

Why Do You Need to Calculate ACH? 

Air changes per hour are important in determining if a space is getting adequate ventilation. Additionally, if you have chosen to implement air purifiers to improve air quality and lower the risk of COVID-19, ACH will help you determine which devices are appropriate for your application, as well as how many you need. 

Ventilation is important for the indoor air quality (IAQ) inside a building. Improper ventilation can lead to a buildup of pollutants in indoor spaces, which is detrimental to the health of building inhabitants. According to the EPA (1), negative health effects of poor indoor air quality include:

• Irritation of the eyes, nose, and throat.
• Headaches, dizziness, and fatigue.
• Respiratory diseases, heart disease, and cancer.

Polluted air also leads to moodiness and decreased productivity for employees and reduced learning for students in school.

Additionally, the EPA asserts that increasing ventilation is an important strategy for decreasing the spread of COVID-19 in public spaces (2). 

What Is the Formula for Calculating Air Changes per Hour? 

To calculate air changes per hour, you will need to find two numbers. 

CFM – First, the volume of air in CFM (cubic feet per minute) currently delivered to the room in question by the building’s air handling unit or when evaluating an air purifier, the CFM of the unit with the air filters installed. 

ROOM VOLUME – Secondly, you will need to find the volume of the room in cubic feet, which is calculated by finding the height, length, and width of the room in feet and multiplying these numbers together.

• Multiply the CFM by 60. This number will tell you how many cubic feet of air your device moves per hour. 
• Divide the number you get by the room’s volume in cubic feet. You now have a number telling you how many times per hour the device moves the total volume of the room. This number is your air changes per hour, or ACH.