What Is the Formula for Calculating the Air Change Rate?

To calculate the air change rate, you will need to find two numbers.:


First, we need the flow volume in cubic meters per hour (m3/h or CMH) currently delivered to the room in question by the building’s air handling unit, or when evaluating an air purifier, the flow volume of the unit with the air filters installed. Sum up all the flows if there are multiple devices being operated in the room.


Secondly, you will need to find the volume of the room in cubic meters (m3), which is calculated by finding the height, length, and width of the room in meters and multiplying these numbers together.

From there, the calculation to find air change rate is simple:

Divide the (total) flow volume you get by the room’s volume in cubic meters.



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 Change Rate in Air Changes per Hour,