What is CFM and how does it relate to Air Purifiers?

CFM stands for "cubic feet per minute" of air passing through the unit. All HEPA air purifiers have a CFM rating. For example, if an air purifier has a rating of 300 CFM, this means that 300 cubic feet of air passes through the unit every minute or 18,000 cubic feet per hour.  It is recommended to have a minimum of two complete air charges per hour.  For remediation work, six air charges per hour are recommended.

Room Air Cleaner Capacities and Sizing:

Determine the size of your room by measuring how wide and how long it is.  Multiply the width versus length as this is the total square feet of the room space.  To find the volume of the room, multiply the floor space by the room height.  To compare room volume to air cleaner capacities, determine the number of air charges an hour (ACH) you desire.  Indoor Air Quality professionals and Allergists recommend 2 room chargers an hour, designated as 2 ACH.  Spaces with high pollutants may require higher air changes to reduce pollutant levels.

To determine how much air must be moved through the air cleaner to provide 2 ACH, you take the total volume of the room and multiply it by 2.  Air cleaners provide air flow performance in cubic feet per minute, so you must take your total volume of air for 2 ACH and divide 60 minutes to get a value for cubic feet per minute (CFM).

Note: The location of the air purifier in the room can affect performance.  Try to place the unit in a central location.

Air Cleaner Sizing Example:

Room Size
12’ x 20’ = 240 square feet of floor space (sq. ft.)
Ceiling height is 8’
240 sq.ft. x 8’ = 1920 cubic feet of air volume (cu. Ft.)
Two Room Air Changes per Hour
1920 cu.ft. x 2 = 3840 cu. ft. per hour (60 minutes)
Volume per Minute
3840 / 60 = 64 cubic feet per minute (CFM)

In the previous example, air cleaner manufacturers provide air flow performance rating when the fan and motor are under no load or not in the system.  Air cleaner system air flow rates can diminish up to 60% of the no load air volume when the filter media is installed due to the resistance to the air flow through the filter media.  Please consider this air flow rate reduction when sizing air cleaners with untested system air flow rates.