What is Air Changes Per Hour (ACH) & How to Calculate

Blast Mini HEPA filter air purifier in office for protection against COVID coronavirus

Air changes per hour (ACH) is an important metric to ensure a room has good air quality and proper ventilation.  But what is air changes per hour and how exactly do you calculate ACH?

What is Air Changes Per Hour?

Air changes per hour is simply the amount of times all of the air in a room is replaced with completely new air in one hour.  Therefore, the stronger an air purifier is, the more times the air gets replaced with new clean air and the higher the ACH. For example, an air purifier with 3 ACH for a given room will replace all of the air in the room with new air 3 times an hour.

Read More: ASHRAE Recommended Air Changes Per Hour For Offices, Schools, Residential, and More

How to Calculate Air Changes Per Hour?

To calculate air changes per hour of a given air purifier in a room, all you need is two numbers: 1) CADR of the air purifier 2) total volume of the room.

Step 1: Find your air purifier’s CADR

CADR measures how much air comes out of your air purifier.  The rate is typically given as cubic feet per minute (CFM).  Many air purifiers will provide three different CADR numbers for smoke, dust, and pollen.  We suggest you take the lowest of these three CADR numbers for your calculation.

Note: If your CADR is given in m3/h, convert this to cubic feet per minute by multiplying by .588.

How to calculate ACH from CADR

Step 2: Measure the volume of your room

Next, measure the area of your floor in square feet.  If you are using metric units, multiple the area in square meters by 10.76.  Next, multiply this number by the height of your room (in feet) to get the total volume of your room.


Calculating Air Changes Per Hour (ACH)

Step 3: Calculate the amount of air changed per hour by purifier with CADR

Multiply our air purifiers CADR rating (measured per minute) by 60.  If our CADR is 300, the air purifier changes 300*60=18000 cubic feet of air per hour.

Step 4: Calculate air changes per hour (ACH)

Final step! Take the number from step 3 (air changed per hour by purifier) and divide it by the volume of your room.

How to calculate ACH (Air Changes Per Hour)

Air Changes Per Hour (ACH) and CADR Calculator

For your convenience, Smart Air has created a calculator to help you with ACH and CADR calculations.  Just input two fields and the calculator does the work for you!  The calculator takes both feet and meters as input options.

Smart Air Air Changes Per Hour (ACH) and CADR Calculator


How to Calculate Air Changes Per Hour (ACH) and CADR


Example: Calculating ACH for the Smart Air Blast Mini Air Purifier

Let’s go through an example with the Smart Air Blast Mini.  The Blast Mini has a CADR rate of 585 m3/h, which is quite powerful for an air purifier.  Lets calculate this air purifier air changes per hour for a 50 sq meters (538 sq feet) room with 9 feet ceilings.

Calculating the air changes per hour (ACH) for the Blast Mini air purifier
The Smart Air Blast Mini Air Purifier
  1. CADR of air purifier is 585 m3/h.  Since given in m3/h, we multiply this by .588 to get 344 cfm. 
  2. The volume of our room is our area (538 sq feet) multiplied by our ceiling height (9 feet)= 4842 cubic feet.
  3. Calculate air moved per hour by air purifier by multiplying our CADR per minute (344 cfm) by 60 to get 20640 cubic feet per hour.
  4. Divide the number in step 3 by the volume of the room (20640/4842) to get 4.26 air changes per hour (ACH).


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