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

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 do you calculate it?

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.  A value of ‘5’ air changes per hour means a room’s entire volume of air is replaced with new air 5 times in one hour.

Air purifiers purifiers do not replace the air in a room with ‘new’ air. Instead, they clean the air already in the room. This is why the term ‘e-ACH’ or ‘equivalent air changes per hour’ is sometimes used for air purifiers. For an air purifier, the more air it cleans, the higher it’s e-ACH. For example, an air purifier with 3 ACH for a given room will clean the volume of air in the room 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 “clean” air comes out of your air purifier.  The rate is given in either cubic feet per minute (CFM) or cubic meters per hour (m3/h). Here’s an example of an air purifier’s CADR rating:

Step 2: Measure the volume of your room

Next, measure the area of your floor in square feet or square meters.  Next, multiply this number by the height of your room (in feet or meters) to get the total volume of your room.


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

If your CADR value is given in CFM, you need to multiply the air purifier’s CADR rating by 60, to get it in hours.  For example, if the CADR is 300cfm, the air purifier changes 300*60=18000 cubic feet of air per hour. If the CADR value is given in cubic meters per hour (m3/h) then your number does not change.

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 from step 2.


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


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.

  1. CADR of air purifier is 585 m3/h.  Since given in m3/h, we multiply this by 0.588 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).

How I Protect Myself

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