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Size Matters - Is Your Air Purifier "Big" Enough?

Air Purifier Size


An critical factor in determining the effectiveness of an air purifier is room size. There is no point in buying a unit that has no capability to clean all the air in a room – you’ll waste your money. Finding the right model for your home doesn’t seem complicated, because manufacturers indicate a “recommended coverage area“ in the description of the air purifier. The maximum coverage area is calculated based on the assumption you have a room with a 8 feet high ceiling and you are running the unit at the highest fan speed. If a machine is using the traditional filter technology, the highest fan speed often translates to very loud and noisy. If you have a room taller than 8 feet or want to operate at less than the highest fan speed, you will need a higher capacity air purifier. Are you ready for some math?

Formula #1 - Use CFM (Cubic Feet Per Minute) - A More Precise Calculation

  • Maximum Room Size = (CFM x 60) Divide By (ACH x Room Ceiling Height)
  • Use the CFM at the noise level you will likely operate most of the time

    An Example:

    • A CFM of 300 gives you max. room size of 450 sq. ft. if you have a 10 feet high ceiling and need ACH of 4. [450 sq. ft. = (300 CFM x 60) / (4 ACH x 10 feet]
      • ACH stands for Air Change Per Hour. It tells you how many times the entire room air is filtered/cleaned within an hour. ACH of 4 and above is recommended for allergy/asthma relief. However depending on the technology, this may not always be necessary. If a unit is designed to run 24 x 7, one could argue that it is more important to have the most effective air cleaning / filtration than how fast it is cleaning.
      • Check your air purifier's spec for CFM if it's available to you
      • If not, lower the manufacturer's room size by 10% to 20%  

      Formula #2 - Use CADR (Clean Air Delivery Rate) - A Simple Formula to Get You In The Ballpark

        • Maximum Room Size = CADR x 1.55
        • A CADR of 300 gives you max. room size of 465 sq ft
        • Most air purifiers come with three CADR numbers, smoke, pollen and dusk. Pick the lowest number for the math.

        UNDERSTAND "Free Flow"/"Before Filter" CFM vs. Actual/"After Filter" CFM

        • Free Flow or Before Filter indicates air flow without filters
        • Actual or After Filter is the air flow with filters in the unit
        • Some manufacturers publish only the Free Flow CFM in their spec sheet.
        • When calculating the coverage area, use Actual or "After Filter" CFM if it's available to you. 
        • If only "Free Flow" or "Before Filter" CFM is available to you, reduce it by at least 15%

        Dwell Time vs. CFM - How Does That Impact Coverage Area Calculation?

        Dwell Time

        • The amount of time indoor air pollutants spend going through an air filter or cleaning unit
        • You want longer dwell times when you turn on your air purifier. The longer harmful indoor air contaminants dwell inside the air purifier, the more filtration/cleaning is taking place.
        • One suggestion is to operate the air purifier at slower speed, which also helps to reduce white noise. 
          • When an air purifier has multiple fan speeds/settings, lower setting translates to lower CFM, but longer dwell time that often means more effective cleaning. 
          • If "longer" dwell time is desired (more effective filtering or killing airborne pollutants), choose a lower setting CFM for coverage calculation.
          • Most people do not run their units on the highest fan speed constantly as the maximum noise level associated with it is typically too loud.


              In most cases you will notice some discrepancies in these numbers for reasons we are not going to delve into here such as testing conditions and various other assumptions. I recommend picking the lowest number. If this is confusing, feel free to give us a call at 1866-626-5969 or send us an email at We will be happy to help.

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