Air purifier CADR and air-change calculator

Enter a room and this returns the minimum smoke CADR that EPA's own published table and AHAM's sizing rule each call for, and the air changes per hour they work out to. Enter a unit you are considering as well and it does the more useful thing: it converts the coverage area printed on the box into an air-change figure, so you can see what that claim is actually promising.

Disclosure and scope. This page contains affiliate links; we may earn a commission on qualifying purchases at no cost to you. This tool reproduces published sizing tables and rating definitions so you can check them yourself. It is arithmetic on figures other people published, not a health recommendation and not a test of any product. We do not own or test air cleaners. If you have a specific health concern, that is a conversation with a clinician, not with a calculator.

The room

Both published sizing rules assume 8 ft. Above that they understate what you need.
EPA and CDC point at 5 for reducing airborne virus particles indoors, counting filtration toward the total. The two consumer sizing rules land at 4.9 and 5.0 in an 8 ft room, which is the same place by a different road.

What you are trying to deal with

A specific unit you are considering

Optional, but this is where the tool earns its keep. Leave it blank to get sizing only.

Where these numbers come from

Nothing on this page is our own determination about air quality. Every figure below is reproduced from a published source and linked. Where the sources differ, the tool shows both. Where a number is our arithmetic on top of a published figure, it says so.

  1. EPA's own sizing table. Guide to Air Cleaners in the Home gives minimum CADR by room area: 65 cfm for 100 sq ft, 130 for 200, 195 for 300, 260 for 400, 325 for 500, 390 for 600. The guide notes: "The CADRs are calculated based on an 8-foot ceiling. If you have higher ceilings, you may want to select a portable air cleaner with a higher CADR." EPA Guide to Air Cleaners in the Home (PDF)
  2. AHAM's sizing rule. "The CADR of your air cleaner should be equal to at least two-thirds of the room's area", and "if your ceilings are higher than 8 feet, an air cleaner rated for a larger room will be necessary." CADR itself "indicates the volume of filtered air an air cleaner delivers, with separate scores for tobacco smoke, pollen and dust." AHAM Verifide, Air Filtration Standards
  3. The rating is measured flat out. EPA: "The rating is typically measured at the air cleaner's highest speed", and separately, "An air cleaner will filter less air if it is set at a lower speed." AHAM's own consumer guidance frames sizing as the way to keep noise tolerable: "Choosing an air cleaner properly sized for the space is the best way to ensure that the noise level is appropriate." AHAM consumer guidance
  4. Air changes per hour, and the 5 target. EPA defines ACH as "the airflow per hour divided by the volume of the space", recommends aiming for "5 ACH using outdoor air ventilation alone", and states that filtration and other treatment can be compared to ventilation as equivalent air changes per hour, with a target of "5 eACH using a combination of ventilation from outdoor air, filtration, and other air treatment." EPA, Preventing the Spread of Respiratory Viruses in Public Indoor Spaces
  5. CADR covers particles only. EPA: "there are no widely used performance rating systems for portable air cleaners or filters designed to remove gases. The CADR rating system is for particles only." And on the alternatives: "Research has not yet shown that other technologies such as plasma, photocatalytic oxidation, or ultraviolet (UV) light can remove gases effectively in portable residential air cleaners." Same EPA guide as above.
  6. What HEPA means. EPA: HEPA is "high efficiency particulate air" and such filters "can theoretically remove at least 99.97% of dust, pollen, mold, bacteria, and other airborne particles with a size of 0.3 microns"; the 0.3 micron figure "corresponds to the worst case; the most penetrating particle size". MERV values "report a filter's ability to capture larger particles between 0.3 and 10 microns". EPA, What is a HEPA filter?
  7. Ozone. EPA: "At concentrations that do not exceed public health standards, ozone has little potential to remove indoor air contaminants", and "When inhaled, ozone can damage the lungs." EPA, Ozone Generators Sold as Air Cleaners. California requires portable indoor air cleaners sold in the state to be CARB certified against an ozone emission limit of 0.050 ppm, and publishes the list — while stating plainly that certification "does not reflect air cleaner effectiveness". CARB certified device list
  8. Wildfire smoke. EPA: place the unit "where people spend the most time and run it as often as possible on the highest fan speed", choose one "sized for the room in which you will use it", use MERV 13 or the highest your HVAC will take, and "make sure the portable air cleaner does not produce ozone." EPA, Wildfires and Indoor Air Quality
  9. What the labels do not mean. EPA "neither certifies nor recommends particular brands of air filters or home air-cleaning devices", and the ENERGY STAR label on a portable air cleaner "does not refer to its air-cleaning effectiveness in any way" — the packaging disclaimer reads "EPA does not endorse any manufacturer claims of healthier indoor air from the use of this product." Same EPA guide as above.

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