Air Purifier Sizing: CADR and True HEPA, Explained
Match an air purifier to your room by its CADR, at least two thirds of the room's square feet on the EPA's chart; true HEPA filters are defined at 0.3 microns.
By Elissa SanciContributing writer.
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The short answer
To size an air purifier:
- Multiply the room's length by its width to get square feet.
- Look for a CADR (clean air delivery rate) of at least about two thirds of that number. A 300 square foot room calls for a CADR of 195 or more. For fine particles, read the tobacco smoke CADR.
- Choose a larger unit if the ceiling is higher than 8 feet or the room opens into other spaces.
The EPA's chart, which this rule follows, assumes an 8 foot ceiling.
What does CADR mean?
CADR is the clean air delivery rate: how much filtered air a portable air cleaner delivers, in cubic feet per minute (cfm). It is usually measured at the highest fan speed. A higher CADR means the unit removes particles faster and can serve a larger room.
Many boxes show three CADRs:
| CADR for | Stands for |
|---|---|
| Tobacco smoke | Small particles, including fine particles (PM2.5) |
| Dust | Medium particles |
| Pollen | Large particles |
The EPA says to pick a high smoke CADR if you want to remove the smallest particles. CADR covers particles only; there is no widely used rating for how well a unit removes gases and odors.
What size air purifier do you need?
The EPA's minimum CADR by room size:
| Room area (sq ft) | Minimum CADR (cfm) |
|---|---|
| 100 | 65 |
| 200 | 130 |
| 300 | 195 |
| 400 | 260 |
| 500 | 325 |
| 600 | 390 |
Most boxes also print a maximum room size. Compare it with your measured area, and for an open floor plan count the whole connected space, not just the corner where the unit sits. Because CADR is measured on the highest speed, a unit run on a quieter, lower speed cleans less air than its rating; a model rated above your minimum leaves room to run it quietly.
True HEPA vs HEPA-type
HEPA stands for high-efficiency particulate air, a filter grade defined by the U.S. Department of Energy. The EPA says a HEPA filter can in theory remove at least 99.97 percent of particles 0.3 microns in size. That size is the hardest to catch, so larger and smaller particles are captured even better.
Labels such as "HEPA-type" or "HEPA-like" are not that defined grade. If the filter does not state the 99.97 percent at 0.3 microns figure, treat it as an ordinary filter and judge the unit by its CADR. Of the 59 air purifiers listed on Wiki Shopping on October 26, 2026, 28 are listed as HEPA air purifiers. Browse them under home and kitchen.
Avoid ozone
Some devices sold as air cleaners make ozone on purpose. The EPA advises avoiding them: ozone irritates the lungs, and no federal agency has approved these devices for occupied spaces. The California Air Resources Board and the Association of Home Appliance Manufacturers keep lists of units that emit little or no ozone. On Wiki Shopping, 7 air purifiers are listed as air ionizers; check the maker's ozone statement for any ionizer before buying.
How we count
CADR figures and advice are the EPA's, linked under Sources; the two thirds rule is our reading of the EPA chart (65 cfm per 100 square feet). Air purifier counts come from our database on October 26, 2026, and change as listings are added or removed.