Air purifier boxes carry two numbers that seem to say the same thing: a CADR rating and a room size. They don’t agree as often as you’d expect, and the room size is usually the more flattering of the two.
This article covers how to read CADR and size a purifier yourself. For the other devices in this category, start with the home wellness equipment guide.
Short answer: ignore the room size on the box. Find the CADR, then use the EPA’s chart: about 65 of CADR for every 100 square feet of floor, with an 8-foot ceiling.
What CADR measures
CADR stands for clean air delivery rate. The US Environmental Protection Agency describes it as the rating that helps you “select an air cleaner based on the size of the area it will be placed in.” A higher number means more particles removed and a larger area served.
The figure is in cubic feet per minute. Think of it as the volume of fully cleaned air the unit hands back to the room each minute. Makers usually publish three: one each for smoke, dust, and pollen. Smoke particles are the smallest, so the smoke figure is the one to size by.
The EPA chart
| Room area | Minimum CADR |
|---|---|
| 100 sq ft | 65 |
| 200 sq ft | 130 |
| 300 sq ft | 195 |
| 400 sq ft | 260 |
| 500 sq ft | 325 |
| 600 sq ft | 390 |
From the EPA’s “Guide to Air Cleaners in the Home.” The EPA says these figures are calculated for an 8-foot ceiling.
To size your own room, multiply the length by the width in feet, then multiply by 0.65. A 12 x 15 foot bedroom is 180 sq ft, so it needs a CADR of about 117.
Run the numbers on a real purifier
The Coway Airmega 400 is the one purifier in our catalog. Its listing gives a CADR of 328 for smoke, 328 for dust, and 400 for pollen. Coway rates it for rooms up to 1,560 sq ft.
By the EPA chart, a smoke CADR of 328 fits a room of about 500 sq ft. That is a third of Coway’s figure. Both are honest. They assume different things.
Here’s what the spec sheet won’t tell you: the gap comes from air changes per hour. Coway’s 1,560 sq ft is stated at 2 air changes per hour. The EPA chart works out to a little under 5 (65 cubic feet a minute is 3,900 an hour, into a 100 sq ft room that holds 800 cubic feet). Cleaning the air five times an hour takes a much bigger machine than cleaning it twice.
My rule of thumb: size by the EPA chart for a bedroom or any room where smoke, dust, or allergies are the reason you’re buying. Treat the maker’s larger figure as the most the unit can cover, not what it does well.
Three things that change the answer
Ceiling height. The chart assumes 8 feet. The EPA says to choose a unit sized for a larger area if your ceilings are higher. For a 10-foot ceiling, add about a quarter.
Open floor plans. A purifier can’t tell where one room ends. The EPA says to consider the whole connected space.
Fan speed and noise. The EPA notes that “higher fan speeds and longer run times will increase the amount of air filtered.” The Airmega 400 lists 22 to 52 dB(A) across its speeds. If the top speed is too loud to sleep next to, you’ll run it lower and filter less air. Buying a size up and running it quietly is a fair trade.
What CADR leaves out
- Gases and odors. The EPA says there are “no widely used performance rating systems” for removing gases. It suggests activated carbon filters with a large amount of material. The Airmega’s filter set includes activated carbon, but our data doesn’t say how much.
- Filter cost. A purifier only delivers its CADR with a clean filter. The Airmega’s main filter set is rated for 1 year, and the pre-filter is washable. We don’t have the replacement price, so check it before you buy.
- Ozone. The EPA advises against air cleaners that intentionally produce ozone, which is a lung irritant.
How to shortlist
- Measure the room and multiply by 0.65.
- Compare that with the smoke CADR, not the room size on the box.
- Add margin for high ceilings and open plans.
- Check the noise at the speed you’ll use at night.
- Price a year of filters.
Bottom line: CADR is the number you can compare across brands. The room size on the box depends on an assumption the box doesn’t print.
Sources
- U.S. Environmental Protection Agency, “Guide to Air Cleaners in the Home,” last updated October 1, 2026: epa.gov
- Coway Airmega 400 listing (CADR, coverage, noise, filters), checked September 2026