What CADR means and how to use the two-thirds rule

CADR stands for clean air delivery rate. AHAM Verifide, the certification program run by the appliance makers' trade group, describes it as the volume of filtered air a purifier delivers, in separate scores for tobacco smoke, pollen and dust. Higher is faster. AHAM's directory page says certified models are verified through testing at a third-party laboratory.

The numbers in this article come from AHAM, the EPA, makers' listings and HouseFresh's tests. I haven't measured anything myself.

The rule, with a tape measure

AHAM calls this a rule of thumb: CADR should equal at least two-thirds of the room's area. Its own example is a 10 by 12 ft room, 120 sq ft (11 m²), which needs a smoke CADR of at least 80.

The EPA's chart is close to the same, calculated for an 8 ft ceiling:

• 100 sq ft (9 m²): 65 cfm
• 200 sq ft (19 m²): 130 cfm
• 300 sq ft (28 m²): 195 cfm
• 400 sq ft (37 m²): 260 cfm
• 500 sq ft (46 m²): 325 cfm
• 600 sq ft (56 m²): 390 cfm

HouseFresh's buying guide rounds differently, asking for at least 150 cfm under 200 sq ft (19 m²) and around 260 cfm up to 400 sq ft (37 m²). The small-room advice is stricter than the EPA's. For a small room, I'd go with HouseFresh and buy the extra margin.

Use the smoke number. The EPA explains that the three scores stand in for small, medium and large particles, with tobacco smoke representing the smallest.

Two worked rooms

Say you have a 16 by 20 ft living room: 320 sq ft (30 m²). Two-thirds of that is about 213. The Blueair Blue Pure 311i Max has a rated smoke CADR of 250 cfm, so it clears the bar.

Now take a 20 by 25 ft open-plan room: 500 sq ft (46 m²). You need about 333. The 311i Max falls short, and the Blueair Blue Pure 211i Max, rated at 410 cfm, doesn't.

Change one thing about that first room and the answer moves. AHAM recommends a smoke CADR equal to the full square footage for wildfire smoke, so 320 sq ft now asks for 320, and the 311i Max no longer fits. For ceilings above 8 ft, AHAM and the EPA both say to size up, and neither gives a multiplier. Scaling in proportion is my own arithmetic, not a standard: a 10 ft ceiling means 1.25 times the volume, which turns 213 into about 267. Close, and over the 311i Max's rating.

Open plans catch people out too. The EPA says to count the whole space the purifier serves.

Where the number misleads

It's a top-speed figure. The EPA says the rating is typically measured at the highest speed, and HouseFresh's guide says the same of CADR tests. The speed you'd sleep through delivers much less. HouseFresh estimates the 311i Max at 108 cfm at a quiet 37 dBA speed. Apply two-thirds and that's a room of about 160 sq ft (15 m²), a long way from the top-speed answer. HouseFresh's results are estimates from its own particle test, not AHAM certifications.

Particles only. The EPA says CADR doesn't cover gases, and that no widely used rating exists for gas filters. A high number tells you nothing about cooking smells.

Coverage claims stretch. The largest figure on a listing often assumes fewer air changes. Abt shows "up to" 1,618 sq ft (150 m²) for the 211i Max, while its footnote assumes 635 sq ft (59 m²) at 4.8 air changes. Sharp's store spells out how this works with its FP-K50UW: 1,253 sq ft (116 m²) at one air change per hour, 259 sq ft (24 m²) at 4.8. Look for the 4.8 figure, which HouseFresh's guide also says it and AHAM use. The two-thirds rule works out to 5 air changes an hour under an 8 ft ceiling, by my arithmetic, so they're nearly the same thing.

Rated isn't measured. The CADR on a box is a rating. Where a tester has published a figure, compare the two. HouseFresh estimated 220 cfm for the 311i Max from its own PM1 test, a different method from AHAM's, and recommended 330 sq ft (31 m²).

More than one purifier

Too big for any single unit? The EPA suggests using several, and HouseFresh's guide says you can add their CADRs together. That only helps if the air can reach both, so I'd treat two closed rooms as two separate sums.

What I'd do

Measure the room, take two-thirds, then ask whether you'd run it at top speed. If you wouldn't, buy the next size up. Checked November 2, 2026, the 311i Max is $229 and the 211i Max is $349.

Air purifiers sorted by room size applies this to five models, the Blueair against the Sharp shows it deciding between two, and the home sizer does the sum for your floor area.