The short answer
Multiply your room’s square footage by two-thirds. That’s the minimum smoke CADR your air purifier needs — a 300-square-foot living room needs a smoke CADR of at least 200. This is the sizing rule from AHAM, the industry body that runs the standardized CADR test, and the EPA’s own sizing guidance matches it.
| Room area | Minimum smoke CADR |
|---|---|
| 100 sq ft (small bedroom) | 65 |
| 200 sq ft (large bedroom) | 130 |
| 300 sq ft (living room) | 195 |
| 400 sq ft (large living room) | 260 |
| 500 sq ft (open living/dining) | 325 |
| 600 sq ft (large open-concept) | 390 |
What CADR actually is
CADR — Clean Air Delivery Rate — is the volume of filtered air a purifier actually delivers, measured in cubic feet per minute under a standardized test. It’s reported for three particle types (smoke, dust, pollen); smoke is the smallest and hardest, which is why sizing uses the smoke number. What makes CADR worth your attention is that it’s measured, not claimed: units carrying the AHAM Verifide mark have been tested to the same procedure, so two purifiers’ CADR numbers can be compared directly.
Adjustments that matter
- Ceilings above 8 feet: size up — both AHAM and the EPA flag this. The table assumes 8-foot ceilings; a 10-foot ceiling means roughly a quarter more air in the same footprint.
- Open floor plans: size for the whole connected space, not the corner you sit in. A purifier can’t hold a clean bubble in one end of an open living/kitchen area.
- Wildfire smoke or allergy season: the rule is a minimum. Oversizing lets you run a lower, quieter fan speed for the same clean-air delivery — the practical difference between a purifier you run and one you turn off.
The number to distrust
“Covers up to 1,000 sq ft” on the box is not a CADR. Coverage claims have no single required test standard behind them — manufacturers choose their own assumptions, and generous ones make small machines sound big. CADR is the standardized, comparable number. A listing that leads with a large coverage figure but doesn’t state CADR is telling you something.
One more filter on the filters
The EPA cautions against air cleaners that intentionally produce ozone, and notes that ionizers, electrostatic precipitators, and some plasma or UV devices can emit it as a byproduct — ozone is a lung irritant, the opposite of the job. It’s why our picks clean mechanically: HEPA-grade media plus carbon, no ionization the machine depends on.
Sized your room? The work of choosing the machine is already done: The 3 Best Air Purifiers (2026) — a premium, an all-around, and a budget pick, each verified against named independent testing.
Related reading
- HEPA vs. ionizers: which actually cleans your air?
- Air purifiers for wildfire smoke: what actually works
- What air purifier filters really cost per year
Sources
- EPA — Guide to Air Cleaners in the Home (sizing table, ozone caution)
- AHAM Verifide — Directory of Air Cleaners (CADR definition, the 2/3 sizing rule)
Last verified 2026-08-13. No affiliate links on this page — how and why is on How We Pick.
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