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An air purifier’s box usually lists a maximum room size, but that figure assumes an 8-foot ceiling and a modest cleaning goal. If you have allergies, pets or deal with wildfire smoke, you’ll want more air changes per hour. Enter your room below to see the clean air delivery rate (CADR) you need.
Two ways to size an air purifier
- AHAM’s 2/3 rule: the smoke CADR should be at least two-thirds of the room’s floor area in square feet (for 8 ft ceilings). A 300 sq ft room needs a smoke CADR of 200+. This roughly equals 5 air changes per hour. See AHAM Verifide.
- Air changes per hour (ACH): CADR (CFM) × 60 ÷ room volume (cubic feet). This accounts for tall ceilings and lets you choose your own target.
Recommended air changes per hour
| Goal | Target ACH | CADR for 12 × 14 ft room, 8 ft ceiling |
|---|---|---|
| General freshness, cooking odors | 2 | 45 CFM |
| Allergies, pet dander | 4 | 90 CFM |
| Asthma, wildfire smoke | 5 | 112 CFM (matches the AHAM 2/3 rule) |
The US EPA’s guide to air cleaners in the home explains CADR and why the smoke rating is the most useful single number.
FAQ
Which CADR number should I use — smoke, dust or pollen?
Smoke. It’s usually the lowest of the three because smoke particles are the smallest, so a purifier sized for smoke will handle dust and pollen too.
Is one big purifier better than two small ones?
For open-plan spaces or L-shaped rooms, two smaller units placed apart often clean more evenly than one large unit in a corner.
Why does my purifier feel weak on low?
CADR is measured at the highest fan speed. On low, most purifiers deliver a third to a half of their rated CADR — size for the speed you’ll actually tolerate at night.
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