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Match a purifier to your room size.

The most common buying mistake is undersizing. A purifier that's too small never catches up; one that's too big just runs quieter on a lower fan speed. Here's the math that keeps you on the right side of that line.

The two-thirds rule

Smoke CADR ≥ ⅔ of your room's square footage

AHAM's rule of thumb delivers roughly 4–5 air changes per hour — the rate allergists recommend for meaningful symptom relief. A 300 sq ft bedroom needs at least 200 CFM of smoke CADR. When in doubt, round up.

The sizing table

Room size	Minimum smoke CADR	What to look for
150 sq ft — small bedroom or office	100 CFM	Compact desktop units
300 sq ft — large bedroom or den	200 CFM	Most mid-size tower purifiers
500 sq ft — living room or open plan	330 CFM	Full-size consoles
800 sq ft — great room or basement	530 CFM	High-CADR flagships, or two mid-size units

How the index stacks up

Of the 35 models we track with a published smoke CADR, here's how many clear each sizing tier — bigger rooms shrink the field fast.

≥ 100 CFM31 models · 89%

≥ 200 CFM18 models · 51%

≥ 330 CFM7 models · 20%

≥ 530 CFM1 models · 3%

Edge cases that change the math

High ceilingsAbove 8 feet? Size upCADR ratings assume 8-foot ceilings. For 10-foot ceilings, add roughly 25% to the CADR target.
Open floor plansCount the whole connected spaceAir doesn't respect the imaginary line between kitchen and living room. Add the areas together, or run two units.
BedroomsCheck noise on low, not maxA bedroom purifier lives on its quiet setting, where effective CADR drops. Look for under 30 dB on low — and enough headroom that low is still useful.

Ready to shop?

Filter the index by coverage.

Every model we track lists room coverage and smoke CADR side by side — sort by either.

Compare air purifiers Read the full buyer's guide