What size air purifier do I need?

Match a purifier’s smoke CADR to your room using the AHAM 2/3 rule, then see picks that clean the air fast enough.

How air purifier sizing works

Air purifiers are rated by CADR (Clean Air Delivery Rate) in CFM. The AHAM “2/3 rule” says a purifier’s smoke CADR should be at least two-thirds of your room’s floor area — so a 300 sq ft room wants ~200 CFM. Taller ceilings need more, since there’s more air to move. Buy at or above the number the calculator gives you, and you’ll get roughly 4–5 air changes per hour.

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