What Size Air Purifier Do You Need? CADR Explained With Our Test Numbers
The CADR rule of thumb, the makers' coverage claims against what we measured, and one purifier per room versus one big one.
The rule
Smoke CADR ≥ two-thirds of the room's square feet gives about five air changes an hour at an 8 ft ceiling. A 200 sq ft bedroom: CADR 135 (Levoit Core 300S, 141). A 400 sq ft living room: CADR 270 (Coway Mighty at 233 is close; Winix 246; Levoit 600S 410 comfortably).
Makers' coverage numbers
Often assume two air changes an hour, not five. The Core 300S is "219 sq ft" and took 24 minutes to clear our 400 sq ft room; the Levoit 600S "635 sq ft" took 9.
One big or several small?
Purifiers clean the room they are in. Two Core 300S in two bedrooms beats one Airmega 400 in the hall. One big unit only wins in genuinely open-plan space.
Running it
Leave it on auto or low 24/7; the particle count climbs back within an hour of switching off.
» See the full rankings: Best Air Purifiers of 2026, Best Air Purifiers for Large Rooms, Best Air Purifiers for Bedrooms (Quiet) .