Find the right air purifier CADR for a room size.
An air purifier is rated by CADR — clean air delivery rate in cubic feet per minute — and the honest way to match one to a room is to work out how much air has to move. Room volume multiplied by the air changes you want, divided by 60, gives the airflow in cfm. AHAM's two-thirds rule (CADR at least 0.66 times the floor area) covers roughly 4.8 air changes an hour at an 8 ft ceiling, so this calculator takes whichever of the two figures is larger. Match the smoke CADR rather than the dust or pollen number: smoke is the smallest particle of the three and always gives the lowest rating, so sizing to it means the unit is adequate for everything else too.
Air Purifier Size
Volume = length x width x ceiling height; airflow cfm = volume x ACH / 60; recommended CADR = max(airflow, 0.66 x floor area)
Sizing guidance only. Air purifiers reduce airborne particles but are not medical devices and do not treat respiratory conditions — follow your clinician's advice on air quality if you have asthma or allergies.
The smoke CADR. Smoke particles are the smallest of the three test aerosols, so the smoke figure is always the lowest — a unit that meets your target on smoke automatically meets it on dust and pollen.
The two-thirds rule bakes in an 8 ft ceiling. A room with a 12 ft or vaulted ceiling holds 50% more air, so the ACH-based figure will be the larger of the two and is the one this calculator reports.
Usually yes. An oversized purifier hits your target ACH on a low fan setting, which is quieter and extends filter life, whereas an undersized one never reaches the target no matter how long it runs.