🌬️What CADR does this room need from an air purifier?

At least 333 CFM by the two-thirds rule, so a unit rated 350 or above. For wildfire smoke or a household with allergies, doubling that to around 650 is the difference between five air changes an hour and ten, and it is the change people notice most.

Clean Air Delivery Rate: the volume of thoroughly filtered air a purifier delivers per minute, in cubic feet. It combines airflow with filter efficiency into one number, so a big fan with a weak filter and a small fan with a superb filter can score the same. AHAM tests it independently for smoke, dust and pollen, and the seal shows all three.

No. HEPA describes the filter’s efficiency at capturing particles; CADR describes how much clean air the whole machine actually produces. A HEPA purifier with a small fan can have a lower CADR than a non-HEPA unit that moves far more air. The filter class matters, but the CADR is what tells you whether it can handle your room.

Four to five for general use, which is what the two-thirds rule gives at an eight-foot ceiling. Six to eight for allergies or pets, and ten or more during wildfire smoke or if someone in the house is immunocompromised. Air changes are the honest way to compare two purifiers in two different rooms.

Yes, because purifiers clean volume rather than floor. A room with a twelve-foot ceiling has half again as much air as the same footprint at eight, and the CADR needs to rise with it or the air change rate falls. The two-thirds rule assumes eight feet, which is why this tool asks.

Two smaller ones in two rooms beat one large one in a hallway, because air does not travel around corners on its own. Within a single open space, one large unit is usually quieter than two small ones running hard — noise is what makes people turn purifiers down, and a purifier on its lowest setting has a fraction of its rated CADR.