A purifier’s room-size claim looks like a fixed spec, but it’s really the output of a specific test condition the manufacturer rarely spells out.
The assumption behind the number
Xiaomi rates the Smart Air Purifier 4 Compact for rooms up to 27 m (89 ft)² based on a 230 m (755 ft)³/h CADR — a figure almost certainly measured near maximum fan speed. Run mostly on a quiet setting, the effective coverage is smaller than the printed number.
Levoit’s own listings don’t fully agree
The Core 300S is rated up to 41 m (135 ft)² on one market listing and up to 50 m (164 ft)² on another, for the identical product — a real inconsistency, and a reminder to treat any single room-size claim as approximate.
Size to the lower figure when in doubt
Where a manufacturer’s own listings disagree, or where you plan to run the unit quietly rather than at full speed, sizing to the more conservative figure is the safer approach — see our full guide on reading a CADR figure.
What we would not do
Assume a purifier rated for your exact room size will clear it quickly on a low, quiet setting. The rated coverage and quiet-mode performance are two different questions, and only one of them is usually printed clearly.
The questions that keep coming up
Should I size up if I plan to run the purifier on a low setting?
Yes. The stated room coverage assumes a speed close to maximum in most cases. If quiet, low-speed operation matters to you, look for a unit rated well above your actual room size, or check noise and CADR at the specific low speed you'll use.
Last reviewed 24 August 2026