How to read an air purifier's CADR and room size
CADR, the clean air delivery rate, estimates how much effectively cleaned air a purifier delivers for a particular type of particle. The room size on the box is worked out from the CADR using an assumption about how often the room's air should be cleaned. Knowing that assumption tells you whether the room size is generous or strict.
What CADR measures
CADR is tested separately for smoke, dust and pollen, and given in cubic feet per minute in the US or cubic metres per hour elsewhere. Higher is more air cleaned. It describes particles only: it says nothing about drying a room, bringing in fresh air, or removing gases and most smells.
Where the room size comes from
For purifiers certified by AHAM, the US appliance association, the suggested room size is the smoke CADR multiplied by 1.55, assuming an 8-foot ceiling. That works out to the room's air passing through the purifier about 4.8 times an hour (AHAM). Other brands' room sizes may use a lower rate and often don't say which. Two machines both "for rooms up to 40 m²" can be very different if one assumes fewer air changes.
Check it against your room
- Find the smoke CADR. Without a CADR figure it's much harder to make a fair like-for-like comparison.
- Work out your room's volume: floor area times ceiling height.
- Divide the CADR (per hour) by the room volume, using the same units for both (cubic metres per hour with cubic metres, or convert cubic feet per minute to per hour first). The result is an estimate of equivalent clean air changes per hour on the top setting. It assumes the room's air mixes well; it isn't a guarantee that every bit of air passes through the machine.
- Remember that top setting is usually the loudest. If you'll run it on a quieter setting overnight, the real rate is lower.
For bushfire smoke, size it smaller
AHAM uses a stricter rule for wildfire smoke: the room size equals the smoke CADR, rather than 1.55 times it (AHAM). If the room size on the box follows AHAM's standard rating, that means planning on a room about two-thirds of the printed area (1 ÷ 1.55 is about 65%). If the box doesn't say how its room size was worked out, start from the smoke CADR instead: AHAM's wildfire rule is a room of up to the smoke CADR figure in square feet, where CADR is in cubic feet per minute.
What a purifier won't do
- It won't dry a damp room or stop condensation. That's moisture, not particles.
- It won't add fresh air to a stuffy room. It cleans and recirculates the same air.
- It does little for most smells, which are gases rather than particles.
If your problem is any of those, the four questions will point you at what to check first.
Sources
Each source below was opened and checked against this page on 30 October 2026.