Quick answer: Multiply room area by ceiling height and your chosen air-change target, then divide by 60. This gives a clean-air delivery target in CFM. Compare it with the same particle category of CADR on each product, rather than comparing the largest room-size claims on the boxes.
Calculate a room-size target
Start with one room or a reasonably connected space. The default 4.8 ACH is a sizing comparison reference, not a personalized health recommendation. You can change it to inspect a different scenario.
What the result means
CADR target (CFM) = area (sq ft) × ceiling height (ft) × ACH ÷ 60. For 300 sq ft, an 8-foot ceiling and 4.8 ACH, the result is 192 CFM. A 10-foot ceiling raises that to 240 CFM. The calculator rounds up to a whole CFM so that rounding does not reduce the target.
AHAM explains its 4.8 ACH room-sizing reference. This calculation describes equivalent clean-air delivery from filtration, not outdoor-air ventilation. Furniture, obstructions, layout, particle sources and operating speed affect performance in a real room. It does not measure gas or odor removal.
Compare like-for-like smoke CADR
These three US models illustrate how to use the calculation; they are not a complete market shortlist or a ranking. Manufacturer specifications checked September 22, 2026. Only examples whose published smoke CADR meets the calculated target appear in the result.
| Example model | Published smoke CADR | Official source |
|---|---|---|
| Coway AP-1512HH | 233 CFM | AP-1512HH specifications |
| Coway Mighty2 AP-1512N | 240 CFM | Mighty2 specifications |
| Coway Airmega 400 | 328 CFM | Airmega 400 specifications |
At a target of 240 CFM, the AP-1512HH’s 233 CFM is below the target. At 350 CFM, none of these examples qualifies alone. The Airmega 400’s higher pollen CADR is not a substitute for its smoke CADR. Do not assume that a model delivers its published rating in sleep or low-speed mode.
Next: check costs and model tradeoffs
Once a model meets your size target, compare replacement-filter budgets and electricity costs. For controls, maintenance and other purchase tradeoffs, see our Mighty2 versus original Mighty comparison and Airmega 400 review. The model guides include affiliate links.
Frequently asked questions
Does a bigger model guarantee quiet cleaning?
No. To know whether a quieter setting still meets your target, you need CADR or comparable clean-air output at that setting. Maximum ratings alone do not answer that question.
Can I simply add two purifiers together?
Adding their CADRs can describe a nominal combined delivery rate, but separate rooms and obstructed airflow prevent this from guaranteeing uniform cleaning. Consider each occupied space and the placement instructions instead of treating a combined number as a whole-home result.
Why did no model appear?
The examples stop at 328 CFM of smoke CADR. A higher target requires other options or a separately assessed arrangement; the calculator does not automatically recommend an undersized model.