What Size Air Purifier Do You Need? CADR Explained
Updated 2026-09-25
The single most useful number on an air purifier is its CADR — clean air delivery rate. It tells you how many cubic feet of filtered air the purifier delivers per minute on its highest setting. Everything else — room size claims, "covers up to 1,500 sq ft" — is derived from it, often generously.
Why box room sizes mislead
Many brands quote the room size a purifier can clean once per hour. One air change per hour barely dents allergens or smoke, because new particles keep coming in. Look for the room size at 4–5 air changes per hour instead. That's the standard AHAM, the appliance industry's certification body, uses for its room-size ratings: about 4.8 changes per hour in a room with 8-foot ceilings.
The quick rule
For a room with standard 8-foot ceilings:
Choose a smoke CADR of at least two-thirds of the room's square footage.
A 300 sq ft living room needs a smoke CADR of about 200 cfm. That rule works out to roughly 5 air changes per hour — the same rule of thumb the EPA points to for portable air cleaners.
For more precision:
CADR needed = room area × ceiling height × air changes per hour ÷ 60
| Goal | Air changes per hour |
|---|---|
| Everyday dust and odors | 4 |
| Allergies, asthma, pets | 5 |
| Wildfire or cigarette smoke | 6 |
Smoke, dust and pollen CADR
AHAM-verified purifiers list three CADRs because filters catch different particle sizes differently:
- Smoke — the smallest particles and the hardest to catch. The most conservative number, and the one to use for general sizing.
- Dust — medium particles.
- Pollen — the largest particles; usually the highest figure.
Some brands publish a single CADR figure instead. We use it for all three on AirSheet and say so on the product page.
Size up if you want quiet
CADR is measured on the highest fan speed, which is also the loudest. If the purifier is for a bedroom, choose one with about 50% more CADR than you need, so it can do the job on a quieter medium setting. The room matcher has a "needs to be quiet" option that does this for you.
Next step
Put your room into the room matcher — it calculates the CADR you need and shows which purifiers have it, cheapest to own first.