Room size → recommended CADR
Based on AHAM's rule of thumb. If a purifier only lists coverage in sq ft, check what air-change rate that figure assumes — many use a gentle 1× per hour, which is weaker than the 4–5× per hour that allergy and asthma guidance suggests.
The most useful number on an air purifier is its CADR — how much clean air it actually produces — and the most common buying mistake is getting one that's underpowered for the room. The calculator above turns your room's dimensions into the exact CADR to shop for.
Key takeaways
- The AHAM rule of thumb: a smoke/dust CADR of at least two-thirds of your room's square footage for everyday air.
- For wildfire smoke, aim higher — a CADR roughly equal to the room's square footage.
- Ignore vague "covers up to 1,500 sq ft" claims until you know what air-change rate they assume. CADR is the honest number.
Why two-thirds of the square footage?
That ratio gets you roughly four to five air changes per hour in a standard-height room — the turnover rate allergy and asthma guidance points to. A purifier rated for your room at a gentle single air change per hour will technically "cover" it, but it cleans the air far more slowly. Sizing by CADR sidesteps the marketing math.
What to do with the number
Once the calculator gives you a target CADR, shop for a purifier whose smoke CADR meets or beats it. If you want to double-check a specific model against your room, the air changes per hour calculator tells you exactly how many times an hour it will clean your space. New to these terms? Start with what CADR and CFM mean.
Frequently asked questions
How do I know what size air purifier I need?
Match the purifier's CADR (clean air delivery rate, in CFM) to your room. The rule of thumb from AHAM is a smoke/dust CADR of at least two-thirds of the room's square footage. Enter your room's length and width in the calculator above and it returns the exact target.
Should I size up for wildfire smoke?
Yes. For wildfire smoke, AHAM suggests a smoke CADR roughly equal to the room's square footage — about 50% more than the everyday target. The calculator has a toggle for this.
Is a higher CADR always better?
A higher CADR than you need isn't wasteful — it lets you run the purifier on a lower, quieter speed and still clean the air fast enough. The only downsides of an oversized unit are price and footprint.
Embed this calculator (free)
Run a home, health, or air-quality site? Drop this tool into any page — it stays free and links back here. Paste this HTML where you want it.
<iframe src="https://www.pureairscout.com/embed/air-purifier-room-size-calculator-what-cadr-do-you-need" title="Air Purifier Room Size Calculator: What CADR Do You Need?" width="100%" height="560" style="border:1px solid #e5e7eb;border-radius:12px;max-width:680px" loading="lazy"></iframe>
