Every air purifier box has a big square-footage number on it, "covers up to 800 sq ft," "great for rooms up to 1,700 sq ft," and it's tempting to just match that number to your room and call it done. Most of the time, that number isn't a lie exactly, but it's answering a much easier question than the one you're actually asking. There's a real, independently tested number hiding underneath it, and it usually tells a very different story.
What CADR Actually Measures
CADR stands for Clean Air Delivery Rate. It's measured in cubic feet per minute (CFM) and tested separately for three things: smoke, dust, and pollen, since different particle sizes filter differently. Most reputable brands get this number independently verified through AHAM (the Association of Home Appliance Manufacturers), which is the closest thing this category has to a trustworthy, apples-to-apples spec. If a listing doesn't mention CADR or AHAM certification at all, that's worth noticing.
The smoke CADR is usually the most conservative of the three, since fine smoke particles are the hardest to capture, so it's the number worth anchoring to if you want an honest read on real-world performance.
Why the "Covers Up To" Number Is Almost Always Inflated
Here's the part manufacturers don't lead with: that headline square-footage claim is typically calculated using just one air change per hour, the loosest, slowest standard that still technically counts as "covering" a room. One air change per hour means the purifier eventually cycles all the air in that space once, given enough time, not that it's doing anything you'd notice.
Meaningful, actually-clean air generally calls for four to five air changes per hour. AHAM's own published guidance uses a simple formula for this: take the smoke CADR and multiply it by 1.55. That gives you the room size the unit can realistically keep clean, not the room size it can technically reach eventually.
Run that math against a few popular models and the gap gets obvious fast. A purifier marketed as covering "up to 810 sq ft" can check out at closer to 170 sq ft once you apply the honest formula, a nearly 5x difference between the number on the box and the number that actually matters for your room.
It's what CADR × 1.55 gives you. Match that number to your room, not the marketing figure, and skip straight to a pick sized for the space you actually have.
Three Real Models, Marketed vs. Honest Coverage
| Purifier | Box Claims | Real Coverage (CADR × 1.55) | Price | |
|---|---|---|---|---|
| Coway Airmega AP-1512HH | Up to 1,748 sq ft (current Amazon listing) | ~361 sq ft | ~$155 | Shop on Amazon → |
| Levoit Core 300 | Up to ~1,095 sq ft | ~388 sq ft | ~$90 | Shop on Amazon → |
| Coway Airmega 100 | Up to 810 sq ft | ~167 sq ft | ~$120 | Shop on Amazon → |
Real coverage calculated from each model's published smoke CADR × 1.55, AHAM's own two-thirds rule. Full picks and testing notes are in the apartment air purifier guide.
The AP-1512HH used to be the odd one out. Coway marketed it by its honest 361 sq ft figure, which matched the real math almost exactly. Its current Amazon listing now leads with "up to 1,748 sq ft," the same loose one-air-change standard everyone else uses, and nearly five times the real number. The purifier did not change; the marketing did. That is the whole case for running the CADR through the formula yourself.
If your room is under 200 sq ft (a bedroom or home office)
The Coway Airmega 100 is genuinely sized for this, despite its box claiming coverage nearly 5x larger. Don't let the "810 sq ft" print talk you into thinking it's overkill for a small room, that number was never meant to describe daily performance.
If your room is 300 to 400 sq ft (a living room or open studio)
Both the Coway AP-1512HH and the Levoit Core 300 land in this range once you use the real formula. The Coway costs more but adds an actual air quality sensor and Auto Mode; the Levoit is the budget-conscious pick if you're fine running it manually.
If your room is bigger than that
Don't just size up to a purifier whose box claims a bigger number, run its actual CADR through the same math first. A lot of "large room" units are still using the same inflated one-air-change standard, just with a bigger headline number attached.
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FAQs
CADR (Clean Air Delivery Rate) measures how many cubic feet of filtered air a purifier delivers per minute, tested separately for smoke, dust, and pollen. It's an independently verified number, usually through AHAM certification, unlike the square-footage claim printed on the box.
Most manufacturers calculate their headline coverage number using just one air change per hour, the loosest standard available. Meaningful air cleaning generally calls for four to five air changes per hour, which means the realistic coverage area is often a third or less of the number printed on the box.
Multiply the smoke CADR by 1.55. That's AHAM's own two-thirds rule for meaningful air changes, and it's a far more honest number than the "covers up to" figure most brands lead with on the packaging.