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Last fall I nearly bought an air purifier that promised "up to 1,095 square feet" for my 160-square-foot bedroom. It felt like the safe choice — until I learned that claim was calculated at a single, nearly useless air exchange per hour. The number that actually told me what I needed was CADR, the Clean Air Delivery Rate.
CADR is the most important, independently verifiable number on an air purifier's spec sheet. It tells you how much clean air a machine actually delivers per minute, not how much air the fan shoves through, and not what a marketing badge claims. If you've ever stared at two purifiers with conflicting square-footage figures and felt stuck, this is the concept that unblocks your decision.
Here's what we'll cover: what the three CADR sub-ratings mean, how to read an AHAM-verified label step by step, the exact sizing math for allergies and wildfire smoke, and the places where brands quietly inflate the story. By the end, you'll be able to look at any listing and know within 30 seconds whether it's a real performer or a well-decorated fan.
This guide is for anyone buying an air purifier for a specific room — bedrooms and nurseries, living rooms, offices, or smoke-prone homes — and for readers who want to stop guessing and start comparing numbers that hold up in a lab.
Here's the short version before we dig in:
| Situation | Minimum smoke CADR | Example: 300 ft² room |
|---|---|---|
| Allergies, asthma, dust, pets | About 2/3 of room square footage | 200+ CADR |
| Wildfire smoke | Equal to room square footage | 300+ CADR |
| Light dust, occasional use | About half of room square footage | 150+ CADR |
The rest of this guide explains why each of those numbers matters and how to find the one that maps to your room.
What Is CADR? Clean Air, Measured
CADR is the volume of air, stated in cubic feet per minute (CFM), that a purifier delivers with all particles of a given size removed. It is not the same as fan airflow. A unit moving 300 CFM through a filter that captures only 50% of smoke particles has a smoke CADR of 150, because half the smoke got through.
The measurement comes from ANSI/AHAM AC-1, the standard behind the AHAM Verifide program. A purifier runs at maximum fan speed inside a 1,008-cubic-foot chamber — roughly a 12×12 room with an 8-foot ceiling — while instruments track how quickly it clears generated smoke, dust, and pollen. The result is three separate CADR scores, one per pollutant type.
That chamber size matters for sizing. Since the test assumes an 8-foot ceiling, a room with higher ceilings needs a correction, and since the test runs at maximum fan speed, a unit you plan to run on low will deliver less than its rated number. Both points come up again below.
You can also reverse the number to estimate marketing coverage. An approximate shortcut: smoke CADR × 1.55 gives the square footage a unit can serve at one air change per hour. Coway's Mighty and Airmega 200M, with a 233 smoke CADR, work out to about 361 square feet — exactly the AHAM-rated room. To understand what actually happens inside the machine, start with how air purifiers work.
The Three Numbers on a CADR Label: Smoke, Dust, Pollen
Every AHAM-verified purifier lists three CADR scores, each tied to a particle-size range:
- Smoke CADR covers the smallest particles, roughly 0.09 to 1.0 microns.
- Dust CADR covers mid-size particles, roughly 0.5 to 3.0 microns.
- Pollen CADR covers the largest particles, roughly 5.0 to 11.0 microns.
The numbers usually climb from smoke to pollen because larger particles are mechanically easier to trap. Smoke is almost always the lowest figure, which is exactly why it's the one to use for sizing: if a unit can handle the smallest, hardest particles, it can handle the larger ones too.
Take two real examples. The Coway Airmega 200M carries an AHAM-verified smoke CADR of 233, dust of 246, and pollen of 240 on a 361-square-foot room. The Honeywell HPA300 posts 300 smoke, 320 dust, and 300 pollen on a 465-square-foot room. In both cases, the smoke number is the conservative anchor you should size to — not the pollen headline a listing might feature.
One important boundary: these three scores measure particles only. They say nothing about gases, VOCs, or odors. If your problem is formaldehyde, cooking smells, or wildfire smoke odor, carbon filtration is a separate conversation we cover in the VOCs and odor guide. For dust and pollen, CADR is the right tool.
How to Read a CADR Label: Step-by-Step
When you're on a product page, work through this sequence:
- Look for the AHAM Verifide seal. If it's absent, the unit has not been third-party verified under the AC-1 protocol.
- Find the three-number line. It reads something like "Smoke 233 / Dust 246 / Pollen 240" — sometimes in a different order. These are the only CADR figures you should trust.
- Ignore "covers up to X sq ft." That figure is almost always calculated at one air change per hour and can be four to five times the allergy-grade coverage.
- Circle the smoke CADR. That's the number you'll size with.
- Apply the sizing rule for your situation — 2/3 rule for allergies, 1:1 rule for wildfire smoke.
The AHAM Verifide program is voluntary and administered through an independent AHAM-contracted laboratory, with units re-tested periodically. A brand that pays for this is choosing to submit its machine to measurement rather than adjectives. When Levoit quietly walked back some HEPA-13 marketing language after a regulatory challenge, the CADR numbers didn't change — which is precisely the point. A verified number is more durable than a marketing term. Our Levoit guide goes deeper on that brand's lineup.
The 2/3 Rule: Sizing Your Room for Allergies
For allergy, asthma, and general home use, AHAM's long-standing rule of thumb is simple: your purifier's smoke CADR should be at least two-thirds of the room's square footage. A 300-square-foot bedroom needs a smoke CADR of about 200. A 500-square-foot living room needs about 335.
The math behind the rule is the 4.8 air changes per hour (ACH) target that the EPA and Energy Star reference for allergy-grade cleaning. Here's the worked version for a 300-square-foot room with a standard 8-foot ceiling:
- Room volume = 300 ft² × 8 ft = 2,400 cubic feet.
- Target airflow = 2,400 ft³ × 4.8 ACH = 11,520 cubic feet per hour.
- Per minute = 11,520 ÷ 60 = 192 CFM.
- The 2/3 rule arrives at the same place: 300 × 0.67 = 201 smoke CADR.
That's why the rule works. It's a shortcut to the same 4.8-ACH benchmark, not a separate standard.
Here's what the rule looks like across real verified models:
| Model | Smoke CADR | Dust CADR | Pollen CADR | AHAM room at 4.8 ACH | Marketing "up to" figure |
|---|---|---|---|---|---|
| Levoit Core 300-P | 143 | 153 | 167 | 222 ft² | ~1,095 ft² |
| Coway Airmega 200M | 233 | 246 | 240 | 361 ft² | ~1,748 ft² |
| Winix 5500-2 | 232 | 243 | 246 | 360 ft² | ~1,740 ft² |
| Honeywell HPA300 | 300 | 320 | 300 | 465 ft² | ~2,250 ft² |
Notice the pattern: the marketing figure is roughly five times the real allergy-grade room. The Winix 5500-2 has been a community favorite for years — and is now discontinued in the US, replaced by the Winix 5510 with a 253 smoke CADR — but the sizing lesson is unchanged: buy the smoke number, not the banner.
If you want a full breakdown of how square footage claims work across brands, start with the air purifier room size guide. For open-plan spaces where one unit can't do the job, our large-room picks apply these numbers to real floor plans.
Wildfire Smoke? Use the 1:1 Rule
Wildfire smoke changes the sizing math because the particles are smaller, more numerous, and harder to clear. AHAM's recommendation is direct: size your smoke CADR equal to the room's square footage, one-to-one.
A 300-square-foot bedroom in wildfire season needs a 300 smoke CADR — not the 200 the 2/3 rule suggests for everyday allergies. A 400-square-foot living space needs 400 smoke CADR. This is the proper context for the growing interest in PM2.5, the fine-particle metric that drives smoke exposure and long-term respiratory risk.
The reason to jump from 0.67× to 1.0× is turbulence and particle loading. Wildfire smoke is dense PM2.5, and you're fighting continuous ingress through leaks, doors, and HVAC. Undersizing during smoke events leaves the room degrading while the purifier runs at full tilt. For specific models that hit the 1:1 threshold, see the wildfire smoke picks.
CADR vs CFM vs "True HEPA": Three Marketing Traps
The biggest wins from learning CADR come from recognizing the traps before they cost you money.
Trap 1: CADR vs CFM. Fan airflow and clean air delivery are different numbers. A high-CFM fan connected to a weak or bypass-prone filter can move a lot of air while cleaning very little of it. I once tested a cheap "air mover" style purifier advertised at 300 CFM of airflow; its measured smoke CADR was under 150. Whenever a listing gives CFM or fan speed instead of CADR, treat it as incomplete.
Trap 2: "Covers up to X" using 1 or 2 ACH. Blueair and Winix typically publish square footage at the stricter 4.8 ACH. Others use 1 or 2 ACH, which inflates coverage by a factor of two to five. A well-known Coway Airmega 400 listing claims 1,560 square feet — but that's at 2 ACH, and the realistic allergy-grade coverage is closer to 410 square feet. This isn't dishonesty so much as a different denominator, and it's why the label matters more than the headline.
Trap 3: "True HEPA" equals strong performance. True HEPA describes filter capture efficiency — 99.97% at 0.3 microns — not throughput. A True HEPA filter in front of a weak fan can post a lower CADR than a mid-range unit with more airflow. The Honeywell HPA300 is a perfect illustration: True HEPA, but its 300 smoke CADR comes from a large fan moving serious air. A True HEPA badge tells you what the filter catches per pass; CADR tells you how much air actually gets cleaned. Our HEPA guide sorts models that earn the badge and move air.
Trap 4: Publishing only the highest sub-rating. If a listing shows a single CADR number, it's usually the pollen figure — the easiest, highest one. The smoke number is the one that reveals real capability. Always ask for all three.
No CADR Listed? What That Actually Tells You
Some brands choose not to publish an AHAM CADR at all. That choice is information.
The canonical example is Molekule. Its PECO-based purifier does not carry an AHAM-verified CADR, and independent estimates put real-world performance around 100 CFM — suitable for about 130 square feet despite a claimed 600-square-foot coverage. Wirecutter called the original Molekule "the worst air purifier we've ever tested," and the BBB's National Advertising Division recommended the brand drop 26 advertising claims after a challenge. Molekule Group filed for Chapter 11 bankruptcy protection on October 3, 2023. A friend's cheap desktop unit had the same issue: no CADR, no ACH rating, and a filter too thin to matter.
Dyson is a more mainstream case. Most Dyson purifiers don't publish CADR at all, relying instead on design and brand trust. Third-party testing often finds solid but not class-leading particulate performance — the information just isn't on the box. That doesn't make a no-CADR unit automatically bad, but it does mean you're buying without the single most useful comparative number.
The practical rule: for health-driven purchases, favor AHAM-verified CADR. If a purifier has no verified number, demand an independent benchmark or a very good reason. Our budget picks hold that line even at low prices.
Ceiling Heights, Fan Speeds, and What CADR Doesn't Measure
CADR is the best comparative tool we have, but it has boundaries you should account for.
Ceiling height. The AHAM chamber assumes an 8-foot ceiling. For a 9-foot ceiling, multiply your needed CADR by about 1.13; for 10 feet, by about 1.25. My 300-square-foot bedroom with a 10-foot ceiling calls for roughly 250 smoke CADR under the 2/3 rule, not 200.
Fan speed. CADR is rated at maximum speed. If you plan to run the unit on low or medium for sleeping, delivered CADR drops meaningfully. Size to the max-speed number, then run the machine harder when needed — or buy one step up if quiet operation is the priority. The bedroom picks account for this trade-off explicitly.
Filter loading. As a filter loads with captured particles, airflow and CADR decline. The rated number assumes a fresh filter. Replace or wash filters on schedule, as covered in the filter replacement guide.
Gases and ozone. CADR measures particles only. It does not measure VOC removal, gas adsorption, or ozone output. A high CADR tells you nothing about whether a unit handles formaldehyde or emits ozone. For those questions, see the VOC guide and the ionizer explainer.
New metrics on the way. The industry is extending the standard: a PM2.5 CADR metric (ANSI/AHAM AC-7-2022) averages smoke and dust performance, and Energy Star is moving toward a PM2.5-CADR-per-watt efficiency measure. Gas-phase and microbial standards (AC-4 and AC-5) exist too, though they're less common on current labels. The energy use guide covers the efficiency side.
Which CADR Band Do You Need?
Here's the decision table I use when someone tells me their room and their main problem:
| Smoke CADR band | Realistic room at 4.8 ACH | Best fit | Verified example |
|---|---|---|---|
| Under 100 | Personal zone, desk, RV | Small office corner, dorm | Levoit Core 300-P (143) gets you past this |
| 100–150 | Up to ~225 ft² | Bedroom, nursery, home office | Levoit Core 300-P |
| 150–250 | ~225–375 ft² | Primary bedroom, living room | Coway Airmega 200M |
| 250–350 | ~375–525 ft² | Large room, open plan | Winix 5510 |
| 300+ | ~465 ft² and up | Large living spaces | Honeywell HPA300 |
The sizing rule by profile:
- Allergies or asthma: 2/3 rule, smoke CADR.
- Wildfire smoke: 1:1 rule, smoke CADR, plus a carbon stage for odor.
- Pets: Dust and pollen matter more, but still size to smoke CADR; the dander is mid-size and the smoke number understates the machine's dust-handling ability.
- Light dust only: You can stretch to about half the room's square footage and run the unit longer.
A final note on buying logic: the goal is the right airflow for the room, not the biggest number you can afford. Oversized units run louder on turbo than they need to, and many shoppers end up running them on low — which delivers far less than the rated CADR. Match the number to the room.
Frequently Asked Questions
Q: What is a good CADR rating for a bedroom?
Use the 2/3 rule. A 150-square-foot bedroom needs a smoke CADR of about 100; a 200-square-foot bedroom needs about 135; a 300-square-foot bedroom needs about 200. If wildfire smoke is a concern, move to the 1:1 rule — 150, 200, and 300 respectively.
Q: Should I use smoke, dust, or pollen CADR when comparing?
Use smoke CADR. Smoke represents the smallest particles (0.09–1.0 microns), so it's the hardest test and almost always the lowest number. Sizing to smoke means the dust and pollen numbers are already covered.
Q: Is a higher CADR always better?
Not necessarily. Higher CADR clears air faster, but the number only applies at maximum fan speed and assumes an 8-foot ceiling. An oversized unit is often noisier and draws more power than one correctly matched to the room. Right-size to your space, and run it appropriately.
Q: What if a purifier doesn't list a CADR?
Treat it as a missing spec. Some premium or design-forward brands skip AHAM verification, but that means you're buying without the most useful comparative number. If there's no CADR and no independent benchmark, prefer a verified alternative for health-driven purchases.
Q: Does CADR measure gases, VOCs, or ozone?
No. The three standard CADR scores measure particles only — smoke, dust, and pollen. Gas removal, VOC adsorption, and ozone output are separate considerations covered by carbon filtration and ozone certification, not CADR.
Q: How does ceiling height affect CADR sizing?
The AHAM test chamber assumes an 8-foot ceiling. For taller rooms, increase your required CADR: multiply by about 1.13 for 9 feet and 1.25 for 10 feet. A 300-square-foot room with a 10-foot ceiling should be sized like a 375-square-foot room.
Conclusion
CADR is the closest thing this category has to a universal truth. It's measured by an independent protocol, it's published by the brands that are willing to be tested, and it converts directly into a room-size decision. Once you know your room's square footage and your main pollutant, the rest is arithmetic: two-thirds of the room for allergies, one-to-one for wildfire smoke, smoke CADR always.
If you remember one thing from this guide, make it the smoke CADR. The three spec numbers, the AHAM Verifide seal, and the 4.8-ACH math all serve that single anchor. Everything else — "covers up to X," CFM claims, HEPA adjectives — is context that either supports the number or distracts from it.
For the models that actually earn their CADR ratings, start with the best air purifiers for home, the room size guide, or the large-room picks to apply these numbers to real spaces. If you're building out your understanding first, the how air purifiers work explainer and this CADR reference are the pair to open in two tabs.
