Air Purifiers · Guide
CADR Explained: How to Read an Air Purifier's Real Coverage
Every air purifier listing leads with a square-footage number, and almost none of them mean what buyers think they mean. The number underneath it - CADR, or Clean Air Delivery Rate - is the one piece of air purifier data measured under a published standard, in cubic feet per minute, in a third-party lab. Everything else on the box is derived from it, usually generously. The Honeywell HPA300 is the clean example: a 300 smoke CADR, an AHAM room rating of 465 sq ft, and a headline claim of 2,250 sq ft. Same machine, same test, three different stories. This guide takes CADR apart - what the test actually does, why smoke is the number to read, the two-thirds rule AHAM uses to turn CFM into square feet, the arithmetic that inflates that figure four- or fivefold on the box, and what to do when a manufacturer declines to publish a CADR at all.
What CADR actually measures
CADR is the output of the ANSI/AHAM AC-1 test. A purifier runs in a sealed test chamber, the chamber is seeded with a test particle, and the rate at which particle concentration falls is compared with the natural decay in the same chamber with the machine switched off. The difference, converted into cubic feet per minute, is the CADR. Read it as a plain statement of output: this many cubic feet of air per minute leave the machine with essentially all particles removed. A 400 CADR unit like the Blueair Blue Pure 211i Max is producing roughly 400 cubic feet of particle-free air every minute. What CADR is not: it is not fan airflow, and it is not filter grade. A machine can push a lot of air through mediocre media, or a little air through superb media, and land at the same CADR. That is why the Medify MA-40, which advertises H14-grade True HEPA, scores 6 out of 10 on clean air at 224 CADR while the Levoit Core 400S scores 6 out of 10 at 256 CADR. Filter grade sets the ceiling on what a single pass removes. CADR tells you how much air actually gets that pass each minute. Buyers who shop grade and ignore CADR routinely pay more for less clean air.
The three test pollutants, and why smoke is the one to read
AC-1 runs the same procedure three times with three different particles: tobacco smoke, dust, and pollen, in ascending order of particle size. Smoke is the finest of the three and the hardest to capture and hold; pollen is the coarsest and the easiest. That is why one machine reports three different numbers. The Levoit Vital 200S lists 250/254/289 smoke/dust/pollen. The Honeywell HPA300 lists 300/320/300 smoke/dust/pollen. The Coway AP-1512HH lists 246/233/240 dust/smoke/pollen. Same unit, same session, spreads of up to 39 points. Every CADR figure in our tables is the smoke number, for two reasons. It is the conservative one, and it maps to what people actually want gone - wildfire smoke, cooking particulate, combustion soot, the fine fraction of dander. When a listing quotes a single CADR without saying which test it came from, assume it is the flattering one. It matters more than it sounds: run the room-size math on the Vital 200S using its 289 pollen figure instead of its 250 smoke figure and the coverage claim grows by about 60 sq ft without anything changing inside the machine. Read the labels rather than the position, too - listings print the triplet in whatever order suits them.
The two-thirds rule: how AHAM turns CFM into square feet
AHAM's room-size recommendation is one piece of arithmetic. The smoke CADR should be at least two-thirds of the room's floor area in square feet. Run it backwards and any coverage figure is just CADR divided by 0.65. This catalog confirms it almost line by line. Levoit Core 300: 141 CADR, 219 sq ft, and 219 x 0.65 = 142. Coway AP-1512HH: 233 CADR, 361 sq ft, and 361 x 0.65 = 235. Honeywell HPA300: 300 CADR, 465 sq ft, and 465 x 0.65 = 302. GermGuardian AC4825E: 99 CADR, 153 sq ft, and 153 x 0.65 = 99. Levoit Core 200S: 118 CADR, 183 sq ft. The coverage number is not extra information. It is the CADR wearing different units. That rule assumes an 8-foot ceiling, and the result works out to about 4.8 air changes per hour - which is precisely why the AHAM figure is useful and the headline figure is not. Two consequences follow. First, if you know the smoke CADR you never need the brand's coverage claim. Second, when the two do not reconcile, the coverage claim is the one that is wrong.
Why the box says 2,250 sq ft and the spec sheet says 465
The headline coverage figure on most listings is the same machine measured at one air change per hour instead of AHAM's roughly five. The arithmetic is simple: CADR x 60 minutes gives cubic feet of clean air per hour, and dividing by an 8-foot ceiling gives the floor area the unit can turn over exactly once in that hour. The Honeywell HPA300 makes it visible. 300 x 60 / 8 = 2,250, and 2,250 sq ft is exactly the headline on its listing, against a 465 sq ft AHAM rating. The Blueair Blue Pure 211i Max shows 3,235 sq ft at one air change against 635 sq ft AHAM. The Levoit Core 400S shows 1,733 sq ft per hour against 433. The Medify MA-40 shows 1,793 against a 420 sq ft class. The multiple is not arbitrary - it is the number of air changes you are giving up. Across this catalog it runs about 4x to 5x, and it stretches toward 8x when a brand quotes coverage "in 30 minutes" or runs the math on its pollen CADR instead of smoke. One air change an hour is dilution, not cleaning. Allergy and asthma guidance generally calls for four to five.
The air changes you actually need, and the CADR that buys them
Do the volume math once and CADR shopping becomes mechanical. Floor area x ceiling height gives room volume in cubic feet. Multiply by your target air changes per hour, divide by 60, and you have the CADR you need. A 150 sq ft bedroom with 8-foot ceilings is 1,200 cubic feet. Four changes an hour needs only 80 CADR, and the 141 CADR shared by the Levoit Core 300, the Levoit Core 300S and the Blueair Blue Pure 411i Max delivers about seven changes an hour in that room. There is no argument for spending more on throughput there. A 300 sq ft living room is 2,400 cubic feet, so four changes needs 160 CADR and five needs 200. The Levoit Core 400S at 256 reaches 6.4 changes an hour, the Coway AP-1512HH at 233 about 5.8, the Winix 5500-2 at 232 about the same. The Core 300's 141 manages 3.5, below target - which is exactly why it scores 4 out of 10 on clean air despite being the long-running category best-seller. A 500 sq ft open living space with 9-foot ceilings is 4,500 cubic feet. Four changes an hour needs 300 CADR flat, and that single requirement eliminates most of this catalog.
CADR per dollar, per watt, per decibel
CADR on its own picks the wrong machine about as often as the right one, because a purifier only cleans while it is running, and three things decide whether it keeps running: price, power draw, and noise floor. Per dollar, the cheap workhorses win. The Winix 5500-2 costs $0.69 per CADR point, the Coway AP-1512HH $0.73, the Levoit Core 200S $0.76. The Dyson Purifier Cool TP07 costs $5.30 per point against its conservatively scored 100 CADR, roughly seven times the Coway. Per watt, the small smart units dominate. The Blueair Blue Pure 411i Max moves 141 CADR on 10 W - 14.1 CADR per watt, the best here. The comparison that should bother anyone shopping strictly on sticker price: the $100 Levoit Core 300 and the $150 Core 300S deliver the identical 141 CADR, but the Core 300 draws 45 W against the 300S's 23 W. Run it continuously and the cheaper unit stops being cheaper. Per decibel is where the category is really settled. The GermGuardian AC4825E has a 40 dB floor and scores 2 out of 10 on quiet. The Winix 5500-2 sits at 28 dB, the HPA300 at 32 dB. A purifier that irritates you gets switched off, and a switched-off purifier has a CADR of zero.
Comparing a unit that publishes no CADR
Two machines here decline the test: the Dyson Purifier Cool TP07 at $530 and the Purifier Cool Gen1 TP10 at $380. The absence of a number is itself information - AHAM testing is open to any manufacturer, and a company that skips it is choosing to. The method for comparing anyway: take the maker's own room claim, apply the two-thirds rule in reverse, and see what CADR it implies. Dyson rates the TP07 for 300 sq ft, which under AHAM arithmetic would require about 195 CADR. Our engine does not grant that. With nothing published, and independent throughput evidence placing it below same-price towers, the TP07 is scored at a conservative 100 CADR, 5 out of 10 on clean air, and flagged as an estimate. The TP10 sits at 90 CADR and 4 out of 10. That is not a verdict against the machines, only against paying for throughput nobody can verify. The TP07 earns its 7.7 overall on other axes: a perfect 10 on smart features, a sealed H13 system, sensors, an app, and a bladeless fan you will genuinely use in July. Buy it knowing that. If clean air per dollar is the goal, the Winix 9800 publishes three times the throughput for $230 less.
AHAM Verifide versus a number typed into a listing
Not every CADR carries equal weight. AHAM Verifide means the unit was measured in an accredited lab under AC-1 and remains subject to spot checks. The Levoit Core 300S and Core 200S are explicitly Verifide, and the Core 300's 141 CFM is an AHAM figure. A CADR that appears only in listing copy is self-reported until something confirms it. So run the two-thirds check on it. The Winix 9800's listing states 500 sq ft, but two-thirds of 500 is 325 against a published class figure of about 300 CADR - generous by the rule, though not wildly so. The Medify MA-40 is the sharper case: a stated 420 sq ft class against roughly 224 CADR, where the arithmetic gives 345 sq ft. Its 6 out of 10 clean-air score follows the CADR, not the coverage claim, and its 5.0 overall reflects a 31 dB floor and manual controls alongside it. The Levoit Core 400S errs the other way: its 433 sq ft is a four-air-change figure derived from Levoit's own 1,733 sq ft per hour, while two-thirds of its 256 smoke CADR gives 394 sq ft. Close enough to call honest. When arithmetic and marketing disagree, trust the arithmetic.
When the smaller number is the right buy, and when to buy nothing
Oversizing is a real mistake, just a quieter one than undersizing. In a closed 150 sq ft bedroom, the 141 CADR of the $140 Blueair Blue Pure 411i Max already delivers about seven air changes an hour, at an 18 dB floor on 10 W - a 10 out of 10 quiet score and the best CADR per watt in this catalog. A 400 CADR machine in that same room is throughput you will never use, because you would keep it on low regardless. The $100 Levoit Core 300 covers the identical bedroom for less, paying in manual controls and no sensor, which is why it scores 3 out of 10 on smart and 5.0 overall. That is an honest trade rather than a bad buy. Three situations argue for buying nothing at all. If the room cannot be closed, coverage math stops meaning much and you are effectively filtering a whole floor. If filters will not get replaced on schedule, delivered CADR decays and the machine becomes furniture. And if the concern is gases - VOCs, radon, carbon monoxide - CADR is silent on them, because it is a particle test. The GermGuardian AC4825E makes the point from the bottom: $90, 99 CADR, a 40 dB floor, 3.0 overall.
The 300 CADR club, and what it costs to join
Only three machines in this catalog reach the 300 smoke CADR that a 500 sq ft open space needs for four air changes an hour, and they are not interchangeable. The Blueair Blue Pure 211i Max: 400 CADR, 635 sq ft AHAM, a 23 dB floor, 45 W, $350, and 8.7 overall with a 9 out of 10 on clean air. It is the only unit here that buys large throughput without paying for it in noise or power - 8.9 CADR per watt, better than anything else above 200 CADR, with app, sensor and auto mode included. The Winix 9800: 300 CADR, 500 sq ft stated, 26 dB, 90 W, $300, and 7.7 overall. Four-stage True HEPA, Wi-Fi and auto mode, at 3.3 CADR per watt - twice the Blueair's draw for three-quarters of the air, but $50 less. The Honeywell HPA300: 300 CADR, 465 sq ft, 32 dB, 130 W, $250, and 5.3 overall. The cheapest 300 CADR on the board at $0.83 per point, and the trade is blunt - 5 out of 10 quiet, 4 out of 10 smart, manual controls, 2.3 CADR per watt. Raw throughput is cheap. Quiet, efficient throughput is what costs money.
Our picks for this
HEPASilent throughput that embarrasses same-price towers
$350
8.7/10
The big Winix: 4-stage, Wi-Fi, 500 sq ft AHAM
$300
7.7/10
Brute-force CADR for extra-large rooms
$250
5.3/10
FAQ
What CADR do I need for my room?
Multiply floor area by ceiling height to get room volume, multiply by the air changes you want (four to five for allergies), then divide by 60. A 300 sq ft living room with 8-foot ceilings needs about 160 CADR for four changes an hour. The Levoit Core 400S at 256 clears that comfortably; the Levoit Core 300 at 141 does not. Always use the smoke CADR, never the pollen figure.
Which of the three CADR numbers should I use?
Smoke. It is the finest of the three AC-1 test particles and the hardest to capture, so it is both the conservative figure and the one matching what buyers actually want removed - wildfire smoke, cooking particulate, fine dander. Pollen is the coarsest particle and always the flattering number. The Levoit Vital 200S reads 250 on smoke against 289 on pollen, and using the higher figure inflates its coverage claim by roughly 60 sq ft.
Why does the box say 2,250 sq ft when the spec sheet says 465?
Because the two figures measure different things. The AHAM rating assumes roughly five air changes an hour; the marketing figure assumes one. The Honeywell HPA300 shows both at once: 300 CADR times 60 minutes divided by an 8-foot ceiling is 2,250 sq ft turned over once per hour, against a 465 sq ft AHAM rating. One air change an hour is dilution rather than cleaning, so ignore the larger number entirely.
What exactly is the two-thirds rule?
It is AHAM's rule of thumb: a purifier's smoke CADR should be at least two-thirds of the room's floor area in square feet, so a 300 sq ft room wants 200 CADR. Reversed, it makes every coverage claim checkable - divide CADR by 0.65. The Coway AP-1512HH's 233 CADR gives 358 sq ft against a stated 361, and the Honeywell HPA300's 300 gives 461 against 465. Close agreement means the claim is honest.
Should I buy a purifier that does not publish a CADR?
Only if you want it for something other than throughput. Dyson publishes no CADR for the Purifier Cool TP07 or the Gen1 TP10, so our engine scores them conservatively at 100 and 90 CADR, 5 and 4 out of 10 on clean air, with the estimate flagged. The TP07 still earns 7.7 overall on smart features, sealed filtration and its fan. Just do not pay $530 expecting the air movement a $300 Winix 9800 publishes.
Does a higher HEPA grade beat a higher CADR?
No. Filter grade governs what a single pass removes; CADR governs how much air receives a pass each minute. The Medify MA-40 advertises H14-grade media and scores 6 out of 10 on clean air at 224 CADR, the same clean-air score as the Levoit Core 400S at 256 CADR - for $70 more, a 31 dB floor and manual controls. Above True HEPA, extra grade buys very little in a home.
Do 9-foot ceilings or an open floor plan change the math?
Yes, and both cut the same way. AHAM's rating assumes an 8-foot ceiling, so a 9-foot room holds about 12 percent more air for the same floor area. An open plan has no walls to contain treated air at all. In a 500 sq ft open space at 9 feet, four air changes an hour requires 300 CADR, which is why only the Blueair Blue Pure 211i Max, Winix 9800 and Honeywell HPA300 qualify here.
Is one big purifier better than two small ones?
CADR adds, so two 141 CADR units equal one 282 CADR unit on paper, and they cover two rooms instead of one. The costs are two filter schedules, two noise sources and two power draws. Inside a single room, one machine usually wins: the Levoit Core 400S delivers 256 CADR on 38 W at a 24 dB floor for $220, which is quieter and leaner than a pair of Core 300s at 45 W each.
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