Guide
CADR Explained
One number, one rule, and a worked example. This is the arithmetic that tells you whether a purifier is big enough for your room.
By Scooter M. · Published · Last updated
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The short answer
AHAM’s guidance is to “choose an air cleaner with a smoke CADR rating that is two-thirds the size of the room’s area.” Their own worked example: “a room that is 200 square feet would require an air cleaner with a smoke CADR of at least 132.” For wildfire smoke specifically, AHAM recommends a smoke CADR equal to the room’s square footage instead.
What CADR actually is
Clean Air Delivery Rate is measured in cubic feet per minute, and it is the volume of air per minute the unit delivers free of a given contaminant. It combines two things a spec sheet usually reports separately - how much air the fan moves, and how much of the contaminant the filter removes - into one number.
That combination is what makes it the only purifier figure genuinely worth comparing on. A unit can have an excellent filter and a feeble fan, or a strong fan and a mediocre filter; CADR reflects the result of both.
It is published in three flavors because different contaminants are different sizes:
- Smoke CADR - the smallest particles, roughly 0.09 to 1 micron. The hardest test, and the one to size on.
- Dust CADR - mid-sized particles, roughly 0.5 to 3 microns.
- Pollen CADR - the largest, roughly 5 to 11 microns. The easiest, and therefore usually the highest of the three.
Manufacturers who publish only one figure tend to publish the pollen one. Size on smoke.
The two-thirds rule, worked
AHAM’s rule is: required smoke CADR = room area x 2/3.
| Room area | Minimum smoke CADR | Which of our picks clears it |
|---|---|---|
| 100 sq ft | 67 CFM | All of them |
| 150 sq ft | 100 CFM | Levoit Core 300S (141), Core 400S (231), Coway (233), Winix (232) |
| 200 sq ft | 132 CFM | Levoit Core 300S (141) just clears it; everything larger comfortably |
| 250 sq ft | 167 CFM | Core 400S, Coway, Winix. The Core 300S no longer clears it. |
| 350 sq ft | 233 CFM | Coway (233) exactly; Winix (232) is marginal; Core 400S (231) marginal |
| 500 sq ft | 333 CFM | None of the units on this site. You need two, or a larger class. |
That last row is the useful one. A 500 sq ft open-plan space needs 333 CFM of smoke CADR, and the popular mid-range units all sit around 230 - which is why “covers up to 1,700 sq ft” on the box and 358 sq ft in the honest spec are both true statements about the same machine.
The ceiling height adjustment
The two-thirds rule assumes an eight-foot ceiling, because CADR is about air volume and area only stands in for volume when height is fixed. If your ceilings are higher, scale up:
- 9 ft ceilings: multiply the required CADR by 1.125
- 10 ft ceilings: multiply by 1.25
- 12 ft or a vaulted ceiling: multiply by 1.5, and treat the result as a minimum
A 250 sq ft room with 10 ft ceilings therefore needs 167 x 1.25 = 209 CFM, not 167.
The wildfire exception
AHAM recommends a different approach for wildfire smoke: a smoke CADR equal to the room’s square footage rather than two-thirds of it. That is a 50% increase in required capacity, and it is because smoke ingress is continuous while the event lasts - you are not cleaning a fixed volume once, you are running against a source. See wildfire smoke indoors.
What CADR does not tell you
- Anything about gases or odors. CADR measures particles. A purifier’s ability to remove smells depends on its activated carbon stage, which is not covered by CADR at all - HEPA vs carbon filters.
- What it costs to run. CADR is measured at maximum fan speed, which is also maximum power and maximum noise. A unit rated 233 CFM at 75 W is a very different proposition from one rated 231 CFM at 38 W.
- Whether you will actually run it that fast. If the top speed is too loud to sleep next to, your effective CADR is whatever the quiet setting delivers, which is much lower.
Verifying a CADR figure
CADR is measured under AHAM’s AC-1 standard, and AHAM maintains a directory of units whose figures have been verified to it. A manufacturer stating a “CADR” without AHAM verification is stating a number they arrived at themselves. That does not automatically make it wrong, but a verified figure has had a third party check it and an unverified one has not.
What we would buy for this
Chosen from the same registry as our roundups, on published specifications and computed running cost.
Levoit Core 300S
Bedrooms up to about 220 sq ft
The right size for an actual bedroom. 23 W and 22 dB on low means you can run it all night without noticing it on the bill or on the pillow.
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Published specifications
| Specification | Value | Source |
|---|---|---|
| CADR | 141 CFM (240 m3/h) | Levoit published specification |
| Coverage | 219 sq ft at 4.8 air changes/hr (1,051 sq ft at 1 ACH) | Levoit published specification |
| Rated power | 23 W | Levoit published specification |
| Noise | 22-50 dB | Levoit published specification |
| Filtration | 3-stage | Levoit published specification |
| Dimensions | 8.7 x 8.7 x 14.2 in, 5.95 lb | Levoit published specification |
What it costs to run
About $15-18 a year in electricity at 12 hours a day.
23 W / 1000 x 12 hours x 365 days = 100.7 kWh. At 18.34 cents/kWh that is $18.47 a year at rated power, and less on the lower fan speeds you would actually use overnight.
What is good about it
- 141 CFM clears AHAM's two-thirds rule for any room up to 211 sq ft, which covers most bedrooms
- 23 W is the lowest draw of anything we list with this much CADR
- Small enough to sit on a nightstand rather than take a corner of the floor
What is not
- Not enough for a living room - the useful 4.8 ACH figure is 219 sq ft, not the 1,051 sq ft headline
- No published filter replacement interval in the specification we could find
Who should skip this
Skip this if the room is over about 220 sq ft, or if you are trying to cover an open-plan downstairs. It will run flat out and still lose.
How it scored
- Clean air per watt9.5
- Honest coverage claim9.5
- Quiet operation9.0
- Filter economics7.5
Overall 8.8 / 10. This is our editorial score from published specifications and computed running cost. It is not a test result, and it is deliberately not marked up as a rating in this page’s structured data.
Levoit Core 400S
Living rooms and open-plan spaces
The one to buy if you want a purifier that genuinely covers a living room, and one of the few spec sheets in this bracket that publishes coverage at a useful air-change rate rather than only the flattering one.
#ad We do not hold a current price for this product, so we do not show one. How we make money.
Published specifications
| Specification | Value | Source |
|---|---|---|
| CADR | 231 CFM | Levoit published specification |
| Coverage | 358 sq ft at 4.8 air changes/hr (1,718 sq ft at 1 ACH) | Levoit published specification |
| Rated power | 38 W | Levoit published specification |
| Noise | 22-52 dB | Levoit published specification |
| Filtration | 3-stage: pre-filter, HEPA, activated carbon | Levoit published specification |
| Dimensions | 10.8 x 10.8 x 20.5 in, 11 lb | Levoit published specification |
What it costs to run
About $25-30 a year in electricity at 12 hours a day, before filters.
38 W / 1000 x 12 hours x 365 days = 166.4 kWh. At 18.34 cents/kWh that is $30.52 a year at rated power - and the unit only draws its full 38 W on the top fan speed, so a mid setting lands nearer $25. Add the replacement filter, which you should price before you buy rather than after.
What is good about it
- Levoit publishes coverage at both 1 ACH and 4.8 ACH, which is unusually straight - most brands quote only the flattering 1 ACH number
- 38 W is low for this CADR, which is what keeps the running cost down
- 22 dB on its lowest setting is quiet enough to sleep beside
- Carbon, pet-allergy and smoke-specific replacement filters are sold for the same body
What is not
- We could not find a published filter replacement interval in Levoit's specification, so we are not quoting one
- The 1,718 sq ft headline is a single air change per hour, which is not a useful rate for allergies
Who should skip this
Skip this if your room is under 200 sq ft. The Core 300S covers that at lower power and lower cost, and the extra CADR buys you nothing in a small bedroom.
How it scored
- Clean air per watt9.0
- Honest coverage claim9.5
- Quiet operation8.5
- Filter economics7.5
Overall 8.6 / 10. This is our editorial score from published specifications and computed running cost. It is not a test result, and it is deliberately not marked up as a rating in this page’s structured data.
Coway Airmega AP-1512HH Mighty
Running continuously in a medium room
The most efficient unit here at low speed - 3.7 W on speed one - which matters enormously if you leave a purifier running all day and almost not at all if you do not.
#ad We do not hold a current price for this product, so we do not show one. How we make money.
Published specifications
| Specification | Value | Source |
|---|---|---|
| CADR (smoke) | 233 CFM | Coway published specification |
| CADR (dust) | 246 CFM | Coway published specification |
| CADR (pollen) | 240 CFM | Coway published specification |
| Coverage | 361 sq ft at 4.8 air changes/hr (1,748 sq ft at 1 ACH) | Coway published specification |
| Power draw by speed | 0.3 W standby / 3.7 W speed 1 / 8.7 W speed 2 / 75.3 W speed 3 | Coway published specification |
| Filtration | Pre-filter, odor filter, Green True HEPA, bipolar device | Coway published specification |
What it costs to run
About $6 a year running 24/7 on speed one, or $121 a year running 24/7 on speed three.
Speed 1: 3.7 W / 1000 x 24 x 365 = 32.4 kWh = $5.94 a year. Speed 3: 75.3 W / 1000 x 24 x 365 = 659.6 kWh = $120.97 a year. That twenty-fold spread is the most important number on this page, and it is why we chase per-speed figures instead of accepting a single rated wattage.
What is good about it
- Coway publishes power draw per fan speed, which almost nobody does and which is the only way to compute a real running cost
- 3.7 W on speed one is remarkable - cheaper to run continuously than most nightlights
- Coverage published at 1, 2 and 4.8 air changes per hour, so you can size it honestly
What is not
- 75.3 W on speed three is the highest draw here; how the auto mode behaves decides your bill
- The bipolar device is a stage we would rather do without; check it can be disabled
Who should skip this
Skip this if you only run a purifier for an hour or two at a time. Its whole advantage is low-speed efficiency over long hours, and short bursts on high speed give that straight back.
How it scored
- Clean air per watt9.5
- Honest coverage claim9.5
- Quiet operation8.0
- Filter economics7.5
Overall 8.7 / 10. This is our editorial score from published specifications and computed running cost. It is not a test result, and it is deliberately not marked up as a rating in this page’s structured data.
How we ranked these
We have not tested these units. Nobody here owns one. What we did was compile each manufacturer’s published specifications, check the coverage claims against AHAM’s room-sizing rule, and compute what each one costs to run for a year at the current US average electricity price. Every figure on this page is attributed to whoever published it, and the arithmetic is shown so you can redo it.
How we rank these sets out the full method, including what would change a ranking.
Questions people ask
+−What is a good CADR rating for smoke?
For ordinary indoor smoke, two-thirds of your room’s square footage - AHAM’s rule, with their own example being 132 CFM for a 200 sq ft room. For wildfire smoke, AHAM recommends a smoke CADR equal to the square footage instead, which is 200 CFM for the same room.
+−Is a higher CADR always better?
Not always. Higher CADR generally means a bigger fan, which means more noise and more power at full speed. If the result is a unit too loud to run at the speed it was rated at, you have paid for capacity you cannot use. Match the room, then compare on power draw.
+−Why do CADR and coverage claims disagree so much?
Because coverage depends on the air-change rate you assume. At 1 air change per hour a unit covers a huge area; at the 4.8 ACH AHAM’s sizing assumes, it covers roughly a fifth of that. Both figures describe the same machine. Only the second one is useful for allergies.
+−Does CADR account for filter age?
No. CADR is measured with a new filter. Real-world performance falls as a filter loads, which is one of the arguments for a washable pre-filter - it keeps the expensive filter closer to its rated state for longer.
+−Do air purifiers without a CADR rating work?
They may, but you have no way of comparing them. An absent CADR means either the manufacturer did not submit for testing or chose not to publish the result. Neither is a reason for confidence when a comparable unit does publish one.