Air Purifier CADR Rating Explained: What the Number Means and How to Match It to Your Room

CADR is the one number on an air purifier box that actually tells you something useful, and it is also the one most people misread. A purifier rated for "300 sq ft" and a purifier with a CADR of 200 are making two different promises, and the gap between them is where a lot of disappointing purchases happen. This guide explains what CADR measures, how to read it honestly, and how to translate it into the room you are standing in.

If you want product recommendations, our air purifier buying guide covers specific models. This page is about the rating itself.

What CADR Actually Measures

CADR stands for Clean Air Delivery Rate. It is the volume of filtered air a purifier delivers per minute, measured in cubic feet per minute (CFM) in the US or cubic meters per hour (m³/h) elsewhere. A CADR of 250 means the unit delivers 250 cubic feet of genuinely filtered air every minute.

The number is set by an independent test (the AHAM Verifide program in the US) rather than the manufacturer, which is what makes it trustworthy. A purifier earns its CADR by running in a sealed chamber while instruments measure how fast it removes pollutants. Because the test is standardized, a CADR of 200 from one brand is directly comparable to a CADR of 200 from another — something almost no other spec on the box lets you do.

One detail that confuses people: there are usually three CADR numbers, not one.

CADR rating	What it tests	Particle size
Smoke	Smallest particles	0.09–1.0 microns
Dust	Medium particles	0.5–3.0 microns
Pollen	Largest particles	5.0–11.0 microns

Smoke CADR is the hardest test and the most useful single number, because a unit that clears fine smoke particles handles dust and pollen easily. If a box lists only one CADR, it is usually the smoke figure. If you have allergies or wildfire smoke is your concern, smoke CADR is the one to read.

The Rule That Connects CADR to Room Size

Here is the part that turns the spec into a decision. The widely used guideline, often called the 2/3 rule, says a purifier's CADR should be at least two-thirds of your room's area in square feet.

Room area (sq ft) × 2/3 = minimum CADR you want

A 300 sq ft living room needs a CADR of at least 200. A 150 sq ft bedroom needs about 100. This rule assumes a standard 8-foot ceiling and, critically, that the unit can clean the room about five times per hour — the air-change rate that actually keeps a space clean rather than just technically "covered."

That five-changes-per-hour target is the gap between CADR and the marketing "coverage area." Manufacturers often print a much larger coverage number based on a single air change per hour, which is enough to make a room slightly less bad but not enough to control allergens or smoke. A purifier advertised for "500 sq ft" might only deliver two air changes per hour in that space. When you size by CADR using the 2/3 rule, you are sizing for real, frequent cleaning.

How to Size a Purifier in Three Steps

1. Measure the room. Length × width in feet. A 12 × 15 bedroom is 180 sq ft.
2. Multiply by 2/3. 180 × 0.67 ≈ 120. That is your minimum smoke CADR.
3. Round up, and go higher if you can. A unit with CADR 150–200 will clean that 180 sq ft room faster and let you run it on a quieter, lower setting most of the time.

That last point matters more than it sounds. An oversized purifier is not wasteful — it is quieter. Running a high-CADR unit on medium is calmer and more effective than running a just-big-enough unit on max all night.

Why "Coverage Area" on the Box Can Mislead

The single biggest mistake is trusting the square-footage claim instead of the CADR. Coverage area is unregulated marketing; CADR is independently verified. Two purifiers can both claim "covers 400 sq ft" while one has a smoke CADR of 240 and the other 130. The first cleans the room roughly five times an hour; the second, barely twice.

When you compare units, ignore the coverage banner and compare smoke CADR directly. If a product lists a coverage area but hides its CADR, that omission is usually the answer to why.

What CADR Does Not Tell You

CADR is the best single spec, but it has blind spots worth knowing:

• It does not measure gases or odors. CADR is a particle test. VOCs, cooking smells, and off-gassing are handled by activated carbon, which CADR ignores entirely. A high CADR unit with a token carbon filter will still let odors through.
• It is a peak number. The rated CADR is usually achieved on the highest, loudest fan setting. The quiet overnight setting delivers a fraction of it.
• It does not account for filter aging. CADR is measured with a fresh filter. A clogged filter months later delivers less, which is why filter replacement schedules matter.

Frequently Asked Questions

What is a good CADR rating for an air purifier? It depends entirely on room size. Use the 2/3 rule: a good CADR is at least two-thirds of your room's square footage. For a typical 300 sq ft room, look for a smoke CADR of 200 or higher.

What does CADR rated mean? A "CADR rated" purifier has had its Clean Air Delivery Rate independently verified, typically through AHAM's program, rather than self-reported by the manufacturer. It means the airflow-and-filtration number on the box passed a standardized test.

Is a higher CADR always better? Higher is better for cleaning speed and lets you run the unit quieter, but only up to the point that fits your room. Beyond that you are paying for capacity you will run on low anyway — useful for quiet operation, not strictly necessary.

How do I match CADR to my room size? Multiply your room's area in square feet by two-thirds. A 240 sq ft room wants a CADR of about 160. This sizes the unit to clean the air roughly five times an hour.

Why is the coverage area bigger than the CADR suggests? Coverage-area claims are often based on a single air change per hour, which barely improves the air. Sizing by CADR with the 2/3 rule targets five air changes per hour, the level that actually controls allergens and smoke.

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Know your target CADR? Our air purifier buying guide matches specific models to room sizes and explains HEPA grades and carbon filtration.