What Is CADR? The Air Purifier Rating That Actually Matters

Walk into any air purifier listing and you will see specs like "CADR: 250/230/260." If those numbers mean nothing to you, this article will fix that. CADR is the single most useful metric for comparing air purifiers, and understanding it takes about five minutes.

What Is CADR?

CADR stands for Clean Air Delivery Rate. It measures the volume of filtered air an air purifier delivers, expressed in cubic feet per minute (cfm).

A CADR of 200 means the purifier outputs 200 cubic feet of particle-free air every minute. Higher CADR means the purifier cleans air faster.

CADR is not a filtration efficiency rating. It combines two factors:

• Airflow rate (how much air moves through the filter)
• Filter efficiency (what percentage of particles the filter captures)

A purifier with a great filter but weak fan will have a low CADR. A purifier with a powerful fan but poor filter will also have a low CADR. You need both working well together.

CADR Sizing: Four Worked Examples

The two-thirds rule is easy to state, but seeing it applied across real room sizes makes it stick. Below we work through four common rooms. In each case we start with the square footage, apply the 0.67 multiplier for the minimum dust CADR, then note the smoke and pollen targets you should confirm on the spec sheet.

The AHAM two-thirds rule assumes standard 8-foot ceilings and targets roughly 4.8 air changes per hour (ACH), which is the level AHAM uses when converting CADR to a suggested room size. If your ceilings are taller than 8 feet, scale the target up proportionally (a 9-foot ceiling adds about 12 percent to the air volume).

Example 1: 100 sq ft home office

100 × 0.67 = 67 minimum dust CADR. This is a small, enclosed space, so almost any entry-level purifier clears the bar. Confirm the smoke CADR is also at least 67 if you burn candles or vape at your desk. Pollen CADR will run higher and is rarely the limiting factor. A unit rated 100+ across all three numbers gives you quiet operation and headroom.

Example 2: 200 sq ft bedroom

200 × 0.67 = 134 minimum dust CADR. Because this is where you sleep, prioritize smoke CADR near 134 as well so fine particles from outside or cooking do not accumulate overnight. Buying above the minimum (say a dust CADR of 240) lets you run the fan on a quiet low or medium speed and still hit your air changes, which matters more in a bedroom than anywhere else.

Example 3: 350 sq ft living room

350 × 0.67 = 235 minimum dust CADR. Living rooms are open, have more furniture to disrupt airflow, and often connect to a kitchen, so aim at or above 235 for all three numbers. If cooking odors and smoke are a concern, treat the smoke CADR as your true minimum since it is the hardest particle class to capture. A mid-tier tower purifier is usually the right call here.

Example 4: 500 sq ft open-plan space

500 × 0.67 = 335 minimum dust CADR. Few single filters reach this on their own, so you have two options: buy one high-output purifier rated 350+ CADR, or run two smaller units whose CADRs add together (two purifiers at 180 CADR each give you a combined 360). Our large-room picks cover units built for this range.

Quick reference: room size to minimum CADR

The pattern is deliberate: your minimum dust CADR sets the floor, and you then verify that the smoke and pollen numbers are not far below it. Well-designed purifiers keep all three numbers close together, so a large gap between them is a signal to look closer at the spec sheet.

The AHAM CADR Rule of Thumb

The AHAM rule of thumb is the simplest way to match an air purifier to your room: your purifier's dust CADR should be at least two-thirds (0.67×) of your room's square footage, assuming standard 8-foot ceilings.

This ensures a minimum of two air changes per hour, enough to meaningfully reduce particle levels.

Quick calculation: Room square footage × 0.67 = minimum dust CADR needed. Or skip the math and use our CADR calculator to get a personalized recommendation based on your exact room dimensions.

Example: You have a 300 sq ft living room. 300 × 0.67 = 201. You need a purifier with a dust CADR of at least 200.

Room Size to Minimum CADR Reference Table

Why Bigger CADR Is Better

A purifier with a higher CADR than your room requires means faster air changes. Instead of running on high speed (loud), you can run it on medium or low (quiet) and still clean the air effectively. This also extends filter life.

Example: A purifier with a CADR of 260 in a 200 sq ft room can clean all the air roughly 5 times per hour on its highest setting, or 2-3 times on a quieter medium setting. Both are more than adequate.

This is especially important for large rooms where you need 300+ CFM to maintain adequate air quality, for bedrooms where noise matters most, and for small apartments where space is limited and every CFM counts.

The Three CADR Numbers

CADR is measured separately for three particle types:

Which CADR Number Matters Most?

Dust CADR is the most practical number for general home use. It represents the mid-range particle size that covers most common indoor pollutants.

Smoke CADR matters most if you are dealing with wildfire smoke, cooking particles, vape aerosol, or tobacco smoke. Because smoke particles are the smallest, this is typically the lowest of the three numbers and the hardest test for the filter.

Pollen CADR is almost always the highest number because large particles are the easiest to capture. It is most relevant during allergy season.

How CADR Is Tested

AHAM (Association of Home Appliance Manufacturers) developed the ANSI/AHAM AC-1 test standard:

1. The purifier is placed in a sealed 1,008 cubic foot test chamber (roughly a 11x11x8 ft room)
2. A specific pollutant (smoke, dust, or pollen) is injected into the chamber
3. The purifier runs on its highest fan speed
4. Particle concentration is measured over 25 minutes using laser particle counters
5. The rate of particle removal is compared to natural decay (settling without a purifier)
6. CADR = the additional particle removal rate attributable to the purifier

The test is repeatable and standardized, which is why CADR is the most reliable way to compare purifiers across brands.

The Limitations of CADR

CADR is the best single number we have, but it is not the whole story. Knowing what it leaves out keeps you from buying on the spec sheet alone.

CADR is a first-20-minutes, max-speed snapshot. The AHAM test runs the purifier flat out with a brand-new filter, and the rate is derived from the fast initial drop in particle concentration. That means the published figure is the best case, not the number you will see running quietly on auto mode with a filter that is three months old.

CADR says nothing about noise. A unit can hit a high CADR by spinning a loud fan. Two purifiers with an identical dust CADR of 240 can differ by 15 decibels or more at top speed. Always cross-check the noise rating (in dB) before assuming a high CADR is livable in a bedroom or office.

CADR ignores filter longevity and running cost. The rating is captured with a fresh filter and tells you nothing about how quickly performance fades or what replacement filters cost over a year. A purifier with a slightly lower CADR but cheaper, longer-lasting filters can be the better buy. Our breakdown of air purifier running costs covers how those numbers stack up.

CADR does not measure gas, VOC, or odor removal. This is the biggest blind spot. CADR only rates particle capture (smoke, dust, pollen). Gaseous pollutants such as formaldehyde, VOCs, and cooking or pet odors pass straight through a HEPA filter and are not reflected in any CADR number.

Carbon filtration is invisible to CADR. The activated carbon stage that adsorbs gases and smells is not part of the AHAM test at all. A purifier can post an excellent smoke CADR (fine smoke particles) while doing very little about the smell of smoke, because the odor is a gas. If odor control matters to you, weigh the amount and quality of carbon separately and lean on independent gas-phase testing rather than CADR.

Important Test Limitations

• Tested on highest speed only. Your real-world CADR on low or auto mode is lower.
• New filter only. CADR decreases as the filter loads with particles over months of use. Follow a regular maintenance schedule to keep performance up.
• 1,008 sq ft chamber. Performance may differ in rooms with different layouts, furniture, or airflow patterns.
• Particles only. CADR does not measure gas or VOC removal. For those pollutants, you need activated carbon filtration.

CADR vs Room Size: Manufacturer Claims

Manufacturers convert CADR into "recommended room size," but the conversion formula varies. Some assume 2 air changes per hour (conservative), others assume 4.8 (aggressive). This is why two purifiers with the same CADR might claim different coverage areas.

Trust the CADR number, not the room size claim. Use the two-thirds rule yourself to calculate the actual room coverage.

Real-World CADR Examples

Want help picking the right one? See our buyer's guide on how to choose an air purifier or browse by need: allergies, pets, smoke, or budget picks under $100. For a side-by-side look at how CADR differences play out between popular budget brands, see our Levoit vs Winix vs Coway comparison.

When CADR Is Not Available

Some brands, most notably Dyson, do not participate in AHAM testing. Their purifiers may still perform well, but you cannot directly compare them using CADR.

In these cases, look for:

• Independent test results from reputable reviewers who measure actual particle removal in controlled conditions
• Airflow rate (cfm or m3/h) combined with stated filter efficiency
• Real-world user reviews mentioning air quality monitor measurements

Be skeptical of proprietary metrics that sound similar to CADR but use different testing methodologies. They exist to make comparison harder, not easier.

Key Takeaways

1. CADR is the best tool for comparing air purifier performance across brands
2. Dust CADR is the most relevant number for general home use
3. Use the AHAM two-thirds rule: room square footage × 0.67 = minimum CADR
4. Look for AHAM Verified to ensure the numbers are independently tested
5. Higher CADR lets you run on lower, quieter speeds while still cleaning effectively
6. Ignore manufacturer room size claims and calculate coverage yourself

For help choosing a specific purifier, see our complete buyer's guide or jump straight to our best air purifiers for your bedroom or apartment.