What Is CADR? The Air Purifier Rating That Actually Matters
Most shoppers ignore CADR — the one number that tells you if an air purifier actually fits your room. See the 2/3 rule, compare 50+ models, and stop guessing.
Table of Contents
- What Is CADR?
- The AHAM CADR Rule of Thumb
- Room Size to Minimum CADR Reference Table
- Why Bigger CADR Is Better
- The Three CADR Numbers
- Which CADR Number Matters Most?
- How CADR Is Tested
- Important Test Limitations
- CADR vs Room Size: Manufacturer Claims
- Real-World CADR Examples
- When CADR Is Not Available
- Key Takeaways
TL;DR
CADR measures how many cubic feet of clean air a purifier produces per minute. Higher CADR means faster air cleaning. The AHAM rule of thumb says your purifier's CADR should equal at least two-thirds of your room's square footage. For a 200 sq ft room, that means a minimum dust CADR of 133. CADR is tested by AHAM using a standardized method, making it the most reliable cross-brand comparison tool.
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.
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.
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
| Room Size (sq ft) | Room Type | Minimum Dust CADR | Example Purifier |
|---|---|---|---|
| 100 sq ft | Small office | 67 cfm | Blueair Blue Pure 411 |
| 150 sq ft | Bedroom | 100 cfm | Levoit Core 300S |
| 200 sq ft | Master bedroom | 133 cfm | Levoit Core 300S |
| 250 sq ft | Large bedroom | 167 cfm | Coway Airmega 200M |
| 300 sq ft | Living room | 200 cfm | Winix 5500-2 |
| 350 sq ft | Large living room | 233 cfm | Coway Airmega 200M |
| 400 sq ft | Open plan room | 267 cfm | Levoit Core 400S |
| 500 sq ft | Large open plan | 333 cfm | See large room picks |
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:
| Particle | Size Range | What It Represents |
|---|---|---|
| Smoke | 0.09-1.0 microns | Fine particles (cooking, tobacco, wildfire smoke) |
| Dust | 0.5-3.0 microns | General household particulate matter |
| Pollen | 5.0-11.0 microns | Large allergens (pollen, mold spores) |
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:
- The purifier is placed in a sealed 1,008 cubic foot test chamber (roughly a 11x11x8 ft room)
- A specific pollutant (smoke, dust, or pollen) is injected into the chamber
- The purifier runs on its highest fan speed
- Particle concentration is measured over 25 minutes using laser particle counters
- The rate of particle removal is compared to natural decay (settling without a purifier)
- 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.
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
| Purifier | Dust CADR | Smoke CADR | Pollen CADR | Realistic Room Size |
|---|---|---|---|---|
| Coway Airmega 200M | 246 | 233 | 240 | Up to 370 sq ft |
| Levoit Core 400S | 260 | 256 | 260 | Up to 390 sq ft |
| Levoit Core 300S | 141 | 141 | 145 | Up to 210 sq ft |
| Blueair Blue Pure 411 | 120 | 105 | 120 | Up to 180 sq ft |
| Winix 5500-2 | 243 | 232 | 246 | Up to 365 sq ft |
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
- CADR is the best tool for comparing air purifier performance across brands
- Dust CADR is the most relevant number for general home use
- Use the AHAM two-thirds rule — room square footage × 0.67 = minimum CADR
- Look for AHAM Verified to ensure the numbers are independently tested
- Higher CADR lets you run on lower, quieter speeds while still cleaning effectively
- 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.