CADR Rating Explained — How to Size an Air Purifier for Your Room
CADR is the only number that tells you how fast an air purifier actually cleans air. Everything else on the box — room size claims, filtration percentages, fancy filter names — is marketing. Here is how CADR works and how to use it.
Last updated: April 2026 · By the CleanAirHomeLab team
What CADR Actually Measures
CADR stands for Clean Air Delivery Rate. It is measured in cubic feet per minute (CFM). The number tells you how many cubic feet of particle-free air a purifier puts out every minute.
The test is run by AHAM — the Association of Home Appliance Manufacturers. They put the purifier in a sealed 1,008-cubic-foot room, fill the room with a specific type of particle, turn the purifier on max, and measure how fast the particle count drops. The faster it drops, the higher the CADR.
Think of it like a faucet. CADR is how fast clean water comes out. A faucet that delivers 5 gallons per minute fills a bathtub faster than one that delivers 2 gallons per minute. Same idea — higher CADR fills your room with clean air faster.
AHAM tests three particle sizes: smoke, dust, and pollen. Each gets its own CADR number because filters catch different-sized particles at different rates.
CADR for Smoke vs Dust vs Pollen
The three CADR numbers test different particle sizes. Most purifiers score differently on each one because smaller particles are harder to catch.
| CADR Type | Particle Size | Examples | Hardest to Filter? |
|---|---|---|---|
| Smoke | 0.09-1.0 microns | Wildfire smoke, cigarette smoke, cooking fumes, smog | Yes — smallest particles |
| Dust | 0.5-3.0 microns | Household dust, dust mites, mold spores, fine pet dander | Medium |
| Pollen | 5.0-11.0 microns | Tree pollen, grass pollen, large pet dander | Easiest — largest particles |
We use smoke CADR as the primary comparison number throughout this site. It is the hardest test, so if a purifier scores high on smoke, it handles dust and pollen easily.
The 2/3 Rule
The industry standard rule: your purifier's CADR should be at least 2/3 of your room's square footage. This gives you about 4 air changes per hour (ACH) — meaning all the air in your room passes through the filter 4 times every 60 minutes.
Quick Formula
Minimum CADR = Room square footage x 0.65
Example: 300 sq ft room x 0.65 = 195 minimum CADR
This assumes 8-foot ceilings. For 9 or 10-foot ceilings, multiply by 0.75 or 0.8 instead.
Why 4 ACH? At lower rates, particles hang around longer. At 2 ACH (what manufacturers use for their room size claims), particles from cooking, pets, or open windows stick around twice as long. At 4 ACH, the air cleans noticeably faster.
Allergists and the CDC recommend 4-6 ACH for spaces where air quality matters. Hospitals use 12+ ACH. For a home, 4 ACH is a solid target that balances cost and results.
Why Bigger CADR Is Not Always Better
More CADR sounds like a no-brainer. But every bump in CADR comes with tradeoffs:
- Noise:A 400 CADR unit on max typically runs at 55-60 dB. That is as loud as a normal conversation. At night, anything above 40 dB can disturb sleep. Most people end up running high-CADR units on low settings, where the actual CADR drops to 150-250.
- Energy:Higher CADR requires a bigger motor. A 200 CADR unit uses 20-30 watts. A 400 CADR unit on max uses 55-65 watts. Running it 24/7 on max adds $40-60 per year to your electric bill.
- Cost:Higher CADR units cost more up front and use larger, more expensive filters. A 200 CADR purifier might cost $120 with $40/year filters. A 400 CADR purifier costs $220+ with $70-100/year filters.
- Size:High-CADR units are physically bigger. A 400+ CADR purifier stands 20-28 inches tall and weighs 16-47 lbs. In a small bedroom, it takes up real floor space.
The sweet spot: buy a purifier with CADR about 30-50% higher than your minimum need. Then run it on medium speed, where noise is tolerable and you still get enough clean air. A 300 CADR unit on medium gives you roughly 180-210 CADR — plenty for a 250-300 sq ft room.
Room Size Lookup Table
Find your CADR below and see the maximum room size it covers at 4 air changes per hour. Remember: this is max speed CADR. For medium speed performance, go one or two rows higher.
| CADR (CFM) | Max Room (sq ft) | Typical Rooms |
|---|---|---|
| 50 | 78 sq ft | Walk-in closet, small bathroom |
| 100 | 155 sq ft | Small bedroom, home office |
| 150 | 232 sq ft | Standard bedroom, nursery |
| 200 | 310 sq ft | Large bedroom, small living room |
| 250 | 387 sq ft | Average living room |
| 300 | 465 sq ft | Large living room, studio apartment |
| 350 | 542 sq ft | Open-concept main floor |
| 400 | 620 sq ft | Large open floor plan |
| 450 | 697 sq ft | Very large open space |
| 500 | 775 sq ft | Loft, small commercial |
Formula: Max sq ft = CADR x 1.55 (at 4 ACH, 8-foot ceilings). Multiply CADR x 1.34 for 9-foot ceilings, or x 1.16 for 10-foot ceilings.
Common CADR Myths
Myth: Higher CADR is always better
Reality: Higher CADR usually means higher noise and energy use on max speed. A 500 CADR unit on max sounds like a box fan. Buy more CADR than you need, then run it on medium.
Myth: Room size on the box is accurate
Reality: Manufacturers often use 2 air changes per hour (ACH). You need 4 ACH for good filtration. That means the real coverage is about half the number on the box.
Myth: CADR does not matter for HEPA purifiers
Reality: HEPA tells you what the filter catches. CADR tells you how fast air moves through that filter. A perfect filter with low airflow cleans a small area around the unit — the rest of your room stays dirty.
Myth: You should always buy the highest CADR you can afford
Reality: If your room is 200 sq ft, a 500 CADR purifier is overkill. It wastes energy, takes up more space, and costs more in filters. Match the CADR to your room.
Myth: Auto mode uses the right CADR for your room
Reality: Auto mode adjusts speed based on the built-in sensor. If the sensor is on the intake side of the purifier, it reads already-filtered air — and may slow down too early. A standalone air quality monitor gives you an honest reading.
Common Questions
What does CADR stand for?
CADR stands for Clean Air Delivery Rate. It is measured in cubic feet per minute (CFM) and tells you how much clean air a purifier produces. CADR is tested by AHAM, an independent industry group, in a standardized 1,008-cubic-foot test chamber.
How do I calculate what CADR I need for my room?
Multiply your room's square footage by 0.65. That gives you the minimum CADR for 4 air changes per hour with standard 8-foot ceilings. A 300 sq ft room needs at least 195 CADR. If your ceilings are higher than 8 feet, multiply by 0.8 instead.
What is a good CADR rating?
For most bedrooms (150-200 sq ft), a CADR of 150-200 is enough. For living rooms (250-400 sq ft), aim for 250-350. For open floor plans over 500 sq ft, you need 350+ CADR or multiple purifiers. Any CADR over 200 is considered good for a single room.
Why do some purifiers not have a CADR rating?
AHAM testing is voluntary, not required. Some brands like IQAir and Austin Air choose not to participate and use their own testing instead. This does not mean they perform badly — it just means you cannot directly compare their numbers to CADR-rated units.
Does CADR account for filter quality?
Yes. CADR measures the end result — clean air output — not just airflow. A high-airflow unit with a bad filter gets a low CADR because the air coming out is not clean. A True HEPA filter combined with a strong fan produces high CADR.
Is CADR tested at max speed or normal speed?
CADR is always tested at the highest fan speed. This is the number on the spec sheet. At medium speed, expect 50-70% of rated CADR. At low or sleep speed, expect 20-40%. That is why we recommend buying a unit with CADR 30-50% higher than your calculated need.
Know Your CADR. Pick Your Purifier.
Now that you understand CADR, check out our tested picks. Every purifier is reviewed with CADR per dollar, noise levels, and real filter costs.
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