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What Is CADR in Air Purifiers? How to Choose the Right Size
Quick Answer
What is CADR in air purifier terms?
You’re standing in an air purifier aisle — or scrolling through websites — and every box claims to clean a different room size.
Some say 500 sq. ft., some say 1,500.
None of them seem to agree.
That’s because most of those numbers are meaningless.
CADR (Clean Air Delivery Rate) is the only metric that actually tells you how fast a unit cleans the air in your room.
It is measured in CFM (Cubic Feet per Minute) and was created by the Association of Home Appliance Manufacturers (AHAM).
Imagine that CADR is like the “speed limit” of an air purifier.
It measures how fast the unit delivers clean air into the room — the higher the number, the faster pollutants are removed.
The more CADR, the more rapid the purification.
Sizing Guideline – How to calculate CADR for air purifier
To determine the appropriate model, and to have a simple CADR to room size formula, apply the 2/3 Rule.
The air purifier’s Smoke CADR must be two-thirds of the area in square feet of the room.
- Formula: Room Square Footage × 2/3 = Minimum CADR
- Example: In a 120 square foot bedroom, the purifying device should have a CADR of no less than 80 (because 120 x 0.67 = 80).
This rating is the most important one to check because it will help you ignore all of the marketing hype and immediately determine whether or not the purifier will work for a given size room.
Key Takeaways
- Performance Over Promises: air purifier CADR rating tells the real story.
A unit might say “HEPA filtration,” but if it has a weak fan and a CADR of 50, it’s useless in a living room.
CADR stacks filter efficiency and airflow, so it gives a true performance score. - Three Scores to Watch: When buying an air purifier, you’ll see three numbers that refer to the CADR: Smoke (small particles, which is the most dangerous), Dust (which is medium size), and Pollen (which is larger size).
The most important to consider is the Smoke CADR as it gives information about the most dangerous and smallest particles (PM 2.5) and most difficult to filter. - The 4.8x Standard: A unit’s CADR rating reflects that it will be able to clean the air in a room 4.8 times per hour.
If you purchase a unit with CADR that is too low for your room size, it means you will have only 1 or 2 full air changes in an hour and that is insufficient for someone with allergies. - New Metrics (2025): The industry is changing.
Look for PM2.5 CADR (a weighted average of smoke and dust) and IEF (Integrated Energy Factor), which reflects clean air produced per watt. - Silence Costs More: CADR is calculated at max fan speed.
For high cleaning efficiency without the jet engine noise, get a unit with greater CADR and run it on medium.
If you are interested also in dehumidifiers, read our article on the best dehumidifiers for basements.
CADR Rating Explained
What is air purifier CADR rating: put simply, it is a measure of how air purifiers can be rated and compared.
It gives clarity to the “Apples vs. Oranges” problem of marketing.
Prior to CADR, a brand could advertise “99% efficient filters,” despite having zero airflow, while the other boasted “High-Velocity Fan,” even if it had no filter, and neither would clean a room.
CADR takes these two factors and combines them.
- CADR = Filter Efficiency (%) x Airflow (CFM)
Standards to this metric are defined by the Association of Home Appliance Manufacturers and can also be cited by the EPA and FTC.
If a unit has the AHAM Verifide® seal, it means that the CADR has been tested in a certified lab as opposed to simply assumed by the manufacturer.
How it Works
CADR test is performed according to the ANSI/AHAM AC-1 standard to provide you with the best and most accurate results possible.
Below is the process step by step:
- The Chamber: Air purifiers are put in a testing chamber with a volume that has been standardized to be 1,008 cubic feet.
- The Contaminants: Exact measured levels of smoke, dust, or pollen are added to the chamber.
- The Decay Rate: The chamber is equipped with a variety of sensors that measure how quickly the particles are removed from the air by natural means (like gravity) in comparison to how quickly they are removed when the air purifier is running.
- The Calculation: air purifier CADR rating is defined as the difference between natural decay rate and the decay rate with the purifier on.
The Three Pollutants
Since the efficiency of the filters varies from one filter to another, AHAM measures three distinct pollutants:
| Pollutant | Particle Size (Microns) | What It Represents | Why It Matters |
|---|---|---|---|
| Tobacco Smoke | 0.09 – 1.0 | PM2.5, Viruses, Vehicle Exhaust, Wildfire Smoke | Most Important. These ultrafine particles penetrate deep into the lungs. This is the hardest score to achieve. |
| Dust | 0.5 – 3.0 | Household Dust, Pet Dander | Represents common allergens found in typical homes. |
| Pollen | 5.0 – 11.0 | Pollen, Mold Spores | Large particles that fall quickly. High airflow is needed to catch them before they land. |
Note: The test usually lasts only 20 minutes.
This measures the unit’s ability to “scrub” a room quickly, a metric AHAM refers to as pull-down performance.
Why CADR Matters in Real Homes
You might wonder, “If I leave a small purifier running all day, won’t it eventually clean the room?”
The answer is often no, and here is why CADR is critical.
The Fight Against Pollution
Your home is not a sealed unit.
You might leave a window open, a door might be ajar, or, your HVAC might turn on.
With any of these, outside “dirty” air is coming in.
Low CADR air purifiers will not be able to keep up with this new influx of pollution.
High CADR air purifiers will be able to keep the air clean and maintain a low allergen count.
Pet owners in particular benefit from a high CADR unit — pet dander is one of the most persistent indoor pollutants.
Our best air purifier for pets guide shows exactly how to apply CADR to a pet home.
Pair it with the best robot vacuum for pet hair to tackle dander from both the air and the floor at the same time.
If you’re also dealing with humidity issues, it’s worth knowing whether you need a dehumidifier or an air purifier — the two solve different problems.
The Amount Of Time Required To Purify Air (Recovery Time)
When you burn toast, or your neighbor’s fire pit smokes, your house becomes filled with tiny particulates.
This makes your air pollution worse.
- Low CADR: it will take up to 2 hours to purify the air of the smoke, and you breathe that smoke the whole time.
- High CADR: in high CADR, the smoke will be cleared in 15 minutes.
With a high CADR air purifier, you will not be exposed to the pollutants for 2 complete hours.
Energy Star V3.0 Compliance
Starting in 2024/2025, the EPA’s Energy Star 3.0 specification has shifted focus to the Integrated Energy Factor (IEF).
The new standard, PM2.5 CADR per Watt, requires better engineering to be used to create more efficient motors.
With the standards of a high CADR and Energy Star, you will be sure to have the best cleaning efficiency for the most inexpensive energy usage.
Typical Values, Standards, and Ranges
Use this table to translate CADR to room size.
This is based on the AHAM recommended 4.8 Air Changes Per Hour (ACH).
CADR to Room Size Summary Table (The 2/3 Rule)
| Room Size (sq. ft.) | Minimum Smoke CADR Needed | Common Room Type |
|---|---|---|
| 80 sq. ft. | 55 | Nursery / Home Office |
| 120 sq. ft. | 80 | Small Bedroom |
| 200 sq. ft. | 130 | Master Bedroom |
| 300 sq. ft. | 200 | Living Room |
| 400 sq. ft. | 270 | Studio Apartment |
| 500 sq. ft. | 335 | Open Concept Living |
| 600+ sq. ft. | 400 | Large Great Room |
Note: The maximum official AHAM rating for Smoke/Pollen is 450 and for Dust is 400. If you see a residential unit claiming “CADR 800,” it is likely extrapolated or untested.
Typical Product Ranges
- Budget/desktop units: CADR 30-100 (these are only good for very small spaces)
If that’s your range, see our picks for the best air purifier under $100, we only include units with verified CADR ratings. - Mid-Range Towers: CADR 100-200 (these are good for standard bedrooms)
If you’re sizing for a bedroom specifically, our best air purifier for bedroom guide does the CADR math for you. - Performance Units: CADR 200+ (Mandatory for living rooms and larger spaces).
Common Myths and Misunderstandings
Many people don’t understand CADR ratings when it comes to the productivity of air purifiers. I will clear this up.
Myth 1: The coverage area is as described in the box.
Reality: The manufacturer also considers it a success if the machine only has to clean air once an hour (1 ACH).
The Association of Home Appliance Manufacturers and healthcare professionals disagree: 4.8 ACH is the minimum to be effective in allergy reduction.
- Example: if a box has the statement, “Cleans up to 1,500 sq. ft!” and it has a CADR of 150, then it is a complete misrepresentation.
This unit is only effective for a 225 sq. ft. room. The CADR should always be trusted over any coverage statements.
Myth 2: CADR measures odors and chemicals.
Reality: Standard CADR only measures solid things (e.g., smoke, dust, and pollen). It does not tell you anything about VOCs, formaldehyde, and even smelly dogs. There is a new standard called c-CADR (Chemical CADR) that is currently being implemented, but it is not widespread.
Myth 3: “I can just buy a huge CADR unit for my small room.”
Reality: You can, and it’s actually a great idea. While you don’t need a CADR 400 unit for a 100 sq. ft. room, buying one allows you to run it on Low speed. It will still deliver a Clean Air Delivery Rate (100 CFM), enough to effectively clean the room, and it will be super quiet. A small unit will be screaming on High.
How to Evaluate CADR When Buying Products
Hopefully, by now you have the answer to your “what is CADR in air purifier” question.
Now, you can use this checklist to avoid being ripped off.
- Find the AHAM Seal: Look for the white AHAM Verified label on the box.
If it is missing, there is a chance the CADR numbers listed are marketing scams. - Calculating CADR for Your Room:
- Let’s say your room’s length is l, and width is w. Your room’s area will be l x w.
How to calculate CADR for air purifier: multiply the room area by 0.67 (2/3). - Example: 250 sq. ft. × 0.67 = 167.
Purchase a unit with Smoke CADR > 170.
- Let’s say your room’s length is l, and width is w. Your room’s area will be l x w.
- Look for “PM2.5 CADR”: This may be listed for newer models (2024+).
It is a specific average for smoke and dust performance, ideal for wildfire smoke concerns. - Disregard “Fan Speed” and “CFM” if there is no CADR: A fan that moves 300 CFM of air through a 50%-efficient filter has a CADR of 150.
You want the CADR, not the fan spec. - Wildfire Adjustment: If you live in a wildfire zone, size up.
Consider the CADR as a 1:1 ratio with square footage (e.g., 300 CADR for 300 sq. ft.) to manage the extreme particle load.
How We Researched This Guide
In drafting this guide, we examined the technical specs and performance criteria from the industry standards, foremost among them the ANSI/AHAM AC-1-2020, and the most current applicable ENERGY STAR® Version 3.0 documents.
Also, we analyzed any available independent laboratory assessments and manufacturer documents for consistency and plausibility and utilized the U.S. EPA Air Cleaners in the Home guidance.
Moreover, for recommendations pertaining to odor control, we utilized the AHAM AC-4-2022 standard for chemical gas removal.
This lets us cut through manufacturer marketing and give you recommendations grounded in actual engineering standards.
Practical Examples
Example 1: The Nursery
- Room: 10 x 10 (100 sq. ft)
- Need: Protection from developing babies lungs
- Calculation: 100 × 0.67 = 67
- Recommendation: A small unit with Smoke CADR of at least 70.
But, buying a unit with CADR 120 means you can run it on “Sleep Mode” (quieter) and still meet the required CADR.
Example 2: The Open-Plan Apartment
- Room: Studio apartment, 500 sq. ft.
- Need: Dust and cooking smoke
- Calculation: 500 × 0.67 = 335
- Recommendation: You need a powerhouse.
For optimal performance, look for a unit with a CADR of 350+ for smoke.
As another option, you can also use two medium units (CADR 180 each).
Placing the units on opposite ends of the apartment will give you better circulation.
A bladeless tower fan running between the two units can help distribute clean air even faster across the open floor plan.
Example 3: The Smoker / Wildfire Zone
- Room: Living room, 300 sq. ft.
- Need: Rapid removal of heavy smoke.
- Calculation: Standard rule is too weak for heavy pollution.
Use the 1:1 rule. - Recommendation: A unit with Smoke CADR 300+.
This guarantees the unit is able to clean the air quickly during smoky periods.
For wildfire-specific picks, see our best air purifier for wildfire smoke guide.
For cigarette smoke, see our best air purifier for cigarette smoke guide.
Frequently Asked Questions (FAQs)
This rating is primarily relative to the size of the room in which the purifier is to be used, but in general:
- 100 – 200: suitable for bedrooms.
- 300+: ideal for living areas and larger spaces.
- 450: ideal for Smoke/Pollen; currently the residential gold standard.
Yes and no.
A CADR 450 unit, leaving aside the concerns of noise and whether you need the unit for its intended purpose, will not ‘clean’ the air in a closet any better than a CADR 100 will.
It will just ‘clean’ faster (in seconds vs. minutes).
A higher CADR unit will almost always be larger and more expensive.
A better strategy is to buy a unit somewhere in the ‘sweet spot’ that is higher than your need so that you can run the unit on one of the quieter lower speed settings.
Some premium brands have lower CADR and explain it like the AHAM CADR test is too focused on speed and doesn’t factor in air distribution.
They use their own testing metrics (like POLAR).
In contrast to these expensive bladeless fans, units like Coway or Winix have been shown to remove particles faster and more effectively so, if your goal is to perform, stick to CADR.
If you’re also comparing filter types, our guide on HEPA vs True HEPA explains why the label matters as much as the CADR number.
Yes, CADR is additive.
If your basement is large (600 sq. ft.) and needs a CADR of 400, you can have two smaller units that are CADR 200.
This is often better for air circulation than having one large unit.
For measuring COVID-19 effectiveness CADR can be used but not directly.
COVID-19, like many other viruses, travels through human respiratory particles, also called aerosols.
Some of these respiratory droplets fall into the 0.1-1.0 micron range, a particle size that the Smoke CADR measures.
Therefore, there is a correlation between a unit’s Smoke CADR score and its ability to lower the level of aerosols that contain viruses.
The higher the Smoke CADR the better the unit is at capturing virus carrying aerosols.
Disclaimer: For informational purposes only. For medical advice or diagnosis, please consult a professional.
Now that you have the answer to your question “What is CADR in air purifier”, check out our article on air purifiers and allergies.
For a complete overview of all our air purifier guides, visit our Air Purifiers category page.
