What CADR Actually Measures
CADR stands for Clean Air Delivery Rate. It measures the volume of air that an air purifier filters and delivers — free of a specific particle type — per unit of time. The unit is typically cubic feet per minute (CFM) in the US, or cubic metres per hour (m³/h) in Europe and Australia.
The key word is delivered. CADR does not measure how good the filter is in isolation. It measures the combined output of the filter and the fan — the actual volume of clean air the purifier produces when running. A purifier with an excellent filter but a weak fan may have a lower CADR than a purifier with a slightly less efficient filter and a more powerful fan. CADR is a whole-system metric.
CADR was developed and standardised by AHAM — the Association of Home Appliance Manufacturers. The AHAM 2040 protocol (the current test standard) specifies the test chamber size (1,008 cubic feet), the particle concentrations used, and the methodology for calculating results. When you see CADR figures from AHAM-certified testing, you are looking at numbers produced by the same methodology across every brand — a genuine apples-to-apples comparison.
How the test works
AHAM tests an air purifier in a sealed 1,008 cubic foot chamber. The chamber is loaded with a known concentration of particles. The purifier runs at its highest setting. Sensors measure how quickly particle concentration drops over time. From the rate of decline — accounting for the chamber's natural particle decay rate — CADR is calculated for each particle type.
This is why CADR figures represent maximum performance at maximum fan speed. Real-world performance at lower settings is proportionally lower. A purifier with a CADR of 250 CFM at full speed might deliver 150 CFM on medium and 70 CFM on low. This distinction matters for bedroom use, where the purifier often runs on low overnight.
The Three CADR Numbers: Smoke, Dust, and Pollen
Every AHAM-tested air purifier receives three separate CADR ratings — one for each of three particle size categories. They are not interchangeable.
| CADR type | Particle size range | What it represents | Difficulty to filter |
|---|---|---|---|
| Smoke CADR | 0.1–1 micron | Tobacco smoke, wildfire smoke, cooking fumes, exhaust particles, fine PM2.5 | Hardest — most particles in this range |
| Dust CADR | 0.5–3 microns | Household dust, skin cells, dust mite particles, some mould spores | Moderate |
| Pollen CADR | 5–11 microns | Pollen, larger mould spores, pet dander, larger dust particles | Easiest — large particles intercepted readily |
The three AHAM CADR particle categories. Smoke CADR is the most demanding test and the most informative single number for general air quality use.
How to Use CADR to Size a Purifier for Your Room
This is where CADR becomes directly actionable. The AHAM guideline states that a purifier's CADR should be at least two-thirds of the room area in square feet.
For a 300 sq ft living room: 300 × 0.67 = 201 CFM minimum
For a 150 sq ft bedroom: 150 × 0.67 = 100 CFM minimum
For a 500 sq ft open-plan space: 500 × 0.67 = 335 CFM minimum
This formula assumes a standard 8-foot ceiling and delivers approximately 4–5 air changes per hour (ACH) — the rate recommended by AHAM for effective air cleaning. ACH is the number of times the entire room's air volume passes through the filter per hour. At 4 ACH, room air is fully filtered every 15 minutes.
When to exceed the minimum
The × 0.67 figure is a minimum for acceptable performance. In several situations, you should exceed it:
- Allergy or asthma management: Target 5–6 ACH, which requires a CADR approximately equal to (not 0.67×) the room area in square feet.
- Wildfire smoke events: The California Air Resources Board recommends purifiers capable of 5+ ACH during smoke events. Double the standard recommendation.
- High ceilings (above 9 feet): Standard formulas assume 8-foot ceilings. For a 10-foot ceiling, multiply the standard minimum by 1.25.
- Open-plan spaces with multiple air inlets: If a kitchen is open to a living room, smoke and cooking particles enter from one side while the purifier draws air from another. A higher CADR compensates for the non-uniform mixing.
- Pet households with multiple animals: Dander loading is continuous and significant. A CADR 30–40% above the room-size minimum is appropriate.
| Room size | Minimum CADR (4 ACH) | Allergy / asthma (5 ACH) | Wildfire smoke (6 ACH) |
|---|---|---|---|
| Small bedroom (120 sq ft) | 80 CFM | 100 CFM | 120 CFM |
| Standard bedroom (180 sq ft) | 120 CFM | 150 CFM | 180 CFM |
| Master bedroom / studio (300 sq ft) | 200 CFM | 250 CFM | 300 CFM |
| Living room (400 sq ft) | 267 CFM | 333 CFM | 400 CFM |
| Large open-plan (600 sq ft) | 400 CFM | 500 CFM | 600 CFM |
Recommended CADR by room size and use case. All figures assume 8-foot ceilings and smoke CADR as the reference number.
CADR Room Size Calculator
Find your required CADR
Enter your room dimensions to calculate the minimum CADR for your use case.
Real-World CADR Examples — What the Numbers Look Like
Abstract numbers are hard to interpret without context. Here is how CADR figures from well-known products translate into practical room coverage.
| Product | Smoke CADR | Max room (4 ACH) | Allergy room (5 ACH) | Noise at max |
|---|---|---|---|---|
| Levoit Core 300 (compact) | 141 CFM | 211 sq ft | 169 sq ft | 24 dB (low) |
| Levoit Core 600S (large) | 410 CFM | 612 sq ft | 492 sq ft | 52 dB (max) |
| Coway Airmega 400 | 350 CFM | 522 sq ft | 420 sq ft | 43 dB (max) |
| Blueair Blue Pure 211+ | 350 CFM | 522 sq ft | 420 sq ft | 56 dB (max) |
| Dyson Purifier Cool TP07 | ~150 CFM (est.) | 224 sq ft | 180 sq ft | 44 dB (max) |
| Winix 5500-2 | 243 CFM | 362 sq ft | 291 sq ft | 27 dB (low) |
CADR figures from AHAM testing where available. Dyson does not publish AHAM CADR — the estimate is calculated from published airflow specs and is approximate. Noise figures are manufacturer-stated at maximum setting.
What CADR Doesn't Tell You
CADR is the best single metric for comparing air purifiers, but it has meaningful limitations. Understanding them prevents you from over-relying on the number.
1. CADR is measured at maximum speed only
The AHAM test runs the purifier at its highest fan setting. For bedroom use — where the purifier typically runs on low or medium overnight — the real-world CADR may be 30–60% lower than the rated figure. A purifier with 400 CFM at maximum but only 100 CFM on its sleep setting may underperform a unit with 250 CFM maximum and 150 CFM on medium.
2. CADR does not measure VOC or odour removal
CADR only measures particle removal. It says nothing about a purifier's ability to remove volatile organic compounds (VOCs), formaldehyde, cooking odours, or tobacco smoke smell. These require activated carbon filtration, which is not captured in any CADR figure. If VOC or odour removal is important, look for carbon filter weight (in grams) separately from CADR.
3. CADR assumes an idealised room
The AHAM test chamber is a perfectly sealed, well-mixed 1,008 cubic foot box. Real rooms have doors, hallways, furniture, dead corners, and multiple particle sources. Placement of the purifier matters significantly — a unit positioned in a corner behind a sofa will perform worse than the same unit positioned centrally or near the particle source.
4. CADR does not capture filter efficiency at different particle sizes within each range
Smoke CADR covers 0.1–1 micron particles as a range. A purifier might capture 99% of 1-micron particles but only 80% of 0.1-micron particles and still report a strong smoke CADR overall, because larger particles in the smoke range are captured at very high rates. For environments with specific ultrafine particle concerns, CADR alone is insufficient.
5. CADR does not measure how quietly the purifier achieves its rated performance
Two purifiers with identical CADR ratings may have very different noise profiles. One might achieve 250 CFM at a tolerable 38 dB. Another might achieve the same 250 CFM at 55 dB — loud enough to disrupt sleep. Always check noise levels at the specific fan speed you will use in practice, not just the maximum-speed noise figure.
Why AHAM Verification Matters
CADR testing through AHAM is voluntary. Manufacturers can — and some do — publish self-reported CADR figures without independent verification. The difference matters because independent testing is repeatable, uses standardised methodology, and cannot be manipulated by the manufacturer.
The AHAM Verified mark on an air purifier package means a third party has tested the product and confirmed the published CADR figures. Self-reported figures without this seal may be accurate, or they may be aspirational numbers from manufacturer testing under more favourable conditions.
Several major brands consistently submit to AHAM testing: Levoit, Coway, Blueair, Winix, Honeywell, and RabbitAir all maintain AHAM-verified CADR data. Dyson, as noted above, does not.
How to Use CADR When Buying
With the above framework in place, here is a practical buying process that uses CADR effectively without overcomplicating the decision.
Step 1: Measure your room and calculate minimum CADR
Length × width × 0.67 (general use) or × 0.83 (allergies/asthma). Use the calculator above. This gives you a hard floor below which a purifier is undersized for your space.
Step 2: Filter to AHAM-verified products only
Eliminate products without AHAM-verified CADR. This immediately removes a significant number of products whose coverage claims cannot be independently verified. The AHAM directory is free to search.
Step 3: Compare smoke CADR, not pollen CADR
Manufacturers sometimes lead with pollen CADR because it is the highest of the three numbers. Always compare smoke CADR across products — it is the honest measure of fine particulate performance.
Step 4: Calculate CADR-per-dollar and CADR-per-decibel
Divide the smoke CADR by the purchase price to get efficiency-per-dollar. A 350 CFM purifier at $180 ($0.51/CFM) compares favourably to a 350 CFM purifier at $280 ($0.80/CFM) if the other specifications are similar. Then check noise levels at the fan speed you will actually use — CADR-per-decibel is the relevant figure for bedroom use.
Step 5: Factor in filter replacement cost
CADR performance degrades as the filter loads. A purifier that requires $70 filter replacement every 6 months will cost $140/year to maintain its rated CADR. This is often more than the electricity cost. Calculate total annual running cost (electricity + filters) and compare it alongside the purchase price.
Common Mistakes When Reading CADR
Comparing pollen CADR across products
Pollen CADR is consistently the highest of the three CADR figures and varies least between purifiers. Comparing smoke CADR gives a more meaningful picture of real-world performance differences.
Trusting coverage area claims over CADR
Manufacturers calculate coverage area using their own formulas — often 2 ACH or even 1.5 ACH, not the AHAM-recommended 4–5 ACH. A purifier rated for "500 sq ft rooms" may only achieve 2 ACH in that space. Always verify the actual CADR figure and calculate ACH yourself.
Not accounting for ceiling height
The room-size formula assumes 8-foot ceilings. A 300 sq ft room with 10-foot ceilings has 3,000 cubic feet of air — 25% more than the same room with 8-foot ceilings. The same purifier achieves proportionally fewer ACH. Multiply your standard minimum by (ceiling height ÷ 8) to adjust.
Assuming maximum CADR equals operating CADR
CADR is measured at maximum fan speed. If you run the purifier on medium or low — which is normal for noise reasons — the effective CADR is significantly lower. Choose a purifier whose CADR exceeds your room's needs at medium speed, not just at maximum.
Ignoring placement
Placing a purifier behind furniture, in a corner, or far from the primary particle source reduces effective CADR. The best position is centrally in the room or near the source of pollutants (e.g. near a window in a wildfire-prone area, or near a pet's sleeping area).
Equating CADR with total air quality improvement
CADR measures particle removal. It says nothing about VOCs, odours, humidity, CO₂, or radon. A high-CADR purifier in a poorly ventilated room with off-gassing furniture may clean particles effectively while VOC levels remain elevated. CADR is one dimension of air quality, not all of it.
How to apply CADR in practice
Measure your room, multiply by 0.67 (or 0.83 for allergy use), and use that number as your minimum smoke CADR. Look for AHAM-verified products that exceed that floor. Compare smoke CADR rather than pollen CADR or manufacturer coverage area claims. Then layer in noise levels at operating speed and annual filter cost to complete the comparison. CADR is not the only number that matters, but it is the most honest one — and starting there eliminates most of the marketing noise in the category.
Frequently Asked Questions
What does CADR stand for?
CADR stands for Clean Air Delivery Rate. It measures the volume of filtered air an air purifier delivers per minute (CFM) or per hour (m³/h). It is independently tested by AHAM for three particle types — smoke (0.1–1 micron), dust (0.5–3 microns), and pollen (5–11 microns).
What is a good CADR rating for an air purifier?
For a typical bedroom (150–200 sq ft), a smoke CADR of 100–135 CFM delivers 4 ACH. For a living room (300–400 sq ft), look for 200–270 CFM. For larger open spaces (500+ sq ft), 335 CFM or higher. The formula is: minimum smoke CADR = room area × 0.67. For allergy or asthma management, use room area × 0.83.
What is the difference between smoke CADR, dust CADR, and pollen CADR?
Smoke CADR measures capture of very fine particles (0.1–1 micron) — the hardest to filter and the most relevant to fine particulate health effects. Dust CADR measures medium particles (0.5–3 microns). Pollen CADR measures large particles (5–11 microns) — the easiest to capture. Smoke CADR is the most demanding test and the most informative single number. A purifier with high pollen CADR but low smoke CADR is not effective against the particles that matter most for air quality.
Is higher CADR always better?
Higher CADR means faster air cleaning, but it comes with trade-offs: more noise at maximum speed, higher energy consumption, and larger physical footprint. A CADR significantly above what your room requires wastes energy without improving air quality proportionally. The goal is to match CADR to room requirements plus a reasonable margin — not to buy the highest available number.
Why do manufacturers sometimes not publish CADR ratings?
AHAM CADR testing is voluntary and independently verified. Some manufacturers skip it because their products perform poorly against independent testing, preferring to use proprietary metrics (airflow in litres per second, "coverage area" calculated at low ACH rates) or vague marketing claims. Absence of an AHAM-verified CADR rating is a meaningful negative signal, particularly at higher price points.
How does CADR relate to air changes per hour (ACH)?
ACH and CADR measure the same thing from different angles. CADR is the volume of clean air delivered per minute (CFM). ACH is how many times the entire room's air volume is filtered per hour. To convert: ACH = (CADR × 60) ÷ room volume in cubic feet. A 200 CFM purifier in a 300 sq ft × 8 ft room (2,400 cubic feet) achieves (200 × 60) ÷ 2,400 = 5 ACH.
Does CADR decrease as the filter gets older?
Yes. As a HEPA filter loads with captured particles, airflow resistance increases. The fan works harder to move the same volume of air, energy consumption rises, and effective CADR drops. Filter replacement at the manufacturer's recommended interval (typically 6–12 months) is necessary to maintain rated CADR performance. Running an air purifier with an overdue filter replacement is significantly less effective than the product's CADR rating suggests.