CADR vs ACH — Two Air Purifier Metrics, One Buying Decision
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CADR (Clean Air Delivery Rate) — cubic feet per minute of particle-free air delivered by the purifier at maximum fan speed, independently tested by AHAM for smoke, dust and pollen separately. A property of the purifier itself, independent of room size. ACH (Air Changes per Hour) — how many times per hour a purifier completely replaces the air volume in a specific room. A derived metric: ACH = (CADR × 60) ÷ room volume (sq ft × ceiling height in feet). ACH is room-dependent — the same purifier delivers different ACH in a 150 sq ft bedroom vs a 400 sq ft living room.
Air Purifier Performance Metrics
CADR and ACH describe the same physical process from two different perspectives. CADR tells you what the purifier produces. ACH tells you what your room receives from it. Neither is more important than the other — they answer different questions and you need both to make a correct buying decision.
The confusion arises because manufacturers use CADR for marketing (it is a single impressive number) while health guidelines use ACH (it relates directly to symptom reduction and clean air delivery timelines). A buyer who only knows CADR does not know if their purifier is adequate for their room. A buyer who only knows a target ACH does not know which purifier achieves it. The conversion between them is simple, and understanding it makes every other air purifier specification easier to interpret.
The relationship: CADR × 60 ÷ room volume (cubic feet) = ACH. For a standard 8-foot ceiling, room volume in cubic feet = square footage × 8. A Coway AP-1512HH with CADR 246 in a 250 sq ft room with 8-foot ceilings delivers (246 × 60) ÷ (250 × 8) = 14,760 ÷ 2,000 = 7.4 ACH. That is the complete picture: one number from the purifier, one number from the room, one result that tells you whether the combination is adequate for your health goal.
What CADR Measures and What It Cannot Tell You
CADR is a property of the purifier, not the room. It measures the volume of air — in cubic feet per minute — that the purifier cleans of particles and delivers per minute at maximum fan speed. The AHAM testing protocol uses a standardized 1,008 cubic foot sealed chamber. The result is a single number: this purifier, at this speed, with a new filter, delivers X cubic feet of clean air per minute.
What CADR tells you: how fast this purifier cleans air. A CADR 246 unit cleans air twice as fast as a CADR 123 unit, regardless of room size. When comparing two purifiers, the one with higher CADR always produces more clean air per unit of time. CADR is the purest comparative metric for particle removal performance.
What CADR cannot tell you: whether a specific purifier is adequate for a specific room. A CADR 300 unit in a 600 sq ft room with 9-foot ceilings delivers 3.3 ACH. The same unit in a 150 sq ft bedroom delivers 13.3 ACH. The purifier is identical. Its adequacy — whether it is meeting your air quality target — depends entirely on the room.
CADR is also measured on a new filter at maximum speed. Real-world CADR is lower: filters accumulate particles and airflow resistance increases over time. Auto mode typically runs below maximum speed. A purifier in a large room on auto mode with a six-month-old filter may deliver 50–60% of its rated CADR. This is why CADR ratings should be treated as a ceiling, not a constant, and why the ACH calculation based on rated CADR gives you the theoretical maximum — not the continuous average.
What ACH Measures and Why It Connects to Health Outcomes
ACH is a derived metric. You cannot look it up on a product listing. You calculate it: (CADR × 60) ÷ (room square footage × ceiling height in feet). For standard 8-foot ceilings, this simplifies to (CADR × 60) ÷ (sq ft × 8). The result tells you how many times per hour the purifier theoretically replaces all the air in that specific room with clean air.
ACH connects directly to health guidance in a way that CADR alone does not. The AHAM baseline recommendation is 2 ACH for general air quality improvement — this is what manufacturer room coverage claims are based on. EPA guidance for allergy and asthma relief suggests 4–5 ACH for measurable symptom reduction. During wildfire events with continuous high-PM2.5 infiltration, more ACH is always better — the goal shifts from maintenance to remediation.
Why ACH predicts health outcomes: at 2 ACH, the entire room air volume is cleaned twice per hour, meaning the average particle spends 30 minutes in the room before being captured. At 5 ACH, the average particle spends 12 minutes airborne. For fine particles like PM2.5 and wildfire smoke that remain suspended for hours without active removal, this difference is clinically meaningful — 5 ACH removes particles fast enough to stay ahead of normal particle generation rates in an occupied room.
ACH has one important limitation: it assumes perfect mixing. In a real room, the purifier draws air from nearby and delivers clean air to one zone. The far corner of a room may receive less than the average ACH if airflow does not circulate adequately. This is why placement matters — a purifier positioned centrally or in the main air circulation path delivers more uniform cleaning than one tucked in a corner. For large or irregularly shaped rooms, two smaller units with better placement coverage often outperform one large unit despite equal total CADR.
The CADR-to-ACH Conversion — Real Examples
The formula is straightforward. The practice of applying it to your specific room before buying is what separates buyers who get the right purifier from those who get one that is too small or unnecessarily large.
Small bedroom (120 sq ft, 8 ft ceiling = 960 cubic feet):
Levoit Core 300 (CADR 141): (141 × 60) ÷ 960 = 8,460 ÷ 960 = 8.8 ACH. Excellent. Far exceeds the 4–5 ACH allergy threshold. For a 120 sq ft room, the Core 300 is the right-sized purifier and you are not paying for excess capacity.
Coway AP-1512HH (CADR 246): (246 × 60) ÷ 960 = 15.4 ACH. This is overkill for a 120 sq ft room. You would be paying for CADR your room does not need. The Core 300 at $100 is the rational choice here, not the Coway at $120.
Standard bedroom (220 sq ft, 8 ft ceiling = 1,760 cubic feet):
Levoit Core 300 (CADR 141): (141 × 60) ÷ 1,760 = 4.8 ACH. Just meets the allergy threshold. Acceptable for mild allergies; borderline for moderate.
Coway AP-1512HH (CADR 246): (246 × 60) ÷ 1,760 = 8.4 ACH. Comfortably above the allergy threshold. The Coway is the right choice here for anyone with allergy or asthma concerns.
Large living room (400 sq ft, 9 ft ceiling = 3,600 cubic feet):
Coway AP-1512HH (CADR 246): (246 × 60) ÷ 3,600 = 4.1 ACH. Meets the allergy threshold but just barely. The extra ceiling height costs you significantly — the same unit delivers 8.4 ACH in the 220 sq ft bedroom and 4.1 ACH here.
Blueair 211i Max (CADR 410): (410 × 60) ÷ 3,600 = 6.8 ACH. Solid performance for the large room. The appropriate tool for this space if allergy or asthma relief is the goal.
Two Coway AP-1512HH units combined CADR 492: (492 × 60) ÷ 3,600 = 8.2 ACH. Better ACH than the Blueair at comparable combined cost — and the flexibility to place one on each side of the room for more uniform coverage.
The recurring insight: the same purifier is the right choice in some rooms and the wrong choice in others. The CADR-to-ACH conversion is the tool that answers the question "right or wrong for my room" before you buy rather than after.
When to Use CADR and When to Use ACH
The two metrics answer different questions and are most useful at different stages of the buying decision.
Use CADR when comparing purifiers: Two purifiers side by side — which cleans air faster? CADR answers this directly. CADR 410 is objectively better than CADR 246 at cleaning a given volume of air. CADR is the cross-brand comparison metric. It works because AHAM-verified CADR numbers are produced by the same test protocol — a Coway CADR 246 and a Blueair CADR 410 were tested under identical conditions, so the comparison is valid.
Use ACH when sizing a purifier to a room: Is this specific purifier adequate for my specific room and health goal? ACH answers this. Calculate ACH for your room with each purifier under consideration. Compare against your target: 2 ACH (general), 4–5 ACH (allergy/asthma), maximum (wildfire/smoke). This prevents both under-buying (purifier too small for room) and over-buying (paying for CADR your room cannot use).
When manufacturer room coverage claims mislead: "Covers up to 360 sq ft" means 2 ACH at 360 sq ft. For allergy relief, the effective coverage is approximately 180 sq ft (4 ACH). For smoke events, effective coverage is approximately 120 sq ft (6 ACH). Room coverage numbers on the box always reflect the most favorable interpretation. Convert to ACH for your actual room before trusting them.
Noise vs ACH trade-off: Maximum speed delivers maximum CADR and therefore maximum ACH. Most purifiers are significantly louder at high speed. In a bedroom at night, most people run at medium speed, which reduces CADR by 30–50% and ACH proportionally. If noise matters (bedroom, home office), target a purifier that delivers your required ACH at medium speed — not just at maximum. Check dB ratings at medium speed, not just the minimum speed they advertise.
ACH Targets by Use Case — A Reference Guide
Rather than a single ACH number, different situations call for different targets. Here is the practical reference guide for applying ACH to real buying decisions.
2 ACH — General air quality maintenance: AHAM baseline. Adequate for homes without specific health concerns, where the goal is reducing dust accumulation and maintaining fresher indoor air. This is what room coverage claims on the box promise. Acceptable for healthy adults in moderate-pollution environments.
4–5 ACH — Allergy and asthma relief: The clinical threshold referenced in EPA and allergist guidance. At 4 ACH, measurable reductions in airborne pollen, dust mite fragments, pet dander and mold spores. At 5 ACH, consistent symptom relief reported in peer-reviewed residential studies. This should be the minimum target for anyone buying an air purifier specifically for respiratory health reasons.
6+ ACH — Wildfire smoke, heavy pollution events, infants: During AQI 150+ wildfire conditions, 6 ACH is the recommended floor. For nurseries with newborns where PM2.5 exposure is a priority concern, 6+ ACH provides a meaningful safety margin above the clinical threshold. A purifier delivering 6 ACH replaces room air every 10 minutes — particles generated by infiltration or occupant activity are removed faster than they accumulate under most conditions.
8+ ACH — Medical or research-grade environments: Hospital isolation rooms, clean rooms, environments requiring near-elimination of airborne particulates. Achievable residentially only with very high CADR units in small rooms, or multiple units. Most households do not need to target this range for everyday air quality — it is listed here to frame the scale and show where residential high-performance units sit relative to medical standards.
The practical takeaway: most buyers should target 4–5 ACH for their primary bedroom and main living area, and accept 2–3 ACH for secondary rooms used less frequently. Calculate for your specific room dimensions before buying. The CADR required for 4 ACH in a 250 sq ft room is 133 — an affordable entry-level purifier. The CADR required for 4 ACH in a 500 sq ft living room is 267 — a mid-range purifier. Room size drives the budget requirement more than the health target does.
How This Changes Your Buying Decision
CADR and ACH together answer the only question that matters: is this purifier adequate for this room and this health goal?
- Always calculate ACH for your specific room before buying. Manufacturer room coverage claims are based on 2 ACH — the minimum AHAM standard, adequate for general freshening but below the clinical threshold for allergy relief. For allergy or asthma: divide your room square footage by 0.78 (4 ACH target) to find the CADR you actually need. For a 300 sq ft bedroom and allergy relief, you need CADR 385 — not the CADR 200 a "300 sq ft coverage" claim implies.
- Use CADR to compare purifiers; use ACH to size them to your room. When comparing two purifiers, CADR is the objective metric — higher CADR always means faster cleaning. When deciding if a specific purifier is right for your specific room, calculate ACH. The same CADR delivers very different ACH in a 120 sq ft bedroom vs a 400 sq ft open-plan living area.
- During wildfire or smoke events, target the highest ACH your purifier can deliver. Set to maximum fan speed, override auto mode. Your CADR-rated output is the ceiling; auto mode often operates below it. Every ACH point matters when outdoor PM2.5 is infiltrating continuously.
- For multiple rooms, calculate per room. A single high-CADR purifier in one room does not raise ACH in adjacent rooms. Two medium-CADR purifiers positioned correctly outperform one large unit in most multi-room scenarios.
Where to go next: CADR explained in full — AHAM testing methodology and calculation tables. ACH guide — clinical thresholds and room-by-room targets. Best purifiers ranked by verified CADR.