CADR vs ACH: The Two Numbers That Tell You If a Purifier Is Big Enough

Two numbers appear on every air purifier listing, and most buyers only understand one of them. CADR gets all the attention — it’s the big number on the spec sheet. ACH barely gets mentioned, and when it is, it’s usually in the fine print of a coverage claim that’s been massaged to look as impressive as possible.

Here’s the thing: neither number means anything without the other. CADR tells you what the machine can do. ACH tells you whether it’s big enough for your room. This guide explains both, shows you the math, and gives you the honest rule of thumb — plus the tool that does the calculation so you don’t have to.

CADR: The Machine’s Output

Measuring a bedroom with a tape measure

CADR (Clean Air Delivery Rate) measures how many cubic feet of clean air a purifier delivers per minute, tested under AHAM standards (the industry’s independent testing protocol). It’s reported as three numbers — smoke, dust, and pollen CADR — because filters perform differently on different particle sizes. The smoke CADR is the number to compare: smoke particles (0.1-1 micron) are the hardest to filter, so smoke CADR is the most conservative spec.

Examples of what CADR means in practice:

Purifier	Smoke CADR	What it means
Levoit Core 300	135 CFM	Filters 135 cu ft of air per minute
Coway AP-1512HH	246 CFM	Filters 246 cu ft per minute
Levoit Core 600S	400 CFM	Filters 400 cu ft per minute
Medify MA-112	490 CFM	Filters 490 cu ft per minute

The key thing to understand: CADR is fixed. It doesn’t change with your room. A 246 CFM purifier produces 246 CFM whether it’s in a closet or a warehouse. What changes is the effect — and that’s where ACH comes in.

ACH: The Room-Specific Result

ACH (Air Changes per Hour) is how many times the purifier cycles the entire air volume of your room in an hour. It’s not printed on the box — it’s a calculation you do with your room’s dimensions:

ACH = (CADR × 60) ÷ Room Volume

Where room volume = length × width × ceiling height (in feet).

Example: a 12 × 12 ft bedroom with 8 ft ceilings = 1,152 cubic feet. With a Coway AP-1512HH (246 CFM):

ACH = (246 × 60) ÷ 1,152 = 12.8 ACH

That’s excellent. The same purifier in a 500 sq ft open-plan living area (4,000 cu ft):

ACH = (246 × 60) ÷ 4,000 = 3.7 ACH

That’s below the recommended minimum. Same machine, same output — completely different result, because the room changed.

Why 4.8 ACH Is the Benchmark

The air purifier industry (via AHAM) recommends a minimum of 4.8 ACH for a room — meaning the room’s air should be fully cycled about once every 12.5 minutes. At this rate:

Particle concentration drops substantially within the first hour of operation
The purifier can keep pace with normal particle generation (people, pets, cooking, dust resuspension)
The room reaches a meaningful equilibrium below the concentration you’d have without filtration

Below 4.8 ACH, the purifier is technically filtering, but it can’t keep up — you’re running a fan with a filter in it, not actually cleaning the room.

For specific situations, aim higher:

Situation	Recommended ACH
General use (AHAM minimum)	4.8
Allergies in the bedroom	6-8
Smoke or strong odors	8-12
During active dust generation (renovation)	12+

The Honest Sizing Rule

Here’s the practical shortcut that gets you 90% of the way without math:

Multiply your room’s square footage by 0.35-0.4. That’s the minimum smoke CADR (CFM) you need for 4.8 ACH with 8-foot ceilings.

150 sq ft bedroom → ~60 CFM minimum (but realistically 150-200 CFM for quiet low-speed operation)
300 sq ft studio → ~120 CFM minimum, realistically 250-300 CFM
600 sq ft open area → ~240 CFM minimum, realistically 400 CFM

Why “realistically” higher? Because purifiers run quieter and longer-lived on low and medium speeds — if the minimum size only works at max speed, you’ll either run it loud or not run it enough. The rule I give people: size for 4.8 ACH at the speed you’ll actually run it. If you’ll run it on medium, size for medium.

The Two Numbers in Practice (How Manufacturers Stretch Them)

Now that you know the math, here’s how to read listings skeptically:

“Covers 1,560 sq ft” on a 356 CFM purifier — that’s at 2 ACH, not the recommended 4.8. At the recommended rate, it covers about 550 sq ft. Most coverage claims use the lowest defensible ACH.
“For rooms up to 200 sq ft” on a 135 CFM unit — that’s 4.3 ACH at 8-ft ceilings, just under the recommendation. Fine for small bedrooms, but the claim is doing its best.
CADR without a third-party test — AHAM verification is the standard; “equivalent” or self-reported CADR isn’t comparable. If a listing can’t name the AHAM test, treat the number as marketing.

The Tool That Does the Math

Rather than doing the arithmetic in your head for every room and purifier you’re considering, we built a tool for exactly this: our Room Size to CADR Calculator converts your room dimensions into the minimum CADR you need, and our Purifier Finder lets you filter our product database by the CFM you actually need — so you can see, instantly, which units hit 4.8 ACH in your room at the speed you’ll actually use.

The One-Line Summary

CADR is the machine’s power; ACH is the match to your room. Size for 4.8 ACH at the speed you’ll run it — and remember that odor removal (carbon) is a separate spec entirely, because CADR says nothing about smells.

The Numbers in Practice: Real Rooms, Real Purifiers

Let’s make the math concrete with the units we actually recommend, in the rooms people actually have:

Room	Dimensions	Volume	Unit (CADR)	ACH at high	ACH at medium
Small bedroom	10×10×8 = 800 cu ft	800	Levoit Core 300 (135)	10.1	~6
Standard bedroom	12×12×8 = 1,152 cu ft	1,152	Coway AP-1512HH (246)	12.8	~8
Master bedroom	16×14×9 = 2,016 cu ft	2,016	Levoit Core 600S (400)	11.9	~7
Studio/living	20×15×9 = 2,700 cu ft	2,700	Coway Airmega 400 (356)	7.9	~5
Open loft	30×20×10 = 6,000 cu ft	6,000	Medify MA-112 (490)	4.9	~3.5

Notice what happens at the bottom of the table: the Medify — the highest-CADR unit we recommend — barely hits the 4.8 ACH minimum in a 6,000 cu ft loft. That’s not a flaw in the unit; it’s the reality that large rooms need either very high output or (better) two units. If your space is bigger than ~600 sq ft with standard ceilings, the honest answer is usually “two properly-sized units” rather than “one enormous unit” — two units give you 2x the redundancy, similar total cost, and better air movement.

Why the “Smoke CADR” Is the Number to Compare

AHAM tests CADR three ways: smoke, dust, and pollen. Most listings quote the dust or pollen number because they’re higher — dust CADR runs about 10-20% above smoke on the same unit, and pollen higher still. Smoke particles (0.1-1 micron, the byproducts of combustion) are the hardest size class to filter, so smoke CADR is the most conservative and comparable spec.

When a listing says “CADR 300” without specifying which test, it’s almost certainly the dust or pollen figure. When a listing is AHAM-verified and quotes the smoke CADR, you can trust cross-brand comparisons. The difference matters in practice: a unit that claims “300 CADR” but has a 220 smoke CADR is, on the number that matters, 25% smaller than advertised.

FAQ

What’s the difference between CADR and ACH? CADR (Clean Air Delivery Rate) measures how much clean air a purifier produces per minute — it’s the purifier’s output spec (e.g., 250 CFM). ACH (Air Changes per Hour) measures how many times the purifier cycles the room’s entire air volume per hour — it’s the result of matching the purifier to a room. The industry recommendation is 4.8 ACH or better.

How do I calculate ACH for my room? ACH = (CADR in CFM × 60) ÷ room volume in cubic feet. For a 12x12x8 ft room (1,152 cu ft) with a 250 CFM purifier: (250 × 60) ÷ 1,152 = 13 ACH. The minimum recommended is 4.8 ACH; for allergy or smoke situations, aim for 6-8 ACH or more.

What’s a good CADR for a bedroom? For a typical 150-200 sq ft bedroom, you want a smoke CADR of at least 150-200 CFM (which gives you 4.8+ ACH at 8 ft ceilings). For larger rooms, size up accordingly: 300-400 CFM for 400-600 sq ft spaces. Use our room size calculator to get the exact number for your room.

Why is 4.8 ACH the magic number? 4.8 ACH (one air change every 12.5 minutes) is the AHAM-recommended minimum for air purifiers — at this rate, the purifier substantially reduces particle concentration within an hour. It’s not a health standard, it’s a practical benchmark: below 4.8, the purifier can’t keep up with normal particle generation.

Can a purifier have high CADR but low ACH? Yes — if the room is too large for the unit. A 250 CFM purifier in a 500 sq ft open room gives you only about 2.5 ACH, which is below the recommendation. That’s why sizing (CADR vs room volume) matters more than raw CADR: the best purifier is the right size for the room.

Does CADR apply to smells or only particles? CADR only measures particle filtration (smoke, dust, pollen) — it says nothing about odor removal, which is handled by activated carbon. Two purifiers with identical CADR can have wildly different odor performance based on carbon weight. Check both numbers: CADR for particles, carbon weight for smells.