| Quick Facts | |
|---|---|
| AHAM two-thirds rule | smoke CADR >= 67% of room area (sq ft) |
| EPA rule of thumb | ~65 CADR per 100 sq ft |
| AHAM worked example | 120 sq ft room needs >=80 smoke CADR |
| Ceiling height assumption | 8 ft (both AHAM and EPA) |
| Wildfire smoke sizing | smoke CADR = full room sq ft (AHAM) |
| Metric conversion | 1 sq meter = 10.76 sq ft |
What CADR actually measures
CADR is AHAM's measure of how many cubic feet of filtered air a purifier delivers per minute, scored separately for smoke, dust, and pollen; the smoke number is the one that matters for room sizing.
Shopping for a specific model? See top-rated air purifier models — 10 models compared on the specs that decide it.
CADR stands for Clean Air Delivery Rate, and it tells you how fast a purifier can keep up with a room, not how good the filter media is. AHAM, the Association of Home Appliance Manufacturers, tests certified units and issues three separate scores, one each for tobacco smoke, dust, and pollen, because those particle sizes behave differently in air. The smoke score is the one worth memorizing: smoke particles are the smallest of the three, so a purifier's smoke CADR is effectively its worst-case processing speed, and that is the figure sizing charts are built around.
It is easy to confuse CADR with filter grade, but they measure different things. A HEPA filter's rating tells you what fraction of particles it traps on a single pass through the media. CADR tells you how much air actually gets pushed through that filter per minute. A purifier can carry an excellent filter and still be undersized for a room if the fan and motor cannot move enough air, which is exactly how buyers end up running a unit on its loudest setting just to keep pace, when a properly sized model could do the same job on low.
What size air purifier do I need? The room-size math, two ways
AHAM's rule says smoke CADR should equal at least two-thirds of the room's square footage; the EPA's version lands close, at roughly 65 CADR per 100 square feet, and both point to nearly the same purifier.
Two of the standards bodies that actually test this stuff agree closely, which is reassuring given how much marketing copy on purifier boxes tends to round in the seller's favor. AHAM's published guidance is a two-thirds rule: a purifier's smoke CADR should equal at least two-thirds of the room's area in square feet. AHAM's own worked example uses a 10-by-12-foot room, 120 square feet total, which needs a smoke CADR of at least 80.
The EPA's Guide to Air Cleaners in the Home lands on nearly the same target through a different route: roughly 65 CADR for every 100 square feet of floor space. Run the numbers for a 300-square-foot living room and AHAM's rule wants about 200 CADR while the EPA's math works out closer to 195, close enough that either formula gets you to a sensible size. Start by multiplying the room's length and width, in feet, to get square footage; that is the number both formulas need, and it is worth doing before trusting a purifier listing's own 'room size' claim, since brands are not always conservative about how they define it.
Sizing up for a small room
In a small room, a bedroom or home office under roughly 150 to 200 square feet, even an entry-level purifier usually clears the CADR bar, so the more useful question becomes noise at night rather than raw capacity.
A small room, a bedroom, nursery, or home office under about 150 to 200 square feet, clears the CADR math with almost any certified purifier on the market, entry-level models included. The math stops being the limiting factor here; comfort does. Run the two-thirds rule on a 150-square-foot bedroom and you land on a smoke CADR requirement of roughly 100, a threshold most compact purifiers clear even on their lowest fan speed.
That headroom is worth using. Buying a unit rated well above what the room technically needs means you can run it on a quieter, lower setting and still hit the CADR your space calls for, useful in a bedroom, where the alternative is a purifier stuck on high just to keep up, humming through the night. Search air purifier small room and most listings you find will already clear this bar; the real filtering happens in fan speed and noise specs, not CADR headroom. Our room air purifier picks span a wide range of CADR tiers, which makes it easier to find one that is comfortably oversized for a small room rather than exactly matched to it.
Sizing up for a large room or open floor plan
A large room or open floor plan, anything past roughly 400 square feet, needs either a single high-CADR purifier built for that footprint or two smaller units placed at opposite ends of the space, since CADR assumes a single well-mixed room rather than a hallway-connected floor plan.
A large room changes the math without changing the formula. You still multiply length by width and hold the purifier to two-thirds of that area's smoke CADR, but the number climbs fast: a 500-square-foot open living and dining space needs a smoke CADR near 333 by AHAM's rule, well outside what compact bedroom units are built for and enough to push you toward purifiers designed specifically for large rooms. Type air purifier large room into a search bar and most of what comes back is manufacturer marketing repeating a square-footage claim from the box; run that claim through the two-thirds math above before trusting it.
Open floor plans complicate things further. AHAM's and the EPA's formulas both assume a single, reasonably enclosed room where air can circulate and mix; a real open floor plan, with a kitchen bleeding into a living room bleeding into an entry hall, does not behave that way. Air near the unit gets scrubbed quickly, while air in a far corner behind a sofa or around a doorway barely moves. For a genuinely open air purifier large space, the more reliable fix is either one purifier rated well above the combined square footage, or two mid-sized units placed at opposite ends of the layout so no part of the space is starved of airflow. Our large-room air purifier guide is built around exactly that math, comparing units rated for the higher square footage this kind of space demands.
Ceiling height and other adjustments that change the number
Both AHAM and the EPA build their formulas around an 8-foot ceiling, so a taller room, an open stairwell, or a vaulted ceiling all mean sizing up a class, and anyone measuring in square meters needs to convert before applying either rule.
Both formulas assume an 8-foot ceiling, standard in most US homes but not universal, and neither AHAM nor the EPA publishes a precise multiplier for going taller; both simply say to size up when ceilings run higher than 8 feet, since a taller room holds more air for the same footprint. A vaulted ceiling, an open stairwell connected to the room, or a loft-style space all count; treat the CADR you calculated as a floor, not a target, in any of those layouts.
Room measurements also trip people up when the source material uses different units. AHAM and the EPA both publish their guidance in square feet, but plenty of purifiers sold outside the US, and a fair number of buyers, work in square meters instead. One square meter is roughly 10.76 square feet, so a 20-square-meter room works out to about 215 square feet before applying the two-thirds rule. Shoppers working from metric room sizes can skip the conversion step: our UK air purifier guide lists coverage in square meters directly.
When to round up regardless of the math
Wildfire smoke, heavy pet shedding, and known allergy or asthma triggers are all good reasons to buy above the calculated CADR; AHAM specifically recommends matching smoke CADR to the room's full square footage, not two-thirds of it, during wildfire smoke events.
The two-thirds rule is a baseline, not a ceiling, and AHAM carves out an exception worth remembering: for wildfire smoke specifically, the recommended smoke CADR is the room's full square footage, not two-thirds of it. That is a meaningfully bigger purifier than the everyday formula suggests, reflecting how much denser wildfire smoke gets compared with routine cooking or dust particles.
Households dealing with regular smoke events, Canadian and western US regions with recurring wildfire seasons are the obvious case, are better served sizing to that higher bar from the start rather than upgrading mid-crisis when the same models are harder to find in stock. The same logic applies loosely to households managing asthma or heavy allergies, where erring toward a bigger CADR than the strict two-thirds math requires buys some margin on bad air days. Our HEPA options for Canadian homes are sized with exactly that kind of buffer in mind, rather than the bare minimum the standard formula returns.