Air Purifier CADR, Explained: Why the Room Size on the Box Is Only Half the Number

CADR — the volume of filtered air a purifier delivers each minute — is the one honest number on the box, but the room size printed beside it hides an assumption about how many times an hour the air gets cleaned, and the same machine can honestly be sold as a 390-square-foot purifier or a 1,875-square-foot one.

An air purifier's room size is only half the number because a square-footage claim is meaningless until you know how many times an hour the machine is assumed to clean the air in that room. The figure that carries the real information is CADR — Clean Air Delivery Rate, measured in cubic feet per minute (CFM) — and the sizing rule behind it comes from the Association of Home Appliance Manufacturers (AHAM): choose a purifier whose tobacco-smoke CADR is at least two-thirds of the room's floor area in square feet. A 300-square-foot room therefore needs a smoke CADR of at least 200. Everything else on the box — the headline room size, the coverage claim, the "cleans up to" figure — is that one ratio, restated with friendlier assumptions.

What CADR actually measures

CADR is not airflow. A fan can move a great deal of air and clean very little of it. According to AHAM, CADR indicates the volume of filtered air an air cleaner delivers — airflow multiplied, in effect, by how much of what passes through the filter actually gets caught — with separate scores for tobacco smoke, dust, and pollen. The three numbers differ because the particles differ: smoke particles are the smallest of the three test pollutants and the hardest to capture, pollen grains the largest and easiest, so a machine's smoke CADR is normally its lowest score and its most honest one. When you size a purifier, size it by smoke.

The numbers are trustworthy only when someone other than the manufacturer measured them. AHAM's Verifide program tests portable air cleaners in independent laboratories against the ANSI/AHAM AC-1 standard and publishes the results in a public directory; AHAM advises shoppers to look for the Verifide mark on the packaging. A CADR printed by a brand that has never submitted to that testing is a claim, not a measurement. Our air purifier ranking treats the distinction the same way: verified CADR first, manufacturer claims labeled as such.

The two-thirds rule, and the arithmetic inside it

AHAM states the rule plainly: as a rule of thumb, the CADR of your air cleaner should be equal to at least two-thirds of the room's area — its own worked example is a 10-by-12-foot room, 120 square feet, which calls for a smoke CADR of at least 80. The U.S. Environmental Protection Agency's Guide to Air Cleaners in the Home publishes a sizing table that lands in nearly the same place: a minimum CADR of 65 CFM for a 100-square-foot room, 195 for 300 square feet, and 390 for 600 square feet, all assuming an 8-foot ceiling. ENERGY STAR repeats the same table for shoppers. Three authorities, one ratio.

The ratio is not arbitrary. It is built to deliver about 4.8 air changes per hour: take the room's volume (floor area times an 8-foot ceiling), multiply by 4.8, divide by 60 minutes, and the two-thirds figure falls out. That pace — the room's entire volume of air passing through the filter roughly every twelve and a half minutes — is what turns a purifier from a pleasant fan into a device that can keep up with a continuous source of particles: a cat that lives in the room, pollen drifting in all day, a wood stove across the hall. AHAM adds one escalation: for wildfire smoke, choose a smoke CADR equal to the room's full square footage, not two-thirds of it.

Why the room size on the box is only half the number

Now run the arithmetic backwards, the way a marketing department does. Take one machine with a smoke CADR of 250 CFM. At AHAM's 4.8 air changes per hour in an 8-foot-ceiling room, it serves about 390 square feet. Assume only 2 air changes per hour — still a defensible-sounding figure — and the identical machine "covers" about 940 square feet. Assume 1 air change per hour and it covers roughly 1,875 square feet. Nothing about the purifier changed; only the assumption did, and the assumption is the part of the claim that is printed in the smallest type, or not printed at all. A room-size claim without an air-changes figure beside it is half a number.

The second hidden assumption is the fan speed. CADR is measured with the machine running at its highest speed — the speed at which, as the National Institute of Environmental Health Sciences notes, the rating will not reflect what the purifier does on a low setting. In a living room at midday, turbo may be fine. In the bedroom where the purifier actually earns its keep, you will run it at the quiet speed you can sleep beside, and at that speed it delivers a fraction of its rated CADR. This is the practical case for oversizing: a machine rated well above your room's minimum can hit the two-thirds target while running slowly and quietly, where a machine sized exactly to the minimum only reaches it at full roar.

Ceiling height is the third quiet variable. Every standard table assumes 8 feet. A 300-square-foot room with a 12-foot vaulted ceiling holds half again as much air, and needs roughly half again as much CADR; AHAM says directly that rooms with ceilings higher than 8 feet need a cleaner rated for a larger room.

What CADR does not measure

CADR is a particle score. It says nothing about gases, odors, or volatile organic compounds — the cooking smells, paint fumes, and formaldehyde that activated carbon handles, slowly, in proportion to how much carbon the machine carries. Two purifiers with identical CADR scores can differ enormously as odor removers, and no number on the box will tell you. The EPA also cautions against air cleaners that intentionally generate ozone, a lung irritant; a particle score is never a reason to accept one.

CADR is also a first-day score. It is measured with a clean filter, and it decays as the filter loads. AHAM warns that off-brand replacement filters may fail to match the original's performance — reducing airflow and letting particles back into circulation — and now runs a Verifide program for replacement filters for exactly that reason. The honest cost of a purifier is therefore the machine plus a genuine filter every six to twelve months, every year you own it; in our air purifier ranking that annual filter cost is counted in every score, the standard our methodology sets for the category. ENERGY STAR certification, finally, certifies energy efficiency only. It is a useful check on the electricity bill and says nothing at all about how much air a machine cleans.

How to size one for a real room

Measure the room, multiply the square footage by two-thirds, and treat the result as the minimum smoke CADR — then adjust honestly: multiply by 1.25 for a 10-foot ceiling, go to the full square footage for wildfire smoke or a serious allergy, and step up one size if the machine will live in a bedroom, so it can do its work at a speed you can sleep through. Check the figure in AHAM's directory rather than on the carton, and price the genuine replacement filter before you price the machine.

Place it in the room where the person and the problem actually are, with the door closed if you can; a portable purifier cleans the room it sits in, and output falls off sharply through a doorway. One correctly sized unit per bedroom plus one larger unit for the main living space beats a single flagship machine asked to clean a house. And remember what a purifier cannot do: it cannot remove the dust already settled into carpet and upholstery, which is why the floor half of the problem belongs to regular vacuuming — our robot vacuum ranking covers the machines that do that job daily. CADR first, room-size claims last, and the arithmetic takes about a minute.

Sources

• Association of Home Appliance Manufacturers (AHAM Verifide) — Air Filtration Standards (CADR defined as the volume of filtered air delivered, with separate scores for tobacco smoke, pollen and dust; the 2/3 rule and its 120 sq ft → CADR 80 example; the wildfire-smoke escalation to a CADR equal to the room's area; higher ceilings need a larger-room rating; the caution on non-genuine replacement filters): https://ahamverifide.org/ahams-air-filtration-standards/
• U.S. Environmental Protection Agency — Guide to Air Cleaners in the Home (portable air cleaner sizing table: minimum CADR 65 CFM at 100 sq ft, 195 at 300 sq ft, 390 at 600 sq ft, at an 8-foot ceiling): https://www.epa.gov/indoor-air-quality-iaq/guide-air-cleaners-home
• ENERGY STAR (U.S. EPA / U.S. Department of Energy) — Air Cleaners (the same EPA sizing chart; CADR as the measure of how quickly a unit delivers filtered air; higher ceilings call for a higher CADR): https://www.energystar.gov/products/air_cleaners
• U.S. Environmental Protection Agency — What Is a HEPA Filter? (HEPA, as defined by the U.S. Department of Energy, removes at least 99.97% of particles at 0.3 microns; 0.3 microns is the most penetrating particle size — the worst case — and larger or smaller particles are trapped with even higher efficiency): https://19january2021snapshot.epa.gov/indoor-air-quality-iaq/what-hepa-filter-1_.html
• ENERGY STAR — Room Air Cleaners Key Product Criteria (CADR measured under the ANSI/AHAM AC-1 standard and the U.S. Department of Energy test method; ENERGY STAR certifies energy efficiency, with ozone limits for emitting models): https://www.energystar.gov/products/air_purifiers_cleaners/key_product_criteria