Air Purifier CADR Calculator

Clean Air Delivery Rate is the Association of Home Appliance Manufacturers’ answer to a problem that used to make air purifiers impossible to compare: a manufacturer could advertise a high airflow with a poor filter, or a superb filter with a fan too weak to move meaningful air through it, and both could claim to clean a room.

CADR collapses the two into one number by measuring the outcome rather than the components. A unit runs in a sealed 1,008 cubic foot chamber, a measured quantity of pollutant is introduced, and the rate of decay is compared against natural settling with the machine off. The difference, expressed as cubic feet per minute, is the volume of perfectly clean air the machine is effectively producing. A CADR of 200 means the unit does whatever 200 cubic feet a minute of outdoor-clean air would do.

AHAM publishes three figures, for tobacco smoke, dust and pollen, because particle size changes how easily a filter catches it. Smoke particles are the smallest and hardest, so the smoke CADR is always the lowest of the three and the most conservative basis for comparison. A unit displaying the AHAM Verifide seal has been independently tested; a number quoted without that seal is a manufacturer’s claim.

CADR describes the machine. Air changes per hour describes what happens in your room, and it is the one that predicts whether you notice anything.

The arithmetic is simple: multiply CADR by 60 to get cubic feet an hour, and divide by the room’s volume. A 200 cfm unit in a 2,400 cubic foot bedroom gives 5 air changes an hour. The same unit in an open-plan living space of 8,000 cubic feet gives 1.5, and will feel like it is doing nothing.

The thresholds worth knowing: around 2 ACH is enough to clear cooking smells and general staleness. Four to five is the range most indoor air quality guidance and most allergists settle on, and it is where people with hay fever or a cat generally start reporting a real difference. Six or more is for actively generated fine particulate — wildfire smoke infiltrating from outside, or a household with asthma. Above eight you are into the territory hospitals use for airborne infection isolation, and reaching it quietly in a domestic room is difficult.

This is also where the manufacturer’s "recommended room size" on the box goes wrong. That figure is very commonly calculated at 2 ACH, which is a defensible general-purpose rate and roughly half what someone buying a purifier for allergies actually wants. A machine sold as covering 900 square feet may genuinely cover 900 square feet at 2 ACH, and only 360 at five.

The rule of thumb promoted by AHAM and repeated widely is that a purifier’s CADR should be at least two-thirds of the room’s floor area in square feet. A 300 square foot room wants a CADR of at least 200.

It is a good rule, and it is worth understanding that it is not a separate piece of advice from the air-change target — it is the same statement in different units. Two-thirds of the floor area, multiplied by 60 minutes and divided by an eight-foot ceiling, comes to exactly 5 air changes an hour. The rule is a 5 ACH recommendation with the ceiling height baked in.

Which is precisely where it fails. In a room with a ten-foot ceiling the rule delivers 4 ACH, and in a vaulted living space with a sixteen-foot peak it delivers 2.5 — while appearing to give the same answer. Because the failure is silent and the arithmetic looks unchanged, this is the single most common way people end up with an undersized purifier in exactly the kind of large open room where they most wanted one.

Air cleaners are effective against a narrower set of problems than their marketing suggests, and being clear about the boundary saves money.

Some air cleaners work by generating ozone, or produce it as a by-product — this includes ozone generators sold explicitly as such, and some ionisers, electrostatic precipitators and "plasma" devices.

Ozone is a lung irritant. The EPA is unambiguous that ozone generators sold as air cleaners are not effective at concentrations that comply with public health standards, and that at concentrations high enough to affect airborne contaminants they are hazardous to breathe. California went further and prohibits the sale of indoor air cleaners emitting more than 0.05 parts per million of ozone, which is why reputable manufacturers now publish CARB certification.

The practical guidance is simple: a mechanical filter — HEPA for particles, activated carbon for gases — has no such trade-off. It removes contaminants by trapping them rather than by reacting with them, and it produces nothing. If a device claims to clean air by emitting something, that is a reason for caution rather than a feature. Check for CARB certification, and be sceptical of anything marketed on "active" technology.

One thing worth knowing before spending several hundred dollars: a box fan taped to one or more MERV-13 furnace filters — the arrangement known as a Corsi-Rosenthal box after the two engineers who popularised it during the pandemic — achieves a CADR comparable to commercial units costing many times more.

The reason it works is that CADR is airflow times single-pass efficiency, and the design trades efficiency for area. A MERV-13 filter is less efficient per pass than HEPA, but five of them arranged as a cube present so much filter area that resistance drops, the fan moves far more air, and the product of the two comes out ahead. Measured builds routinely reach 300 to 600 cfm for a materials cost well under a hundred dollars.

The honest drawbacks are that it is bulky, it is not attractive, a box fan is noisier than a purpose-built purifier at equivalent airflow, and it has no automatic control. For a bedroom people care about the look of, a commercial unit is a reasonable purchase. For a workshop, a basement, a nursery during wildfire season, or anywhere several rooms need covering at once, the DIY version is difficult to argue against on cost per unit of clean air.