Quick facts
- The rule: smoke CADR should be at least two thirds of the room's square footage
- What that buys you: about five air changes per hour at an 8-foot ceiling
- 150 sq ft bedroom: look for a smoke CADR near 100 cfm
- Ceilings above 8 feet: multiply the target by ceiling height divided by 8
Size the machine by CADR, because the square footage on the box is a best case
Every air purifier box prints a room size in square feet. Very few say what fan speed that figure assumes.
The number that survives scrutiny is CADR โ clean air delivery rate, in cubic feet per minute.
It is the volume of genuinely filtered air the machine delivers each minute, measured by an independent lab rather than by marketing.
Certified units carry three CADR figures: smoke, dust, and pollen.
Smoke particles are the smallest of the three, so the smoke number is the honest one to size from.
Two thirds of your floor area is the target
Take the room's area in square feet and multiply by 0.67.
A 150-square-foot bedroom lands near a smoke CADR of 100. A 350-square-foot living room wants about 233.
That ratio is not arbitrary.
With a standard 8-foot ceiling it works out to roughly five air changes per hour.
That is the level most guidance points to for households dealing with allergies, asthma, or wildfire smoke.
Minimum smoke CADR by room size, assuming an 8-foot ceiling.
If nobody in the house has a respiratory issue, two or three air changes an hour is a defensible floor.
That halves the target. Undersizing is still the more common regret, because the shortfall only shows up on bad air days.
Manufacturer claims are voluntary, so look the model up in the independent CADR directory before you trust the box.
Look up a model's verified CADRAHAM Verifide directory of air cleaners โ
High ceilings and open plans break the arithmetic
The two-thirds rule assumes an 8-foot ceiling, and plenty of American homes do not have one.
For a 10-foot ceiling, multiply the target by 10 divided by 8.
A 350-square-foot room under a 10-foot ceiling therefore wants a smoke CADR closer to 291 than 233.
Open floor plans are the harder case.
A purifier has no idea where you drew the line between the kitchen and the living room.
If the space flows without a door, size for the whole connected volume โ or accept that you are only treating one end of it.
Two mid-size units in a large open space usually beat one big one, because they work two separate air paths.
A purifier rated for your room and left on its quietest setting is a purifier rated for a much smaller room.
The rating is measured at the loudest setting
This is the part the box never mentions.
CADR is measured with the fan at maximum.
That setting is often loud enough that people turn it down within a week and never turn it back up.
Running on low can cut effective delivery by half or more, which quietly erases the sizing you just did.
The practical fix is to buy one size up from the calculation and run it on medium.
You get the airflow you planned for, at a noise level you will actually tolerate overnight.
Check the decibel figure at medium speed too, not just the number quoted for sleep mode.
A loaded filter and an open door both shrink the number
CADR is a bench measurement, taken on a new filter inside a sealed chamber.
Your bedroom is neither of those things.
A filter thick with dust moves less air, and it fails slowly enough that you will not notice the day it happens.
An open door adds hallway volume you never counted.
Closing the bedroom door at night is the cheapest capacity upgrade available to you.
The Blueair Blue Pure 311i Max Review: The 5 Numbers Behind a $230 Purifier runs this arithmetic against one machine's published specs.
Read the EPA guide to home air cleanersU.S. Environmental Protection Agency โ
