You've been shopping for an air purifier. You've seen coverage ratings plastered on every box — "covers up to 1,500 square feet!" — and you've been trying to match them to your room. Seems straightforward. But people who've bought purifiers based on room size alone often end up disappointed, running a unit that sounds impressive and still doesn't seem to be doing much.

Here's what those coverage numbers don't tell you — and what to look at instead.

The number most people use is incomplete

When a manufacturer says an air purifier covers 500 square feet, that figure is derived from something more fundamental: how much air the unit can actually move and filter per minute. That underlying number is the CADR, or Clean Air Delivery Rate.

CADR is measured in cubic feet per minute — CFM. It tells you how quickly a purifier delivers filtered air back into a room. A unit with a CADR of 200 CFM moves 200 cubic feet of cleaned air per minute. A unit with a CADR of 400 CFM is moving twice as much.

The square footage rating on the box is calculated from the CADR. Manufacturers take the CFM output and run a formula to produce a coverage estimate. But that estimate almost always assumes an 8-foot ceiling in a perfectly sealed room under ideal lab conditions. It doesn't account for higher ceilings, furniture, open doorways, contaminant load, or the number of people in the space. Two purifiers can have identical square footage ratings and meaningfully different performance because their CADR numbers are different.

CADR is the actual performance number. Square footage is a derived estimate based on assumptions that may or may not apply to your home.

Why air exchanges matter more than coverage

What you actually want from an air purifier isn't just that it can technically "cover" your room. You want it to cycle the air in that room frequently enough to consistently reduce the pollutants you're trying to remove. That's measured in air changes per hour — how many times the room's total air volume passes through the filter in 60 minutes.

For general residential use, four to five complete air exchanges per hour is considered effective. In rooms with higher allergen loads, pets, or people with respiratory sensitivities, the upper end of that range is more appropriate.

Here's where CADR becomes directly useful. You can calculate whether a purifier will achieve adequate air exchanges with straightforward math. Multiply your room's length, width, and ceiling height to get its cubic volume. Then see how many times per hour your purifier's CFM output can move through that volume.

A room that is 15 feet by 20 feet with 8-foot ceilings contains 2,400 cubic feet of air. A purifier with a CADR of 200 CFM delivers 12,000 cubic feet of filtered air per hour. Divide 12,000 by 2,400, and you get 5 air exchanges per hour — solid performance for that space. Push that same purifier into a room twice as large and you're at 2.5 exchanges per hour. Technically "covered" by some marketing standard. Not meaningfully clean in practice.

The contaminant load question that coverage ratings ignore

Here's something box ratings never mention: the same room can require very different levels of air purification depending on what's in it.

A bedroom with no pets, standard furniture, and good humidity control has a relatively low contaminant burden. An identical-sized bedroom with two shedding dogs, carpet, and windows facing a busy street has a much higher one. Both rooms might be 250 square feet. The same purifier will work much harder in the second.

CADR gives you a real performance number to think about in context. If you know your room has elevated particulate levels — because of pets, because of cooking odors that drift in, because of known dust or mold issues — you can deliberately choose a purifier whose CADR gives you more headroom than the minimum calculation requires. Running a unit with excess capacity at a lower speed is typically quieter, gentler on filters, and more effective than running an undersized unit constantly at full speed.

This is also why the iAdaptAir manual specifically notes that sizing should account for the room, occupancy, and contaminant load — not just square footage. That's not a vague recommendation. It reflects the reality that CADR has to be understood in the context of what the purifier is actually being asked to clean.

What CADR doesn't tell you

CADR measures airflow through the filter. It doesn't measure filtration quality on its own. A purifier can have a high CADR and a low-quality filter — moving a lot of air, but not capturing the particles that matter. Conversely, a purifier with highly effective filter technology but insufficient airflow won't cycle the room's air often enough to make a difference.

The combination is what counts. You need both adequate CADR for the space and filtration technology capable of capturing the pollutants you're concerned about. True HEPA filtration captures particles down to 0.3 microns — including fine particulate matter, mold spores, pet dander, and dust. Activated carbon removes VOCs and other gaseous pollutants that HEPA filters don't capture. CADR tells you how fast the air moves through those filters.

Neither the CADR nor the filter spec alone answers the question. You need to evaluate both.

How to actually use CADR when you're shopping

The practical process is straightforward. Measure your room. Calculate its cubic volume by multiplying length, width, and ceiling height. Decide on your target air exchange rate — four times per hour for a standard room, higher if you have allergies, pets, or known air quality issues.

Multiply your cubic volume by your target exchange rate. That gives you the cubic feet per hour of filtered air you need. Divide by 60 to convert to CFM. That's the CADR you need at minimum. Shop to that number, not to a square footage rating.

For example: a 300-square-foot living room with 9-foot ceilings has a volume of 2,700 cubic feet. At 5 air exchanges per hour, you need 13,500 cubic feet of filtered air per hour — or a CADR of 225 CFM. A unit rated for 300 square feet might hit that. Or it might not, depending on how that coverage number was calculated.

The iAdaptAir publishes its CADR directly. The 2S delivers 177 CFM and covers 265 square feet on a 12-minute cycle. The 2M delivers 353 CFM for 530 square feet. The 2L delivers 530 CFM for 795 square feet. The 2P delivers 706 CFM for 1,059 square feet. Those coverage ratings are based on achieving a full air cycle every 12 minutes — that's 5 air exchanges per hour, a real performance standard rather than a marketing floor.

When you know the CADR and the cycle time, you can evaluate the claim. When you're only given a square footage number, you can't.

CADR and more

Square footage ratings are a starting point — a rough guide that's useful only if you understand the assumptions behind it. CADR is the actual measure of performance. It's concrete, comparable across brands, and gives you the information you need to match a purifier to a real room rather than an idealized one.

Find your room's volume. Set a target air exchange rate. Calculate the CFM you need. Then look for a unit that meets it, and that combines filtration technology to address your specific pollutants. That's how you choose an air purifier that actually works.

When you're ready to put those numbers to use, shop Air Oasis and Breathe Better, Live Better.