I've bought four air purifiers over the years, and I sized the first two wrong. Here's the short version: match the machine's smoke CADR to your room's square footage using the 2/3 rule, then confirm it hits 4 to 5 air changes an hour. That's the whole trick.
TL;DR Pick a purifier whose smoke CADR is at least two-thirds of your room's floor area in square feet. A 300 sq ft bedroom needs a smoke CADR near 200 CFM. For allergies, size up so you get 4 to 5 air changes per hour.
What Does CADR Actually Measure?
CADR stands for Clean Air Delivery Rate. It's a number certified by AHAM, the Association of Home Appliance Manufacturers, after independent lab testing. Every certified unit gets three CADR numbers, not one: smoke, dust, and pollen. Each is measured in cubic feet per minute (CFM).
Why three? Because particle size changes how fast a filter clears the air. Smoke particles are the tiniest of the three, roughly 0.09 to 1 micron. Pollen is huge by comparison, 5 to 11 microns. A filter grabs big pollen easily, so the pollen CADR is almost always the highest number on the box.
Here's my rule, and I'll die on this hill: trust the smoke CADR. It's the hardest test to pass, so it's the honest one. If a purifier moves smoke well, it moves everything larger too. Marketing loves the pollen number because it's bigger. Ignore that.
How Do You Match CADR to Room Size?
AHAM publishes a simple guideline called the 2/3 rule. Take your room's floor area in square feet. Multiply by two-thirds. That's your minimum smoke CADR, assuming standard 8-foot ceilings.
So a 150 sq ft room needs a smoke CADR of at least 100. A 300 sq ft room needs about 200. Easy.
But the 2/3 rule only gets you a baseline of roughly 4 to 5 air changes per hour (ACH), which happens to be the target that allergy specialists and the EPA suggest for people with asthma or bad allergies. ACH is exactly what it sounds like: how many times per hour the purifier cycles your room's entire air volume.
Worked example. My bedroom is 12 by 14 feet, so 168 sq ft, with 8-foot ceilings. Air volume is 168 x 8 = 1,344 cubic feet. I want 5 ACH, so I need to clean 1,344 x 5 = 6,720 cubic feet every hour. Divide by 60 minutes and I need about 112 CFM of clean air. So a smoke CADR around 110 to 120 is my floor. Notice that lines up with the 2/3 rule (168 x 0.67 = 113). The two methods agree, which is the whole point.
Want more than 5 ACH for wildfire season or a cat that sheds like a snowstorm? Size up. There's no penalty for extra CADR except a little noise and price.
Room Size to Minimum CADR (Quick Table)
| Room size (sq ft) | Air volume (8 ft ceiling) | Minimum smoke CADR (2/3 rule) | Rough ACH delivered |
|---|---|---|---|
| 150 | 1,200 cu ft | 100 CFM | ~5 ACH |
| 300 | 2,400 cu ft | 200 CFM | ~5 ACH |
| 500 | 4,000 cu ft | 335 CFM | ~5 ACH |
| 700 | 5,600 cu ft | 470 CFM | ~5 ACH |
Two real units anchor this. The Levoit Core 300 carries a smoke CADR around 135 CFM, which comfortably covers a 150 to 200 sq ft bedroom at 5 ACH. Step up to something like the Coway Airmega 400, with a smoke CADR near 350 CFM, and you're covering a 500 sq ft open living area at the same rate.
Does HEPA Change the CADR Math?
Short answer: no, but it matters. True HEPA (sometimes labeled H13) captures 99.97% of particles at 0.3 microns, the hardest size to trap. CADR already bakes in the filter's efficiency and the fan's airflow together, so a high smoke CADR usually means a real HEPA filter sitting behind a strong fan.
Watch the wording, though. "HEPA-type" or "HEPA-like" is not the same as True HEPA. Those knockoff filters can miss the 0.3 micron mark by a lot. If a unit skips AHAM certification and also says "HEPA-type," I put it back on the shelf.
The two mistakes I see most:
- Oversizing to be safe, then running the unit on its lowest, quietest setting. Your 350 CADR machine now delivers maybe 120. You paid for capacity you switched off.
- Undersizing and cranking it to max forever. It's loud, the filter wears out fast, and you still fall short of 4 ACH.
Match the CADR to the room, then run it on medium. That's the sweet spot. For more whole-home thinking, our beginner's guide to smart home setups covers where air quality fits alongside everything else.
How Do You Read the Label Without Getting Fooled?
The big one: "covers up to 1,500 sq ft." That claim almost always assumes a feeble 2 ACH, which is fine for a stuffy office but useless for allergies. Take any "covers up to X" number and, if you actually care about clean air, cut it in half. That gets you back to a real 4 to 5 ACH room.
Look for the AHAM Verifide seal and the three CADR numbers printed together. If a listing hides the CADR and shows only coverage area, that's a tell. You can look up any certified model in the AHAM Verifide directory before you buy.
One more habit worth building: think about pets and dust together. If shedding is your main problem, a good robot vacuum for pet hair removes the settled dander a purifier can't reach, so the two work as a pair. Filters catch what's airborne. Vacuums catch what's landed.
So, what size air purifier do you need? Measure the room, multiply the square footage by two-thirds, and buy a unit whose smoke CADR clears that. Then run it on medium and get on with your day.