CADR vs. Square Feet
If you’ve ever shopped for an air purifier, you’ve seen the big, bold numbers on the box: "Cleans 500 sq. ft." or "Great for rooms up to 1,200 sq. ft." Here is why those numbers are often a marketing trap.
The Problem with Square Footage
Square footage is a two-dimensional measurement of floor space. But air purifiers don't clean floor space—they clean a three-dimensional volume of air. A room with 12-foot vaulted ceilings has 50% more air to clean than a room with 8-foot ceilings, even if they have the exact same square footage. A "500 sq. ft." rating simply doesn't tell you the whole story.
The Secret ACH Assumption
When a manufacturer says a unit is for "1,000 sq. ft.", they are often assuming a very low Air Changes per Hour (ACH)—sometimes as low as 1 or 2 ACH. This means it takes an entire hour to cycle the air in that room twice. For healthy indoor air, experts recommend 4 to 5 ACH. A purifier that is "rated" for 1,000 sq. ft. at 2 ACH is actually only suitable for 400 sq. ft. at the recommended 5 ACH.
Max Fan Speed vs. Real Life
Square-footage ratings are almost always calculated based on the highest fan speed. These settings are typically very loud (55–65+ decibels). In real life, most people run their purifiers on a medium or low setting so they can sleep or watch TV. On these lower settings, the effective square footage of the unit drops dramatically—often by 50% or more.
The Solution: CADR
CADR (Clean Air Delivery Rate) is the only metric that matters. It measures exactly how many cubic feet of clean air a unit puts out per minute (CFM). CADR is independently verified and cannot be manipulated by ceiling height or ACH assumptions. It is a raw measurement of the machine's power.
Do the Math the Right Way
Our sizer uses your actual room volume and target ACH to find the CADR you really need, including a buffer for quiet operation.
Use the CADR Sizer →A Real-World Comparison
Imagine two rooms, both 400 square feet:
- Room A: 8ft ceilings = 3,200 cubic feet.
- Room B: 12ft ceilings = 4,800 cubic feet.
To achieve 4 ACH, Room A needs a CADR of 213. Room B needs a CADR of 320. A "400 sq. ft." air purifier from the store might only have a CADR of 150—leaving both rooms under-filtered.
How to Shop Using CADR
When looking at a product, ignore the large "Room Size" claims. Instead, look for the small print that lists Smoke CADR, Dust CADR, and Pollen CADR. Most experts recommend the "two-thirds rule": your Smoke CADR should be at least two-thirds of your room's square footage (assuming standard 8ft ceilings). But for the most accurate sizing, always use a volume-based calculator.
FAQ
What if a purifier doesn't list its CADR?
Don't buy it. If a manufacturer is willing to list square footage but refuses to publish their independently verified CADR, they are likely hiding a weak motor or inefficient filter.
Is AHAM the only organization that tests CADR?
AHAM (Association of Home Appliance Manufacturers) is the most recognized, but some manufacturers perform their own internal CADR testing. Always look for "independently verified" or "AHAM Verifide" for the most trustworthy data.
Related Guides:
Full Sizing Guide | ACH Explained