Forget Square Footage: The Only Number You Need to Know Before Buying an Air Purifier (It's CADR)

You’ve seen the claims on the boxes of air purifiers, perhaps even on the product page for a unit like the AZEUS GL-FS32: “Cleans up to 1080 sq. ft.!” It sounds impressive. It seems like the most important number to consider. It is also, from an engineering perspective, almost completely meaningless.

That number is the single most misleading metric in the air purification industry. It’s a marketing tool designed to give you a simple, big number to latch onto, but it omits the most critical variable: time.

To understand why, you have to break an air purifier down to its two fundamental components: a filter and a fan. Everyone obsesses over the filter—is it “True HEPA”? Does it have carbon? But they forget the fan. A world-class filter powered by a weak fan is like putting a Formula 1 engine in a rowboat. It has immense potential, but it’s not going anywhere fast. The filter determines how clean the air can get. The fan determines how much air gets cleaned, and therefore, how large a space can be effectively managed.

So, how do we measure this combination of filtration efficiency and fan power? The answer is a single, beautiful, standardized metric: CADR.

The Ultimate Answer: What is CADR?

CADR stands for Clean Air Delivery Rate. It is a performance metric created and verified by the independent Association of Home Appliance Manufacturers (AHAM). Think of it as the horsepower rating for air purifiers.

Its unit is CFM (Cubic Feet per Minute), and it tells you, quite literally, the volume of 100% clean air that a purifier delivers every minute.

It’s not just a measure of the fan’s raw airflow (which is also measured in CFM). CADR is smarter. It is a combined score that represents Airflow (CFM) × Filter Efficiency (%). This is why it’s the ultimate metric. A machine with a powerful fan but a leaky, inefficient filter will have a low CADR. A machine with a great filter but a weak fan will also have a low CADR. Only a machine with both powerful airflow and high-efficiency filtration will earn a high CADR rating.

The “Square Footage” Trap and the Importance of ACH

So why do manufacturers push “square footage” instead of the superior CADR metric? Because they can manipulate it. A tiny, weak purifier could claim to clean a massive room if you’re willing to wait eight hours for it to do so. This is where the concept of ACH (Air Changes per Hour) comes in.

ACH tells you how many times a purifier can completely filter all the air in a given room in one hour. For general air quality, 2 ACH might be fine. But for anyone with allergies, asthma, or serious health concerns, the recommendation from experts is a minimum of 4 to 5 ACH.

Let’s look at that “1080 sq. ft.” claim again. The fine print on the AZEUS GL-FS32 page says it cleans that space in “4 cycles within 1 hour.” This means it provides 4 ACH in a 1080 sq. ft. room. But what if you need 5 ACH? The effective room size suddenly shrinks. This is why a single square footage number is a trap; it’s a floating variable, not a fixed measure of performance.

CADR, on the other hand, is an absolute. It is a standardized, third-party tested value measured at the machine’s highest setting. It doesn’t change. It is the bedrock of performance on which you can make a reliable calculation.

The Engineer’s Toolkit: Your Personal CADR Calculator

Enough theory. Here is the simple, powerful tool you need. It’s a rule of thumb from AHAM, the very organization that created the CADR standard. It’s called the 2/3 Rule.

Your Room’s Area (in sq. ft.) ÷ 1.5 = The Minimum Smoke CADR You Need(This is the same as multiplying by 2/3)

That’s it. Let’s try it.

• Your Bedroom: 12 ft × 15 ft = 180 sq. ft.
• Calculation: 180 ÷ 1.5 = 120
• Result: You need an air purifier with a Smoke CADR of at least 120.

This simple formula ensures the purifier is powerful enough to provide roughly 5 air changes per hour in that room, which is ideal for effectively removing allergens and pollutants.

How to Find and Read a CADR Rating

When you shop, look for the official AHAM Verifide® seal on the packaging or in the product specifications. It will list three CADR numbers: * Smoke: This is the most important number to look at. Smoke particles are the smallest (0.09-1.0 microns) and the most difficult to capture. A purifier that is effective against smoke will be even more effective against larger particles. * Dust: (0.5-3 microns) * Pollen: (5-11 microns)

If a manufacturer doesn’t list a CADR rating, be skeptical. It might be a great product, but they are choosing not to participate in the industry-standard performance benchmark. It’s a red flag that warrants caution.

One final, crucial point: CADR is measured at the purifier’s highest fan speed. Since you will likely run it at a quieter, lower speed most of the time, it’s always wise to choose a model with a CADR rating that is a bit higher than your calculated minimum. This gives you a performance buffer, ensuring you still get adequate air changes even at a whisper-quiet setting.

Stop guessing. Stop letting marketing departments decide what’s right for your space. Measure your room, do the simple math, and find the CADR that meets your needs. By focusing on this single, honest, engineering-based number, you transform from a confused consumer into an empowered expert.