What Is a Good CADR Rating? A Practical Guide to Choosing the Right Air Purifier

If you’ve spent more than five minutes shopping for an air purifier, you’ve been bombarded by one acronym: CADR. You see it plastered on every box, a three-digit number promising clean air. It’s easy to fall into the trap of thinking that, like horsepower or megapixels, bigger is always better. Brands often seem to be in a race to post the highest CADR numbers, and consumers are left feeling that the biggest number must be the best.

But what if that high-powered machine is the equivalent of using a fire hose to water a houseplant? It’s overkill, it’s noisy, and it’s inefficient. Choosing the right air purifier isn’t about finding the highest CADR; it’s about finding the right CADR for your specific space and your personal needs. Let’s decode this number and turn you into a smarter shopper.

CADR 101: The “Horsepower” of Your Air Purifier

CADR stands for Clean Air Delivery Rate. In the simplest terms, it’s a measure of how much clean air an air purifier can deliver in a specific amount of time. It’s a standardized metric developed by the Association of Home Appliance Manufacturers (AHAM) to help consumers compare apples to apples.

You’ll usually see three CADR numbers, like on the GermGuardian AC9600W, which lists Smoke: 255, Dust: 267, and Pollen: 271. These numbers tell you how effective the purifier is at removing those three common types of pollutants. The number itself represents the volume of filtered air in cubic feet per minute (CFM). So, a smoke CADR of 255 means the unit can deliver 255 cubic feet of smoke-free air every minute. When in doubt, it’s always wise to focus on the lowest number (usually smoke, as its particles are the smallest and hardest to capture) as your baseline for performance.

The Metric That Matters: Air Changes Per Hour (ACH)

Now, knowing the “horsepower” is useful, but is a Ferrari engine suitable for a delivery truck? To answer that, we need to introduce a much more practical concept: ACH, or Air Changes Per Hour.

ACH tells you how many times a purifier can clean the entire volume of air in your room every hour. This is the metric that directly impacts the air you actually breathe. A purifier that can achieve 4 ACH in your bedroom means it’s completely replacing the air with freshly filtered air every 15 minutes. This is where the magic happens for allergy sufferers.

The product description for the AC9600W says it “circulates room air over 4x per hour” in its recommended 395 sq. ft. room. This is their way of talking about ACH. But you don’t have to take their word for it. You can calculate it yourself.

Become Your Own Expert: The Super-Simple ACH Formula

Ready to become your own air quality expert? Grab a calculator. Here’s the formula:

ACH = (CADR of your purifier × 60) / (Room Length in ft × Room Width in ft × Ceiling Height in ft)

Let’s use the GermGuardian AC9600W as our example. We’ll use its smoke CADR of 255. Let’s put it in a 15ft by 20ft bedroom with a standard 8ft ceiling.

• Room Volume = 15 × 20 × 8 = 2400 cubic feet
• ACH = (255 × 60) / 2400
• ACH = 15300 / 2400 = 6.375 ACH

In this bedroom, the unit is incredibly powerful, cleaning the air more than 6 times per hour!

A Handy Rule of Thumb:
Don’t want to do the math? For a quick check with standard 8-foot ceilings, use this: For allergy sufferers, a good target CADR is roughly 1.5 times your room’s square footage. For a 300 sq. ft. room, you’d want a CADR of around 450. For general use, a 1:1 ratio is often sufficient.

What’s Your Number? Matching ACH to Your Needs

Not everyone needs the same number of air changes. Let’s break it down:

• Level 1: General Freshness (2-3 ACH): For a standard household without specific health concerns, having the air cleaned 2 or 3 times per hour is great for reducing ambient dust and odors.
• Level 2: Allergy & Asthma Sufferers (4-5 ACH): This is the sweet spot recommended by many experts. Achieving 4-5 ACH means pollen, dander, and other triggers are being removed from the air frequently enough to provide noticeable symptom relief.
• Level 3: High-Sensitivity & Special Cases (6+ ACH): For those with severe allergies, compromised immune systems, or in rooms with high pollutant sources (like a home with a smoker or during wildfire season), maximizing ACH provides the highest level of protection.

The CADR Triangle: Performance vs. Noise vs. Cost

Before you rush out to buy the machine with the highest CADR you can find, remember this: there’s no free lunch. High CADR ratings come with trade-offs, forming an “impossible triangle” of performance, noise, and cost.

1. Performance: The CADR rating is measured at the purifier’s highest fan speed.
2. Noise: The highest fan speed is also the loudest setting. It might sound like a small jet engine, making it unbearable for a bedroom or office. In reality, you’ll likely run it on a medium or low setting, achieving a much lower actual CADR.
3. Cost & Energy: A powerful motor that can move that much air consumes more electricity. While many modern units are Energy Star certified, a high-CADR machine running 24/7 will have a bigger impact on your utility bill than a smaller, appropriately-sized one.

The Smart Choice: Find Your Balance

The goal isn’t to buy the highest CADR purifier; it’s to buy the one that provides the ACH you need for your room size, at a fan speed you can actually live with.

It’s often smarter to choose a purifier that seems slightly “oversized” for your room. This allows you to run it on a quieter, medium setting while still achieving your target 4-5 ACH. You get the clean air you need without the disruptive noise. So next time you see that big number on the box, don’t just be impressed. Ask yourself: “What ACH will that CADR give me, in my room, at a noise level I can tolerate?” Answering that question is the key to breathing easier in every sense of the word.