When comparing air purifiers, you'll frequently encounter CADR ratings. These numbers are among the most important specifications for evaluating performance, yet they're often misunderstood or overlooked entirely. Understanding CADR can help you choose the right air purifier for your space and avoid common purchasing mistakes.

In this guide, we'll explain what CADR means, how it's measured, and how to use it to make informed buying decisions.

What Is CADR?

CADR stands for Clean Air Delivery Rate. It's a standardised measurement developed by the Association of Home Appliance Manufacturers (AHAM) that tells you how much filtered air an air purifier can produce. In simple terms, CADR indicates how quickly a purifier can clean the air in a given space.

CADR is typically measured in cubic metres per hour (m³/h) in Australia and most countries, or cubic feet per minute (CFM) in the United States. A higher CADR means the purifier can clean more air faster.

💡 The Key Insight

CADR combines two factors: how much air flows through the purifier AND how effectively particles are removed. A purifier could have high airflow but poor filtration (low CADR), or excellent filtration but minimal airflow (also low CADR). CADR captures both elements in a single, comparable number.

Three Types of CADR

CADR is measured separately for three different types of particles, each representing common household pollutants:

Smoke CADR

Measures effectiveness against the smallest particles (0.09-1.0 microns). This represents fine particles like tobacco smoke, bushfire smoke, cooking fumes, and some bacteria. The smoke CADR is often the most challenging for purifiers and typically shows the lowest numbers.

Dust CADR

Measures effectiveness against medium-sized particles (0.5-3.0 microns). This includes dust mites, mould spores, and some pollen. Most people dealing with general household dust should pay attention to this rating.

Pollen CADR

Measures effectiveness against the largest particles (5.0-11.0 microns). This covers pollen, large dust particles, and pet dander. The pollen CADR is usually the highest number as these particles are easiest to capture.

📋 Which CADR Matters Most?
  • Smoke CADR: Prioritise if you're concerned about bushfire smoke, cooking fumes, or very fine particles
  • Dust CADR: Most relevant for general home use and dust allergies
  • Pollen CADR: Focus on this if hay fever and pollen allergies are your main concern

How CADR Is Tested

AHAM testing follows a standardised protocol to ensure fair comparisons between products:

  1. The purifier is placed in a sealed test chamber of a standard size (28.5 cubic metres)
  2. Specific quantities of smoke, dust, or pollen particles are introduced
  3. The purifier operates at its highest setting
  4. Particle levels are measured over time
  5. The rate of particle reduction is calculated and converted to CADR

It's important to understand that CADR is tested at maximum fan speed. In real-world use, you may run your purifier at lower speeds for noise reasons, resulting in lower actual performance than the stated CADR.

Using CADR to Size Your Air Purifier

The most practical application of CADR is determining whether a purifier is appropriate for your room size. Here's how to do it:

The Two-Thirds Rule

A commonly used guideline is that a purifier's CADR should be at least two-thirds of your room's floor area. For a 30m² room, you'd want a CADR of at least 200 m³/h.

The ACH Calculation

For a more precise approach, calculate Air Changes per Hour (ACH)—how many times the purifier can filter the entire room's air in one hour:

ACH = CADR × 60 ÷ Room Volume

For effective air cleaning, aim for at least 4-6 air changes per hour. In high-pollution situations (bushfire smoke, strong allergen presence), 8+ ACH is preferable.

🔢 Quick Room Size Guide

For a standard 2.4m ceiling height and 5 ACH:
15m² room: Need CADR of ~180 m³/h
25m² room: Need CADR of ~300 m³/h
40m² room: Need CADR of ~480 m³/h
60m² room: Need CADR of ~720 m³/h

CADR vs. Manufacturer Room Size Claims

Manufacturers often state "suitable for rooms up to X square metres" on packaging. These claims can be optimistic because:

  • They may assume lower ACH than ideal (sometimes as low as 2 ACH)
  • They may be based on maximum fan speed, which is often too noisy for regular use
  • They may not account for real-world air infiltration and pollution sources

It's generally wise to choose a purifier with a slightly higher CADR than the minimum needed. This gives you headroom to run at lower, quieter speeds while still maintaining effective air cleaning.

Understanding CADR Limitations

While CADR is valuable, it has limitations to be aware of:

What CADR Doesn't Tell You

  • Noise levels: High CADR may come with loud operation
  • Energy consumption: More powerful purifiers typically use more electricity
  • Filter life: Higher airflow may mean faster filter degradation
  • Gas and odour removal: CADR measures particles, not gaseous pollutants
  • Real-world performance: Testing is done in ideal lab conditions

Uncertified Products

Not all air purifiers undergo AHAM certification. Some manufacturers provide their own CADR numbers based on internal or third-party testing. While these may be accurate, they're not directly comparable to AHAM-certified ratings. Look for the AHAM Verified mark when comparing products.

⚠ Watch For Inflated Claims

Some manufacturers quote "air processing" rates rather than true CADR. Processing rate measures total air moved through the unit without accounting for filter efficiency. A purifier might process 500 m³/h but only have a CADR of 300 m³/h if its filter captures only 60% of particles. Always look specifically for CADR.

CADR in Practice: Real-World Considerations

Running Continuously vs. Periodically

Higher CADR purifiers can clean air faster, making them suitable for periodic use—running them intensively for a few hours rather than continuously. Lower CADR units work best when run continuously to maintain air quality over time. Consider your usage preferences when choosing.

Multiple Rooms

CADR applies to single enclosed spaces. If you want to purify multiple connected rooms, you'll need higher CADR or multiple units. Air doesn't flow efficiently through doorways, so relying on one purifier for an entire home is rarely effective.

Pollution Sources

If your space has continuous pollution sources—cooking, smoking, pets—you'll need higher CADR to keep up with ongoing particle generation. The "right" CADR depends not just on room size but on how much pollution you need to remove.

Making Your Decision

When comparing air purifiers using CADR:

  1. Calculate your room volume: Length × Width × Ceiling Height
  2. Determine target ACH: 4-6 for general use, 6-8 for allergies, 8+ for heavy pollution
  3. Find required CADR: (Room Volume × Target ACH) ÷ 60
  4. Add buffer: Consider sizing up 20-30% for quieter operation
  5. Compare products: Look at CADR for the particle type most relevant to you
  6. Balance with other factors: Noise, price, features, and filter costs

Conclusion

CADR is one of the most reliable metrics for comparing air purifier performance. By understanding what it measures and how to apply it to your specific situation, you can make more informed purchasing decisions and avoid expensive mistakes. Remember that CADR is just one factor—consider noise, energy consumption, maintenance costs, and specific features alongside performance ratings when choosing your air purifier.

When in doubt, size up. A purifier with higher CADR than you minimally need gives you flexibility to run at quieter speeds while still effectively cleaning your air.

👨

Andrew Kim

Air Quality Engineer

Andrew has worked in HVAC and air quality engineering for over 15 years. He enjoys translating technical specifications into practical advice that helps consumers make informed decisions about their indoor air quality equipment.