CADR is the most useful specification for comparing air purifiers. Unlike vague "room size" claims, CADR gives you a standardized number that actually means something.
What Is CADR?
CADR stands for Clean Air Delivery Rate. It measures how much filtered air an air purifier produces, expressed in cubic feet per minute (CFM). Higher CADR means more clean air delivered faster.
CADR is typically given for three particle types:
- Smoke: 0.09-1.0 microns (smallest particles tested)
- Dust: 0.5-3.0 microns
- Pollen: 5.0-11.0 microns (largest particles tested)
Why CADR Matters
CADR accounts for both filter efficiency and airflow. A filter could be 99.99% efficient, but if airflow is poor, cleaning performance suffers. CADR captures the complete picture.
It's also standardized—tested under controlled conditions by AHAM (Association of Home Appliance Manufacturers). This makes comparison meaningful.
How to Use CADR
The 2/3 Rule
A common guideline: your purifier's smoke CADR should be at least 2/3 of your room's square footage. This provides approximately 4.8 air changes per hour in a room with 8-foot ceilings.
| Room Size (sq ft) | Minimum Smoke CADR | Better Performance |
|---|---|---|
| 100 | 67 | 100+ |
| 150 | 100 | 150+ |
| 200 | 133 | 200+ |
| 300 | 200 | 300+ |
| 400 | 267 | 400+ |
| 500 | 333 | 450+ |
Higher Ceilings
The 2/3 rule assumes 8-foot ceilings. For higher ceilings, you need more CADR because there's more air volume to clean.
Which CADR Number to Use
Smoke CADR is typically the most useful since smoke particles are the smallest and hardest to filter. If a purifier handles smoke well, it'll handle dust and pollen even better.
CADR Limitations
Tested at Max Speed
CADR is usually measured at maximum fan speed. In real use, you might run at lower, quieter speeds. Performance drops accordingly.
Doesn't Measure Gas Removal
CADR only measures particle removal. It tells you nothing about VOC or odor removal from activated carbon filters.
Not All Purifiers Have Ratings
AHAM testing is voluntary. Some manufacturers don't participate. No CADR doesn't necessarily mean a bad product, but it makes comparison harder.
CADR vs Room Size Claims
Manufacturers often claim their purifier covers "up to X square feet." These claims vary in how they're calculated and are often optimistic. CADR is more reliable:
- "Up to 400 sq ft" might mean 2 air changes per hour (slow cleaning)
- CADR-based calculation targets 4-5 air changes (effective cleaning)
- Same purifier, very different "room size" depending on calculation
Use CADR to verify room size claims rather than trusting marketing.
Quick Calculation
To calculate air changes per hour: (CADR × 60) ÷ Room Volume. For a 200 sq ft room with 8-foot ceilings (1600 cubic feet) and CADR of 200: (200 × 60) ÷ 1600 = 7.5 ACH. That's excellent performance.