CADR Explained: How to Compare Air Purifiers for Your Room
Clean air delivery rate, or CADR, describes an air cleaner’s particle-cleaning output under its test conditions. It is useful when comparing equipment, but it is not a guarantee of a particular reading in every room.
Read the number and the unit together
CADR may be reported in cubic metres per hour (m³/h) or cubic feet per minute (CFM). One CFM is approximately 1.7 m³/h. Compare values in the same units and check which particle test they describe.
EPA notes that ratings are typically measured at the highest operating speed. It recommends choosing a particle cleaner with sufficient CADR for the room or area. EPA guide to choosing an air cleaner.
A useful calculation, with limits
For an illustrative 30 m² room with a 2.7 m ceiling, volume is 81 m³. A stated CADR of 300 m³/h corresponds to about 3.7 equivalent clean-air changes per hour: 300 ÷ 81.
That is a sizing calculation, not 3.7 outdoor-air changes. It does not predict the exact PM2.5 concentration or establish ventilation adequacy. Use the manufacturer’s room-sizing guidance and assess the setting you will actually run.
What to compare
| Item | Check before choosing |
|---|---|
| Treated space | Room area, ceiling height and connected open areas |
| Operating speed | Performance and noise at a usable setting |
| Test basis | Reported CADR, units and test method |
| Maintenance | Correct replacement parts, availability and cost |
| Room placement | Clear intake and outlet paths |
| Actual result | Consistent indoor measurements under normal use |
There is no universal “divide the room rating by two for India” rule. Outdoor entry, indoor sources and operation differ between homes. A rating also cannot establish that untested particle sizes pass straight through the filter.
What CADR does not tell you
Particle CADR is not a gas-removal rating or an outdoor-air supply rate. If your concern includes CO₂ or gas pollution, ask a separate question about ventilation or treatment of the particular gas. A larger particle CADR does not answer it.
Likewise, do not compare a fresh-air fan’s free-air volume directly with a purifier’s clean air delivery rate. For aqi0, the technical note distinguishes free-air and effective supply airflow. The installation then needs an assessment of the rooms and airflow paths.
If a device you own is underperforming
Before replacing it, check filters, placement, operating speed and room conditions. The Delhi winter troubleshooting guide provides a sequence for doing that.
If you need filtered outdoor-air supply across the home, compare it with room filtration using our fresh-air system versus purifier guide.