Best for
- Anyone sizing an air purifier
- Rooms with non-standard ceiling height
- People comparing brands with different coverage claims
Watch for
- Using only square feet when ceilings are unusually high
- Comparing 1-ACH coverage against 4.8-ACH coverage
- Treating CADR as a measure of gas removal
The core formula
Multiply room square footage by ceiling height to get cubic feet. Multiply by the number of desired air changes per hour, then divide by 60 to convert the result to cubic feet per minute.
Example: 300 ft² × 8 ft × 5 ACH ÷ 60 = 200 CFM.
Choose the ACH target from the problem
Higher air-change targets are useful when particle load is high or faster cleaning matters. Lower targets may be reasonable for background filtration in cleaner spaces.
Do not confuse whole-room air exchange with a purifier’s fan speed label; the final result depends on actual clean-air delivery.
Use published CADR consistently
When separate smoke, dust and pollen CADRs are provided, use the figure most relevant to the problem. Smoke CADR is commonly used as a conservative particle-airflow reference.
If only a giant room-size number is published, check what ACH assumption produced it before comparing with another model.
Use the same sizing basis across brands. Room-size claims can use different air-change assumptions. When possible, compare published CADR and calculate the airflow your room actually needs.