Room area alone can hide the effect of a high ceiling. Calculate room volume and multiply by a chosen clean-air change rate to compare purifier specifications on a consistent basis. The ACH setting is your design target, not a promise of a particular health outcome.
How to use
- Measure the room length, width and ceiling height in one unit system.
- Choose your desired clean-air changes per hour.
- Compare the required CADR with a verified product rating at a fan speed you can tolerate.
How the calculation works
Volume = length × width × height. CADR (m³/h) = volume (m³) × ACH. CADR (CFM) = volume (ft³) × ACH ÷ 60.
Example
A 4 m × 5 m room with a 2.7 m ceiling contains 54 m³. At 5 ACH it needs 270 m³/h, or approximately 159 CFM, of clean-air delivery.
Factors affecting the result
- CADR is clean-air delivery, not unfiltered fan airflow.
- Lower fan speeds usually have lower CADR than the highest advertised setting.
- Open doors, filter loading, obstructions and incomplete mixing change real room performance.
Frequently asked questions
Is CFM the same as m³/h?
No. CFM is cubic feet per minute. One CFM is approximately 1.699 m³/h; the minute-to-hour conversion is essential.
Does a purifier supply fresh outdoor air?
A recirculating particle purifier filters existing room air. Its equivalent clean-air changes are not outdoor ventilation and do not establish control of carbon dioxide or every gas.
Which CADR should I compare?
Use the rating relevant to the particles you want to filter. Product specifications may list separate smoke, dust and pollen CADRs.
Sources & methodology
Calculation basis: Physics-based airflow calculation. Methodology
- Air Cleaners and Air Filters in the Home — U.S. Environmental Protection Agency
- Guide to Air Cleaners in the Home, second edition — U.S. Environmental Protection Agency
Last updated: September 15, 2026