ACH expresses an hourly clean-air flow relative to room volume. Here it describes equivalent air changes supplied by a purifier, using CADR rather than raw fan airflow. It is a useful comparison of capacity and space, not a measurement of pollutant concentration.
How to use
- Enter the full connected room dimensions.
- Enter the purifier CADR, taking care to choose CFM or m³/h correctly.
- Read the ACH result and use the CADR calculator to explore a different target.
How the calculation works
ACH = CADR (m³/h) ÷ volume (m³), or ACH = CADR (CFM) × 60 ÷ volume (ft³).
Example
A 250 m³/h purifier in a 50 m³ room provides 5 equivalent air changes per hour. If the connected volume doubles to 100 m³, the value falls to 2.5 ACH.
Factors affecting the result
- The calculation assumes reasonably mixed air throughout the entered volume.
- Measured ventilation and purifier equivalent ACH are different quantities.
- Particle sources may continue emitting while the purifier operates.
Frequently asked questions
Does 5 ACH mean all air is clean after 12 minutes?
No. Air mixes, so some air can pass through the filter repeatedly while some has not yet reached it. ACH is a flow-to-volume ratio, not a complete replacement schedule.
Should I enter fan airflow or CADR?
For this purifier calculator, enter CADR. Raw airflow does not account for how effectively the filter removes the relevant particles.
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