Air Changes per Hour Calculator
Enter a room size and an airflow figure to calculate the nominal air changes per hour — how many room volumes of air that airflow would move through the space each hour.
Result
Enter the room size and airflow to see the nominal air changes per hour — how many room volumes of air the entered airflow would move through the space each hour.
How the Calculation Works
Air changes per hour is the volume of air moved through a space in one hour, divided by the volume of the space.
- Imperial: ACH = (airflow in CFM × 60) ÷ room volume in ft³
- Metric: ACH = airflow in m³/h ÷ room volume in m³
Worked example: a room 20 ft long, 15 ft wide and 8 ft high has a volume of 2,400 ft³. With 400 CFM of airflow, (400 × 60) ÷ 2,400 = 10 air changes per hour.
Formula reference: CDC/NIOSH Ventilation FAQ, which states the same air-changes-per-hour formula. This calculator uses the formula only; it does not apply any target rate from that or any other source.
About the Numbers You Enter
The airflow figure in equipment documentation is a nominal rating. The air actually delivered depends on the setup, ducting, filters and condition of the equipment. This calculator only does the arithmetic on the number you enter — it does not confirm what any particular setup delivers.
This tool does not recommend a target air change rate and does not size equipment. Requirements vary by application and are set by the standards and professionals responsible for the job.