Air Changes Per Hour (ACH) Calculator
Convert between room airflow and air changes per hour using room length, width and ceiling height.
Room volume
3,000 ft³
Actual ACH
10 ACH
CFM for target ACH
300 CFM
Turn the calculation into a complete design workflow
Use the free result as a quick engineering check, then continue into MEPFlow for connected system design, sizing and project calculations.
How the Air Changes Per Hour (ACH) Calculator Works
Air changes per hour compares the amount of air delivered or exhausted in one hour with the volume of the room. Enter room dimensions and actual airflow to calculate ACH, or enter a target ACH to calculate the corresponding CFM.
The calculation is geometric: room volume equals length × width × height, and the hourly airflow is CFM × 60. It does not by itself describe ventilation effectiveness, contaminant removal, outdoor-air fraction or air-distribution quality.
Use ACH when the applicable design guidance expresses ventilation or exhaust as room air changes. When a standard instead prescribes CFM/person, CFM/ft² or an exhaust rate, use that method directly rather than converting without justification.
Calculation basis
ACH = CFM × 60 ÷ Room Volume
When to Use This Calculator
- Room ventilation checks
- Exhaust fan airflow checks
- Laboratory and utility-space screening
- Air-cleaner airflow comparison
Engineering Checks Before Final Design
- ACH does not prove good air mixing.
- Supply and exhaust air should not be double-counted for the same air change.
- Use the governing standard for the required ACH target.
Frequently Asked Questions
How do I convert ACH to CFM?
Multiply room volume in ft³ by the target ACH and divide by 60 minutes per hour.
How do I convert CFM to ACH?
Multiply CFM by 60 and divide by the room volume in cubic feet.
Does recirculated air count as outdoor ventilation?
No. ACH can describe different airflow types, but recirculated supply airflow is not the same as outdoor-air ventilation.
Does ACH account for poor mixing?
No. The simple ACH calculation assumes only volumetric turnover; real contaminant removal can be slower when mixing is poor.
Engineering References
MEPFlow uses public engineering references to document the calculation basis. Always verify the final design against the standard, code edition and manufacturer data applicable to your project.