Formula and input notes
Air changes per hour (ACH) = measured airflow (CFM) × 60 ÷ (room length × width × height)
| Input | Practical note |
|---|---|
| Room length | Measure the inside room length in feet; divide irregular spaces into simpler zones if one rectangular volume would be misleading. |
| Room width | Use the matching inside width for the same room zone and keep it in feet. |
| Ceiling height | Use the average ceiling height only when it reasonably represents the occupied room volume; sloped or open-connected spaces may need separate treatment. |
| Measured airflow | Enter the airflow actually supplied to or exhausted from the defined room, according to the air-change question being evaluated. |
Worked air changes per hour example
Air Changes per Hour Calculator example: A 20 ft × 12 ft × 8 ft room has 1,920 ft³ of volume. With 192 CFM of measured airflow, ACH = 192 × 60 ÷ 1,920 = 6 air changes per hour.
Interpret the calculated ACH
The result describes how many room-volume equivalents the entered airflow moves in one hour. It does not show how well air mixes, how much is outdoor air, or whether the rate is suitable for the space.
Define the airflow boundary before comparing rates
Supply, return, exhaust, outdoor, makeup, and recirculated air are not interchangeable measurements. Document where the CFM was measured and which room boundary the volume represents.
Questions specific to this calculation
How are measured CFM and ACH related?
CFM is a volumetric flow rate. Multiplying CFM by 60 gives cubic feet moved per hour; dividing that amount by the room volume gives air changes per hour.
Which airflow measurement belongs in the ACH formula?
Use the flow that matches the question being evaluated, such as supply, exhaust, or outdoor air for the defined room. Recirculated system airflow should not be presented as outdoor-air change without a valid basis.