Formula
CFM = room volume (ft³) × ACH ÷ 60
Room volume is length × width × ceiling height. ACH must come from a defined project, occupancy, process, or applicable requirement; it is not a universal HVAC default.
Step-by-step calculation
- Measure the room length, width, and finished ceiling height in feet.
- Multiply them to get room volume in cubic feet.
- Choose the stated ACH basis and identify whether the result represents supply, exhaust, outdoor, or recirculated flow.
- Multiply volume by ACH and divide by 60.
- Check the result against the system’s load, duct, fan, sound, and pressure requirements.
Worked example
A room is 20 ft long, 15 ft wide, and 8 ft high. Volume is 20 × 15 × 8 = 2,400 ft³. If a documented design basis calls for 6 ACH, CFM is 2,400 × 6 ÷ 60 = 240 CFM. The calculation converts the stated basis to flow; it does not prove that 6 ACH is appropriate for every room.
Common calculation errors
| Error | Correction |
|---|---|
| Using floor area instead of volume | Include ceiling height; ACH is based on volume. |
| Mixing ft³ with m³ | Keep the entire calculation in one system before converting the final airflow. |
| Calling every airflow outdoor air | Identify supply, return, exhaust, outdoor, and recirculated streams separately. |
| Selecting a duct from CFM alone | Check area, velocity, friction, fittings, and available fan pressure. |
FAQ
Can this replace a ventilation design?
No. It is a volume-flow conversion. Ventilation design also needs the applicable requirements, air-stream definition, equipment, and distribution review.
How do I convert CFM to m³/h?
Multiply CFM by 1.69901082. For 240 CFM, the result is about 408 m³/h.