Airflow calculator
Air Changes per Hour Calculator
Calculate the ACH implied by a stated CFM flow and room volume.
Calculate ACH from CFM
Use one clearly identified air stream: supply, outdoor, exhaust, or recirculated airflow.
How to use this calculator
- Enter the inside dimensions of the space.
- Enter the CFM for one identified airflow stream.
- Use the result as the volume-flow ratio for that stream.
Formula
ACH = CFM × 60 ÷ room volume (ft³)
Variables and units
| Term | Meaning | Unit |
|---|---|---|
| CFM | One stated volume-flow stream | ft³/min |
| Room volume | Inside dimensions multiplied together | ft³ |
| ACH | Volume-flow ratio per hour | 1/h |
Example
A 20 ft × 15 ft × 8 ft room has 2,400 ft³ of volume. At 240 CFM, 240 × 60 ÷ 2,400 = 6 ACH.
Result interpretation
ACH describes a volume-flow rate relative to room volume. It does not mean all air is perfectly replaced or mixed each hour. Supply, outdoor air, exhaust, return, and filtered recirculated air answer different questions, so do not substitute one stream for another without a documented basis.
Check the airflow source
Use the Air Changes per Hour Explained guide to identify what the number means. Convert a documented target back to CFM with the CFM Calculator.
Frequently asked questions
Does an ACH result set the required ventilation rate?
No. It only calculates a ratio from the values entered. Follow applicable codes, project criteria, and qualified HVAC design for requirements.
Can I use supply CFM as outdoor-air CFM?
Not unless the system documentation shows they are the same. Supply air may include recirculated air, while outdoor air is a separate quantity.
Does a higher ACH always mean better performance?
No. Air distribution, contaminants, humidity, filtration, pressure relationships, noise, energy, and system purpose all matter.