Air Changes Per Hour (ACH) Calculator
Calculate how many times the air volume in a room is completely replaced per hour. Used to assess ventilation effectiveness for HVAC design, indoor air quality, and code compliance.
Enter values above and click Calculate.
Formula
ACH = (Q × 60) / V
- ACH = Air Changes Per Hour (hr⁻¹)
- Q = Airflow rate (CFM — cubic feet per minute)
- 60 = Conversion factor (minutes per hour)
- V = Room volume (ft³) = Length × Width × Ceiling Height
Example: A 500 CFM system in a 20 ft × 15 ft × 9 ft room (2,700 ft³):
ACH = (500 × 60) / 2,700 = 11.1 ACH
Assumptions & References
- Airflow is measured in CFM (cubic feet per minute) at the supply or exhaust point.
- Room volume is calculated as a simple rectangular box (Length × Width × Height). Irregular rooms should use actual measured volume.
- The formula assumes uniform air distribution throughout the room (well-mixed model).
- Ductwork losses, filter resistance, and actual supply vs. return balancing are not accounted for.
- ASHRAE Standard 62.1 governs minimum ventilation rates for commercial buildings.
- ASHRAE Standard 62.2 governs residential ventilation requirements.
- Typical ACH benchmarks: Residential 0.35–1 ACH (natural), Office 4–8 ACH, Classroom 4–12 ACH, Hospital patient room 6–12 ACH, Operating room 15–25 ACH, Laboratory 6–12 ACH (ANSI/AIHA Z9.5).
- To convert from m³/hr to CFM: 1 m³/hr ≈ 0.5886 CFM.