Air Change Rate (ACH) Calculator
Calculate the actual air changes per hour (ACH) for a room given the supply airflow CFM and room dimensions. Verify ventilation adequacy against ASHRAE standards.
Results
What is it?
ACH (Air Changes per Hour) measures how many times the total air volume in a room is replaced by fresh or conditioned air every hour. It is the key metric for verifying indoor air quality, infection control, and code compliance. ASHRAE 62.1 specifies minimum ventilation rates by occupancy type.
How to use
Enter the measured or design supply CFM along with room dimensions. The calculator gives you actual ACH and CFM per square foot, which you can compare against ASHRAE 62.1 or building code minimums for your space type.
Example scenario
A 20�20 ft office with 9 ft ceilings (3,600 ft�) has 480 CFM of supply air. ACH = 480 � 60 / 3,600 = 8 ACH and CFM/ft� = 480/400 = 1.2. ASHRAE 62.1 requires ~0.06 CFM/ft� + 5 CFM/person for offices, so this space is well-ventilated for a typical occupancy.
Pro tip
During infectious disease outbreaks (e.g., COVID-19), the CDC recommends =6 ACH for occupied spaces. Existing buildings typically achieve only 1�3 ACH with natural ventilation. Use portable air cleaners (HEPA) rated with a Clean Air Delivery Rate (CADR) to supplement mechanical ventilation and boost effective ACH.