AC1, commonly referred to in the HVAC industry as ACH (Air Changes per Hour), is a measure of how many times the total volume of air in a specific room or space is replaced with new air within one hour. This metric is critical for ensuring proper ventilation, maintaining indoor air quality, and controlling humidity or airborne contaminants.
The AC1 Formula
AC1 = (CFM × 60) / (Length × Width × Height)
Where:
CFM: Cubic Feet per Minute (airflow rate of your fan/vent).
60: Minutes in an hour.
Volume: Length × Width × Height of the room in cubic feet.
Recommended AC1 Rates by Room Type
Room Type
Recommended AC1 (ACH)
Kitchens
15 – 60
Bathrooms
8 – 15
Bedrooms/Living Rooms
5 – 10
Classrooms
4 – 6
Example Calculation
If you have a bedroom that is 12 feet long, 10 feet wide, and 8 feet high, your total volume is 960 cubic feet. If you install a bathroom fan rated at 80 CFM, the calculation would be: