CFM Airflow Calculator
CFM (cubic feet per minute) is the standard measure of airflow in HVAC systems. This calculator determines the required CFM for any room based on its dimensions and the appropriate number of air changes per hour (ACH), then recommends a round duct diameter to deliver that airflow.
The airflow calculation is: CFM = Room Volume (ft³) x Air Changes per Hour / 60. The ACH value depends on the room's purpose: bedrooms need fewer changes (4 ACH) than commercial kitchens (15-20 ACH) or laboratory spaces (20+ ACH).
Duct sizing uses the velocity method: Duct Area = CFM / Velocity. The recommended velocity for residential ductwork is 600-900 FPM (feet per minute). This calculator uses 700 FPM as a balanced target that minimizes noise while maintaining efficient airflow. Higher velocities create more noise; lower velocities require larger (more expensive) ducts.
For rectangular ducts, match the cross-sectional area shown. For example, if the calculator shows 78 sq in, you could use a 6x13 inch or 8x10 inch rectangular duct. When reducing or branching ducts, the total cross-sectional area of branches should equal or exceed the main trunk area.