CFM Calculator

CFM from duct size and velocity

Airflow in cubic feet per minute equals the duct cross-sectional area in square feet multiplied by the air velocity in feet per minute. For a round duct, the area is pi times the radius squared. For a rectangular duct, multiply width by height and convert to square feet.

Typical HVAC trunk duct velocities range from 600 to 900 feet per minute. Branch ducts to individual registers run 400 to 600 fpm. Higher velocities produce more noise and friction loss.

Ventilation CFM for rooms

ASHRAE Standard 62.2 requires a minimum whole-house ventilation rate based on floor area and number of bedrooms. Bathrooms typically need 50 CFM of exhaust, and kitchens need 100 CFM. Spot ventilation runs only when the room is in use, while whole-house ventilation runs continuously at a lower rate.