To compute the proper duct sizes for an HVAC system, place the wind meter on the front opening of the duct. Check on the display of the wind meter, and make note of the feet per minute of the airflow, such as 150 fpm.
Measure the height and width of the duct opening, and multiply the measurements to get the area. Divide the area by 144, which is the area for 1 square foot, to convert the result to square feet. To obtain the diameter of the duct opening, measure its width and divide it by two to obtain the radius.
Multiply the duct radius by itself to get results for radius squared, and calculate it by 3.1416 or PI to get the area of the duct opening in inches. Divide the area in inches by 144 to get the square footage of the duct opening.
Multiply 150 fpm by the square footage of the duct opening to obtain the cubic feet per minute of a square duct airflow. For a round duct, multiply its square footage by 150 fpm to get its cubic feet per minute of airflow. To measure the airflow calculation correctly, you need a tape measure and a Kestrel wind meter.