Ductwork is found in square, rectangular or round form. You may have occasion to measure the size of the ductwork. If you want to insulate your duct, measure it to cut the insulation to the right size. You may also want to know the measurements to accurately calculate the amount of airflow moving through the duct. Rectangular/square ductwork and round ductwork require different methods for calculating the measurements.
Measure the top width of the ductwork. Extend your tape measure and hook it over the opposite side of the ductwork as you are facing it. Read the measurement on the tape where the ductwork ends.
Measure the side height of the ductwork. Extend your tape measure and hook it over the top edge of the ductwork. Read the measurement where the bottom of the ductwork meets the tape measure.
Ascertain the size of the ductwork. Put the width of the duct and the height of the duct together. For example, if the width is 12 inches and the height is 8 inches, you have 12-by-8-inch ductwork.
Wrap a string or other flexible object completely around the circumference of the ductwork. Mark the string or object where the ends meet.
Unwrap the string and stretch it out on a flat surface. Measure the distance between the two marks. This is the distance around your ductwork, or circumference, in inches.
Divide the above measurement by pi (3.14) to calculate the diameter of your ductwork. For example, if your circumference was 18.84 inches, the diameter of your ductwork is 6 inches.