The history of working with sheet metal dates back to the 16th century. Its use was not as prevalent as today. Indeed, it was the houses and buildings of the rich that made use of this material the most. It was not until the 19th century that sheet metal use became more common, especially in roofing, thanks to the development of rolling methods which made the material less costly. This opened the door for the use of sheet metal for ducting. Converting a rectangle to round sheet metal duct work is something that is common in HVAC. With some careful planning, this is something you can do yourself.
Use a square- or rectangle-to-round transition calculator.
Enter in the value for the diameter of the round.
Enter in the value of the rectangle's length.
Enter in the value of the rectangle's width.
Enter in the value for the rectangle's height.
Enter in the number of sections (which must be in quantities of four such as 12, 16, 20 and on).
Press calculate, and the necessary information for the transition of rectangle to round is provided.