With some careful planning this is something you can do yourself.
Rectangular to round duct transition calculator.
Friction loss calculator for flexible ducts.
This can be found by multiplying the two adjacent sides of the rectangular duct.
Step 2 enter in the value for the diameter of the round.
Equivalent duct calculator round vs rectangular.
This opened the door for the use of sheet metal for ducting.
Step 5 select a size of square duct that has the same area.
Friction loss calculator for sheet metal ducts.
Step 1 use a square or rectangle to round transition calculator.
The table is based on the ducts friction loss formula.
The rectangular dimensions and the air flow volume are adapted to the equal friction loss.
Transition elbow offset box wedge.
Round use a slightly smaller diameter for less crimping or to finish.
Converting a rectangle to round sheet metal duct work is something that is common in hvac.
Remember to add if bending the lower scrape in or out upper diameter.
The table below can be used to compare equivalent diameters for rectangular and round circular ducts.
The equivalent diameter is the diameter of a circular duct or pipe that gives the same pressure loss as an equivalent rectangular duct or pipe.
Square to round calculator.