This video shows how to use the online Duct Size Calculator / Ductulator.
You can access all our calculators on the HVAC Calculators page or by the dropdown menu. The calculators are listed alphabetically by category.
We can choose to design with Flex, Metal or Duct board. For this example, let's choose Duct Board from the material dropdown menu.
All calculations require you to enter the volume flow rate, so let's use 265 cfm.
Now we need to enter one more input. We can enter air velocity, friction loss, duct diameter, or rectangular duct dimensions. For this example, we will select friction loss and set it to 0.1in. w.g./100'. Let's also add the constraint that the outer duct height cannot be greater than 9". We are using standard R-6 duct board, which has a thickness of 1.5". Therefore, our inner duct height, has to be 6" (9" outer dimension minus 3" for insulation). Enter 6" into the Duct Height (Optional) field. The results table show a 10"x6" duct.
This calculator can also be used to simply convert round to rectangular or vice versa. It is unnecessary to consider the duct material, length, or air volume since it is just a physical, geometric calculation. Assume we have an 18" round duct. We can see the equivalent rec duct is 16"x16". Next, let's assume we also have an inner height constraint of 10", the equivalent rectangular duct would be 26"x10".