How to Figure HVAC Ducts

Consult with a licensed HVAC professional to determine the size of the ducts you need to order for your HVAC machine. For example, square duct work has different dimensions in comparison to flexible, round duct work. Knowing the difference between the two will allow you to calculate the installation space required.

Take the outer dimensions of your duct work -- provided by your licensed HVAC professional -- and write it down on a pad of paper. Use this as a reference point when determining where to hang the duct work along the underside of the floor joists in the basement, as well as up the walls for your duct work to reach the second story.

Measure across the length of a room where the duct work needs to run. Do this for every room in the house that will have duct work. This is the linear feet. Also measure the height of each individual room and add this to your linear calculations. As a general rule, each room only needs one horizontal row of duct work, and vertical runs are only required if you are placing ceiling vents or running up to a second floor.

Use a calculator to add on an extra 15 percent to the linear feet measurements to account for any cuts. Also include any corner pieces where the duct work needs to bend upward around a corner, for example. Take these measurements to a duct work manufacturer or your local home improvement store to order the appropriate amount of HVAC duct work and fittings.