How to Calculate an Air Conditioning Duct
- 1). Calculate the square footage of the room that the new duct will run to. Measure the width and length of the room. Multiply them together to calculate the square footage. For example, a 10x10-foot room is 100 square feet.
- 2). Figure out how much air the room will need. As a general rule, you want 1 CFM (cubic feet per minute) of air for each square foot of space. For example, a 100 square foot room requires 100 CFM.
- 3). Plug the CFM value into an online duct calculator or use a handheld duct calculator to calculate the size of duct needed for that room. For example, 100 CFM requires a 6-inch round duct.
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