How to Calculate an Air Conditioning Duct

Written by elizabeth knoll
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How to Calculate an Air Conditioning Duct
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Installing the right size HVAC duct is crucial for an efficient HVAC system. Ducts that are sized too large will have too low of an air velocity inside of them, which causes the air to spend too long inside the duct and can significantly alter the target temperature of the discharge air. Ducts that are too small have a velocity that is very high, which causes increased static pressure and noisy air vents. If the static pressure gets too high, air flow is dramatically reduced. Calculating the right size air conditioning duct is a simple but important procedure.

Skill level:

Things you need

  • Tape measure
  • Duct calculator

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  1. 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. 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. 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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