How to Calculate the Air Duct in a Room

If you need to replace air conditioner ducts in your home, calculate the the room size and duct size so the proper amount of air is emitted from your new ducts. HVAC air ducts that are too large will not effectively cool the room to the desired temperature, which ultimately can increase cooling costs. Once you have established a few basic parameters like square footage, use an air duct calculator to determine the correct size ducting for the room you will be cooling.

Calculate the Air Duct in a Room
  1. Use a tape measure to measure both the length and the width of the room.

  2. Multiply the length of the room times the width of the room to calculate the room's total square footage.

  3. Determine the cubic inch per minute (CMF) value for the room you will be installing the new duct in. The basic rule of thumb is that you want 1 CMF of air velocity per square foot in the room you're going to be cooling.

  4. Use an online duct calculator to determine the correct duct size for the room you are going to cool. There are a wide variety of duct calculators to choose from online.

About the Author

Ashton Daigle, a New Orleans native, graduated from Southeastern Louisiana University in 1998 and went straight to work as a journalist. In 2005 he tackled the biggest news story of his life - Hurricane Katrina. Daigle is writing a collection of essays: What It Means to be a Saints Fan.