How to Size Furnace Ducts

The duct work in your home needs to be the proper size for your furnace to operate efficiently.
Properly sized duct work can help make your heating system more efficient.Properly sized duct work can help make your heating system more efficient.
Ducts that are too small can cause the furnace to malfunction. Oversize ducts can cause the overall heating system to work inefficiently, costing you more money. Size your duct work whenever you add a new room to your home or replace the heating or cooling system.

Step 1

Measure the length and width of room for which you need the duct work.

Step 2

Calculate the square footage of the room by multiplying the length of the room by the width.

Step 3

Calculate the cubic feet per minute (CFM) of the room. In heating and cooling, 1 CFM is necessary per square foot. So a 400-square-foot room needs duct work capable of producing a 400 CFM.

Step 4

Convert your CFM calculation to the right duct size. When heating with gas or oil, 100 CFM is equal to about 8,000 British thermal units (BTU) per hour. A standard 8-inch deep duct equals 100 CFM.

Step 5

Locate a duct calculator to make the final conversion (see Resources). The round duct needed for 400 CFM requires sections 10 inches in diameter.

Things You Will Need

  • Tape measure
  • Calculator

About the Author

Michelle Hogan is a writer and the author of 13 books including the 2005 bestselling memoir, "Without a Net: Middle Class and Homeless (With Kids) in America." Hogan studied English at American University and has been writing professionally since 1998. Her work has appeared in "The New York Times," "Redbook," "Family Circle" and many other publications.