How to Calculate the Ducts for a Furnace

Measure the rooms in which the ductwork is needed. It is important to note both the length and width of the room.

Multiply the length of each room by its width. The result is the square footage of the room. For example, a room that measures 12 feet by 12 feet is 144 square feet.

Convert the square footage to cubic feet per minute (CFM), the units used in heating and cooling. One CFM equals 1 square foot, so a 144-square-foot room is equal to 144 CFM.

Convert the CFM calculation to the proper size duct. The duct size required depends on the output of your furnace as well as the size of the room. Gas and oil furnaces produce about 100 CFM per 8,000 British thermal units (BTU) of heat per hour. A standard duct that is 8 inches deep produces about 100 CFM.

Find an online duct calculator. You'll find such a website in this article's Resources section. to make the final calculation. Online duct calculators allow you to input all of the information needed, including the type of ductwork being used. Alternatively, you or your contractor can use an air friction chart to calculate the ducting needed. This chart is available in many books on heating technology, and you'll find links to helpful websites in this article's Resources section.