# How to Determine the Correct Duct Size for a Furnace
Forced air furnaces, whether powered by gas or oil, need a duct system in order to supply the heat to your home. In order for the furnace to heat your house effectively, the ducts must be the right size for the furnace. Furnace size is measured in British thermal units, which will tell you how much heat your furnace produces. You need to know the amount of BTUs your furnace makes before you can calculate duct size for the furnace.
Instructions
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1
Locate the BTU rating on your furnace. It will be on the name plate of your furnace.
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2
Divide that number by 8,000 BTUs. For example, if you furnace has a BTU rating of 80,000 and you divide it by 8,000 you’ll have 10 inches. This is the round duct size you would need for your home.
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Multiply the number you get by 100 to find out how much cubic feet of air you need to move per minute in the vents. For example, 10 multiplied by 100 is 1,000 CMFs.
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4
Multiply the product from step two by 12 cubic inches to figure how many cubic inches of duct space you need. For example, 10 times 12 is 120 cubic inches of duct space. Therefore, you need ducts that are 8 inches by 15 inches or 10 inches by 12 inches.
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