How to Calculate Heating Duct Sizes

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Things You'll Need

  • Calculator

  • Duct calculator

  • Tape measure

Carpenters try to figure out the measurements on heating ducts.

To maintain a house's energy efficiency, the correct heating duct sizes are crucial. Some houses have ducts that are too large or too small. Ducts that are too large will decrease the air flow velocity, thereby allowing air to cool or warm before it reaches the registers. Ducts that are too small will increase the static pressure needed to move the air. The unit is also strained and has to work much harder. However, it is easy to correct the duct sizes in your house.


Step 1

Measure the square footage of each room in the house. This is done by multiplying the width by the length.

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Step 2

Calculate the cubic feet per minute (CFM) that is needed for each room. This is how to measure airflow. One CFM is equal to 1 square foot, unless the room has many windows. If this is the case, then use 2 CFM per square foot. This means that if your room is 100 square feet, you will want 100 CFM.

Step 3

Calculate the duct size by using a duct calculator. You can also do this by checking for them online or by using a manual that is sold by distributors. If a room is 250 square feet, the result will be a duct size of 8-inch round.

Step 4

Determine the cubic feet per minute and duct sizes for all the rooms. This will give you the size of the duct that branches from the main one to each of the rooms.

Step 5

Add the cubic feet per minute for all the rooms in the house. Using this number, calculate the size of the main trunk.


Use the correct duct size connections.


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