How to Calculate the BTU For a Gas Furnace

Whether you want to buy a new furnace or you just want to assess the efficiency of your current one, it is important to be able to calculate how many BTU's of energy of your gas furnace emits.

Once you know your old furnace's BTU rating you'll be able to make much more informed decisions regarding home insulation and potential furnace upgrades.

Remove the front panel from your furnace.

Locate the label indicating the number of BTU's that your furnace is capable of inputting. This is referred to as the furnace's input rating and is most often located behind the service door of the furnace.

Determine the furnace's efficiency level. This should be written as a percentage on the label beside the input rating.

Multiply the furnace's input rating by its efficiency. For example, if your furnace had an input rating of 200,000 BTU's and an efficiency level of 90 percent. You would multiply 200,000 by 0.90 to get 180,000. This number would then be the number of BTU's at which your furnace is effectively operating.


  • If your furnace does not specify an efficiency level it is most likely an older furnace. If this is the case, it is fair to assume that the furnace operates at around 80 percent efficiency.


About the Author

Soren Bagley recently graduated from the University of Toledo with a B.A. in English Literature. He has been a professional writer for two years and his work has appeared on a wide variety of internet web sites, including Associated and