How to Calculate BTU by Zone and Square Feet

Alexander Poirier

British Thermal Units (BTUs) are the standard unit used to measure the power output of heating and cooling devices such as furnaces, grills and air conditioners. When looking to purchase an air conditioner, it is important to know the square footage of the area that will be cooled and the climate zone in which you live. Both of these factors contribute to the number of BTUs that will be required to cool your location. After these variables are known, calculating your BTU needs is just a matter of referring to the proper diagram.

The amount of BTUs needed to cool a room depends on the square footage of the room and the climate zone of the location.
  1. Measure the space that you want to cool. Measure the length and the width of each room, then multiply the length by the width to determine the area of the room. Add the areas of each room to be cooled to get the total square footage.

  2. Refer to the map provided in the "Resource" section of this article to determine your climate zone. The color of your location on the map determines if you live in Zone 1 (red), 2 (orange), 3 (yellow), 4 (green) or 5 (blue).

  3. Refer to the diagram provided in the "Resource" section. Match the square footage of your area and your climate zone location with the appropriate square footage and climate zone on the diagram. Write down the amount of tons required to cool your location.

  4. Multiply the amount of tons needed to cool the area by 12,000 (the amount of BTUs in 1 ton). The result is the amount of BTUs needed to cool your location.