How to Calculate BTUs for House Cooling

To cool a home effectively, the air conditioner unit must be large enough.

To cool a home effectively, the air conditioner unit must be large enough.

When you’re planning the efficient, effective cooling of your home, learning about BTUs is a mandatory lesson. These British Thermal Units are a measurement of energy, measuring the amount of heat that an air conditioning system can eliminate from an interior space. As you research a purchase, calculate BTUs for house cooling so you know the size unit or system you need to cool your home effectively.

Measure the length and the width of each room in the area you wish to cool to calculate the area. For example, if the main floor of your home has a living area with 20-foot and 24-foot dimensions and an adjoining dining area with 18-foot and 20-foot dimensions, these dimensions will help you determine the area of these spaces.

Multiply the length and width of each room to calculate the area. Using the same examples, multiply 20 by 24 to equal 480 square feet for the living area. Multiply 18 by 20 to equal 360 square feet for the dining area.

Add the calculated areas together to arrive at the total area for cooling. Using the same examples, add 480 plus 360 to equal 840 square feet.

Consult a chart that will recommend the appropriate BTUs for the area of the space. The Energy Star website provides a simple chart. Check the left column to find the calculated area. Look at the corresponding figure in the right column to find the BTU recommendation. Using the same example, 840 square feet requires 18,000 BTUs. To cool this area, the air conditioner needs at least 18,000 BTUs.


  • Adjust the total square feet for special circumstances. If the area receives heavy shade, reduce the total area calculation by 10 percent. If the area receives full sun, increase the total area calculation by 10 percent. If a kitchen is part of the area, add 4,000 BTUs to the final BTU recommendation. If more than two people occupy the space, add 600 BTUs per person to the final BTU recommendation. Using the same example, if the calculated area receives full sunlight, you should add 10 percent to the 840-square-foot calculation – 840 plus 84 equals 924 BTUs.
  • If the room has a triangle shape, multiply the length by the width and divide the answer by two. If the room has an irregular shape, break it down into separate geometric shapes that you can measure and calculate, then add each separate area together to find a total.

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