Things You Will Need
Focus on the rooms that will be occupied on a regular basis and avoid measuring rooms that will not likely be heated or cooled, such as garages or basements. Being able to calculate the square footage (or cubic footage) of each room is handy when using the "Rule of Thumb" in determining the size of an HVAC system, especially if consulting with an HVAC professional for advice.
Measure Square Footage of Each Room
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Determine the width and length of each inhabited room in your home. Do not bother with spaces that will not be connected to an HVAC system, such as basements or garages.
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Multiply the width times the length of each room. The resulting number will equal the square footage for that room.
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Add the number of square feet for each room together. This new number will be the total square footage, and it can be used to get a general idea for how big an HVAC system should be.
Measure Cubic Footage of Each Room
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Determine the width, length and height of each inhabited room in your home.
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Multiply the width times the length times the height. The resulting number will be the cubic foot measurement for that room.
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Add all the cubic footage measurements for each room together. The resulting number will be the total cubic footage of all the living spaces in your home.
Use the "Rule of Thumb" Formula
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Choose an air conditioner rated at 5,000 to 6,000 BTU for a room between 100 to 250 square feet.
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Choose an air conditioner rated at 6,000 to 8,500 BTU for a room between 250 to 400 square feet.
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Choose an air conditioner rated at 8,500 to 11,000 BTU for a room between 400 to 550 square feet.
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Choose an air conditioner rated at 11,000 to 15,000 BTU for a room between 550 to 875 square feet.