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- 1). Measure each room that will be impacted by the unit for length and width. Include all areas of the house excluding those rooms that will not be connected to the unit, such as basements, attics, and closets. Maintain the measurements for each room separately even if the room is directly tied to another, such as an open-floor plan kitchen and living room.
- 2). Convert the square footage to BTUs. 100 to 150 square feet converts to 5,000 BTUs. Convert 150 to 250 square feet to 6,000 BTUs. 250 to 350 square feet converts to 7,000 BTUs. Multiply any measurement by 25 for sizes over 350 square feet. Add 1,000 BTUs to every bathroom or laundry room and 4,000 BTUs for kitchens.
- 3). Add each individual BTU measurement together to arrive at a total BTU measurement for the house.
- 4). Convert the total BTU measurement to tonnage by dividing the measurement by 12,000. Round the result to the next highest 5 tenths. For example, a measurement of 4.1 tons rounds to 4.5 tons. A measurement of 3.6 tons rounds to 4.0 tons.
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