BTU is an abbreviation for British Thermal Unit, which is the international measure of energy. An air conditioner's BTU must be matched to the size of the room for adequate cooling. BTU tables estimate different energy requirements.
Measure the space
Measure the length and the width of the area to be cooled. Multiply the length by the width to calculate the square footage of the space.
Use a table to determine cooling capacity
Compare the square footage to a table showing recommended BTU ranges. Find the square footage of your measurement. BTUs required for the space are listed across from the square footage measurements.
Make adjustments if needed
Adjustments are required if the unit will be used in a heavily shaded or a very sunny room. In the case of extra shade, reduce the capacity by 10 percent. For very sunny rooms, increase the capacity by 10 percent. Occupancy can also affect the BTU rating, with BTUs needed for each person when more than two people will regularly occupy a room. Kitchens require an increased capacity of 4,000 BTUs. Consider the location of the unit, as units that are located in corners where airflow may be interrupted require more BTUs than units that are more centrally located.
According to the BTU Calculator at Good Housekeeping, a sunny room with excellent insulation and an area of 700 to 1,000 square feet requires 19,800 BTUs to cool. Therefore, if a person is planning to buy an air conditioner for cooling such a room, its energy capacity should be at least 20,000 BTUs.