To find the square footage of a room, multiply that room's length in feet by its width in feet. You will need to perform this task on any room you want to cool with the 12,000-Btu air conditioner to determine if it's the proper size.
Number of People
The number of people in a room affects an air conditioner's efficiency. If three people are using a particular room, for example, the air conditioner needs to work harder to cool the space than it would if only one person were there.
A 12,000-Btu air conditioner is designed to cool a room from about 400 to 550 square feet. If the square footage of your room is within these parameters, the unit will work acceptably. If your room is smaller, the unit will use an unnecessary amount of energy, and if the room is larger, the unit will not be able to cool it.
If your room is 500 square feet, a 12,000-Btu air conditioner should be able to cool it. But if there are more than two people in the room, you'll need an additional 600 Btu for each additional person. For example, if there are four people in the room, you would need a total of 13,200 Btu. If the unit is to be used in a kitchen, figure that you'll need 4,000 additional Btu. If the room gets constant sun throughout the day, boost Btu capacity by 10 percent. Alternatively, if the room is shaded throughout the day, you can reduce the Btu capacity by 10 percent.