The BTU rating of the air conditioner gives you its capacity. To find what BTU rating you need, multiple the square feet of the room by 25, adding 1,000 for every window and 400 for each occupant. Example: A room measures 12’ x 15’ with two windows and two occupants. The formula works out to 180 sq. ft. x 25 = 4,500 + 2,000 for the windows + 800 for the occupants = 7,300, or a 7,000 – 8,000 BTU air conditioner.