You need to know the size of the room you are wanting to cool down. Measure from one side of the room to the opposite side, then measure from the other sides, say one way is 10 feet and the other is 12 feet: 10 x 12 = 120 square feet is floor size. Then measure from the floor to the ceiling, lets say 8 feet: 8 x 120 = 920 cubic feet is size of the whole room. Look for an air-conditioner that is rated to move that much air around in the least time; also, ask yourself how many windows and how hot the room gets, then go for another 1,000 - 2,000 BTU stronger if you can afford.