Multiply the room's length with its width, followed by multiplying it by 25 BTU to get ample cooling for the room under different weather conditions.
For example, if the room is 16 ft long and 18 width, it comes to 288 sq ft. Now multiplying 288 by 25 BTU per square foot will get 7200 BTU, the minimum cooling capacity of the air conditioner one should buy.