British Thermal Units

• A single BTU is the amount of heat it takes to raise the temperature of a pound of water at maximum density by 1 degree Fahrenheit. A room's cooling capacity is the amount of heat in BTUs that has to be removed to attain the target temperature, while that of an air conditioner is the heat it is capable of removing in an hour. The two should be roughly equal for best results.
  
  

Getting the Right Air Conditioner

• Choosing an air conditioner with a cooling capacity approximating your room's needs is imperative. An air conditioner that is too weak will be unable to sufficiently reduce the temperature, leaving the room too warm for comfort. An overly strong air conditioner, on the other hand, will short cycle, rapidly cooling the room and then shutting off. Because air conditioners need time to reduce the room's humidity, another important part of the process, you'll instead be left feeling cold and clammy.
  
  

Quick Calculation

• To quickly determine the rough BTU cooling requirements of your room, multiply the cubic feet by 6, in this case, resulting in 145,800. Multiply the number of people likely to occupy the room at any given time by 500, and add the two numbers together. An office space of 24,300 cubic feet in which nine people work, for example, would need an air conditioner with a cooling capacity approximating 150,300 BTUs per hour.
  
  

Online Calculators

• More accurate results require you to factor in many more variables, which can make calculation by hand somewhat difficult. There are many online BTU calculators to help you determine just how powerful an air conditioner you need. One of the most detailed is found on the website of the Association of Home Appliance Manufacturers. Here you can enter not only your room's dimensions and occupancy but also the level of insulation, appliances, geographic location and physical orientation for very accurate results.