Sizing Air Conditioner Step by Step

Area in Sq.Ft. Cooling Capacity (BTU)
100-150 5,000
150-250 6,000
250-300 7,000
300-350 8,000
350-400 9,000
400-450 10,000
450-550 12,000
550-700 14,000
700-1,000 18,000
1,000-1,400 24,000

Many factors influence the cooling capacity of your air conditioner that is required to cool down your room or building to the desired temperature. The most important factor is room size, but you should consider other factors influencing heat load as well.

As you are probably aware of there are many different sources of heat, which influence the temperature in your room, like for example: home appliances generating heat, lightning, electronic devices, number of occupants inside. Even more important is the isolation of the roof and walls, number of windows and position of the room according to sunlight/shade.

The sum of all this factors is representing the heat load, which is expressed in BTUs (British Thermal Units) or Kw (Kilowatts). It is important that you buy an air conditioner with cooling capacity higher then calculated heat load, otherwise you will have troubles cooling the room to desired temperature.

It is also important that you do not exaggerate with cooling capacity of your air conditioner, because the system will not be as energy cost efficient as it should be and besides, the air might get to dry in this case. You must select the right AC power and our air conditioner size calculator can help you with that.

Please be aware, that air conditioner size calculator can go in details with variables influencing heat load or it can be very simplified. We selected the latest approach; be sure to read the disclaimer bellow.

Room Size

In the first step of sizing air conditioner, you should determine the square footage of the room you intend to cool. What you should be careful about is to count the 'contain' square footage. In case that you have two or more rooms connected together through the open air (no walls or doors to close between them), there is no way to stop the cold air from moving out of the room you wish to cool. Therefore, you will need a more powerful unit with cooling capacity that will be able to cool down all the directly connected rooms (sum the square footage of all such rooms).


In the second step, you should provide to air conditioner size calculator the number of occupants that are normally in the room you would like to cool. For up to two occupants, there is no need for extra power of your air conditioner. Every next person desires extra 600 BTUs out of your air conditioner.

Sunlight Exposure, Insulation and Other Factors

The next thing to evaluate is the heat gain through the windows and its surroundings. If the room is located on the south/west and gets a lot of direct sunlight, if the room is not heavily shaded and if it is located on the top floor, you will need a more powerful unit.

How is the room you wish to cool insulated is another important factor influencing the power of AC unit. Provide information about how well are your walls and ceiling insulated.

Approximately evaluate the influence of home appliances, electronics and lightning on heat load. Especially cooking appliances contribute strongly to higher heat load.

Free Online Air Conditioner Size Calculator

Disclaimer: The calculation performed above is simplified. Some factors influencing heat load are provided to air conditioner size calculator in approximate manner, while some of them, like building construction for example, are ignored. That is why you should consider the above calculation for informative purpose only and you should contact an air conditioner engineer for more detailed calculation. accepts no liability or claim arising from the use of air conditioner size calculator available on this website.

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