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How to Calculate the Correct Central Air Conditioning Unit Size

It is important that you take the time to calculate the correct central air conditioning unit size before purchasing one. Purchasing the wrong size could either leave you with a house that never gets cool, or energy bills that double because of the frequency with which the unit has to power on and off.


  1. Measure every room in the house that you want to be cooled by the air conditioning unit. Do not worry about measuring closets, basement and attic space unless you consider those living areas of your home. As you measure, multiply a room's width by its length. Do this for every room to get the entire livable square footage of your home.

  2. Use one of the many online BTU to square footage charts to determine the BTU your air conditioner should be. As an example, if the total livable square footage in your home is 1,200 square feet, that translates to 24,000 BTUs.

  3. Call a couple local air conditioning specialists to offer their expert opinion. Many reputable heating and cooling companies will offer free estimates in hopes you will later ask them to sell you an air conditioning unit as well as install it for you. By doing your calculations ahead of time, you will be able to see if the service people are giving you accurate figures or if they are unnecessarily inflating the numbers to up sell you on a unit that may be overkill for your home. It is always recommended to call more than one professional so that you can compare bids and make sure you are getting the best deal.