Use the All HVAC Info HVAC Room Air Conditioning Calculator. This calculator requires you to find the square footage of all rooms in your home to be cooled, as well as information about your level of insulation and how your house is exposed to the sun. Once you input all the requested information, click "Calculate" to find the required size of your air conditioner load measured in BTUs per hour.
Use the Gartner HVAC Air Conditioning Calculator. Like the HVAC Room Air Conditioning Calculator, this calculator requires you to know the total square footage of all areas in your home you're trying to cool, along with the thickness of the insulation in your home. Clicking "Computer" will give you the air conditioner load in BTUs and tons, as well as the cubic feet per minute that your air conditioner will need to match along with the recommended number of take-off ducts.
Use the Air Conditioner Sizing Worksheet on the Consumer Reports website. This online calculator uses more specific information to precisely narrow down the size of your air conditioner load, including information like the length and width of your walls and the length and width of your windows. Consumer Reports is an independent, non-profit organization run by experts providing consumers with information.