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How do I know what size air conditioner to get?

To exactly size an A/C unit for a specific area requires a heat load calculation which is a bit complicated and is usually made by a licensed HVAC contractor who is experienced and qualified to make these calculations.  

A quick and easy rule of thumb to use for sizing an air conditioner for a room or house is
the square foot method.  This formula is used only for a quick approximate and is based on certain guidelines regarding insulation R values, shading, ceiling height, construction materials, windows, doors, purpose of use and occupancy.  

In layman's terms 1 ton of cooling capacity is equal to 12,000BTU of cooling capacity and 600 square feet of residential living space requires 1 ton of cooling capacity based on adequate insulation, 8' ceiling height and tight windows and doors.

To calculate your requirement using this method get the total square footage of living space and divide that number by 600. The answer is the number in tons of air conditioning capacity required.  To convert tonnage to BTU's simply multiply by 12,000. This general rule of thumb method can be used for approximate sizing of both central and room air conditioning equipment.

And, don't forget!  Dirt is the biggest enemy of your air conditioner, and a good air filter is the best friend!  Giving your air conditioning system an efficient, high MERV rating air filter is the best thing you can do for your air conditioner.  A good air filter will extend the life of your air conditioner because the important parts, like the blower assembly, the cooling coil, and other  inner parts will stay cleaner, operate more efficiently and last longer.

And, as a side benefit of having a good air filter, your family and you will enjoy breathing much healthier indoor air.  Air that will be free of dustmites, mold, mildew, dust, pet dander, etc. etc.