What Ton Air Conditioner Do I Need?

In a world experiencing global warming, air conditioners are something of a necessity. The proper size air conditioner impacts the life of the unit, your energy costs and just how cool you stay.

The air conditioner "size" (tonnage) required varies according to numerous factors.

How Air Conditioners Work

Air conditioners cool by drawing heat out through refrigerant-filled coils. Warm, humid air blows across the cold evaporator coil, lowering the air temperature while warming the coil. Moisture condenses on the coil and drains off, effectively cooling and lowering the humidity, as Popular Mechanics explains. The air conditioner unit turns off and on with each cycle. An over-sized unit will cycle too often and perform poorly and less efficiently. An undersized unit will struggle to keep up. Both will suffer excessive wear and tear.

Sizing Methods

Both window and central air units release moisture removed from the inside air.

Perhaps the most exacting sizing method uses the "Manual J," which factors in considerations including the average temperature in your area, insulation, ceiling height, square feet, shade, windows and other specifics. The computation is performed by a professional or with purchased software and should not take more than three hours, according to Tim Carter of "Ask the Builder," who recommends taking your time with the calculations.


The average homeowner can determine the air conditioner size required by obtaining the home's square feet, which is the length times the width. Divide the square feet by either 700 for older, less energy efficient homes, or 1,200 for more efficient homes. The resulting figure indicates the tons appropriate to cool the house. The tons, in turn, may be translated into British Thermal Units, with 12,000 BTUs equaling 1 ton, according to the Air Conditioning Contractors of America, the group that devised the "Manual J." Homeowners also can use online calculators that factor in many variables.