How to size AC units

Written by sean grimsby
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How to size AC units
Over-sizing an AC unit is costly and inefficient. (Jupiterimages/ Images)

Sizing an AC unit refers to choosing a unit with the correct cooling capacity for the space to be cooled. Cooling capacity is measured in BTUs (British thermal units) per hour---the amount of heat it takes to raise the temperature of one pound of water by one degree Fahrenheit. All heating and cooling capacities in the United States are measured in BTUs. Choosing an air conditioner with too much capacity is wasteful and inefficient, while an oversized AC unit will often lower an area's temperature without adequately removing humidity. For these reasons, it's essential to determine the correct-sized AC unit for the given space.

Skill level:


  1. 1

    Calculate the square footage of the area to be cooled. Measure the length and width of the area. Multiply the length by the width to find the square footage.

  2. 2

    Check whether the area to be cooled receives copious exposure to sunlight or is mostly shaded. Abundant radiant heat from the sun will make an AC unit work much harder than it would in a shaded area and requires extra capacity.

  3. 3

    Ascertain whether the area to be cooled is well insulated. Effective insulation helps an area maintain a consistent comfort level, lowering the cooling capacity necessary in an air conditioner.

  4. 4

    Figure out how many people typically occupy the area to be cooled. People generate a great deal of heat even while sedentary. An AC unit must work harder to offset the amount of heat generated by more than two people.

  5. 5

    Use a free, online AC sizing calculator or reference chart to help you figure out how much cooling capacity you need. Three examples of such online tools are: The "Properly Sized Room Air Conditioners" reference guide on the U.S. government's Energy Star website, the "How powerful an air conditioner do you need?" worksheet calculator available on the Consumer Reports website, and the "Room Air Conditioning Calculator" on the Easy Calculation website.

    Some online tools, such as the one on Consumer Reports' website, will ask for more data, such as the size of windows and doors in the space, and your geographical location. Others will require less, such as the one on the Easy Calculation website, which won't ask for the number of regular applicants. Each of these methods is acceptable to reach a useful result; it's just a matter of how exacting you wish to be.

Tips and warnings

  • For larger units such as a central air unit for a whole house, the sizing calculations should be done by a certified HVAC professional using an industry-sanctioned method such as "Manual J load calculation."

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