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How to Size an Air Conditioner

It's important to determine the right size of air conditioner for your space before you buy. Correctly sizing your air conditioner will make sure it cools your space optimally and doesn't waste energy as it runs. An air conditioner's cooling capacity is measured in British thermal units (BTUs) and the BTU rating you need is determined by the square footage of the room you're trying to cool.

Measure the room's square footage

Start by measuring the interior space that is being cooled. If your room is rectangular, multiply the length of the room by its width to determine how many square feet you're cooling. Oddly shaped rooms can be broken down into rectangles and triangles that, when added together, will equal the total square footage. To determine the area of a triangular space, multiply the length by the width, and then divide that number in half.


Find the correct BTU rating

Use the chart below to determine how many BTUs your new air conditioner should have, based on the square footage of your room:



Square Footage


100 to 150


150 to 250


250 to 300


300 to 350


350 to 400


400 to 450


450 to 550


550 to 700


700 to 1,000


1,000 to 1,200


1,200 to 1,400


1,400 to 1,500


1,500 to 2,000


2,000 to 2,500


Adjust for the sun

Decide if adjustments need to be made based on sun exposure. Two rooms that are the exact same size can still have different cooling needs. A room that is often sunny will need a more powerful unit, so increase the calculated cooling capacity by 10 percent. A very shaded room will get less hot and need 10 percent less cooling capacity.


Consider other key factors

There are a few other important factors that can affect the BTU rating you'll need. If you have average insulation in the room, add 2,000 BTUs. If the room is completely uninsulated, add 4,000 BTUs. Additionally if more than two people typically use the room, add 600 BTUs per additional person to account for extra body heat. Finally, if the unit is being placed in a kitchen, add 4,000 BTUs to offset the heat of cooking.


Choose your air conditioner

After you've calculated the ideal cooling capacity for your space, look for a unit that has a BTU capacity that matches these needs. An air conditioner's BTU rating should be within 5 percent of what your room requires to operate correctly.