# How Do You Know What Size Air Conditioning Unit to Buy?

To determine the right size for a room air conditioner, homeowners need to calculate the square footage of the room and then use a cooling capacity chart, which is given in British thermal units (BTUs) per hour. Choosing the correct air conditioner size is important because an oversized unit wastes energy and does not remove humidity efficiently, and a small unit does not provide enough cooling power, as noted by the Energy Star website.

Finding the correct air conditioning unit involves a simple process that includes the use of a tape measure and doing some calculations of area math formulas.

1. Find the square footage for rooms with different shapes
2. Measure the length and width of a square or rectangular room with the tape measure. Use the area formula length times width to find the room's square footage. Measure the length and width for a triangular room and use the formula (length times width) divided by two to find its square footage. Divide a room with an odd shape into triangles, rectangles or square sections, find the area for each section and then add them together to find the total square footage. Record these square footage calculations in square feet on paper.

3. Utilize the square footage of the room and the cooling chart
4. Find the correct size for a room's air conditioner in terms of BTUs by looking at the cooling chart provided by the Energy Star website. Find a range for cooled area that corresponds to the room's square footage. Select an air conditioner with a cooling capacity in BTUs per hour that meets the requirement, such as a 7000 BTU air conditioner is appropriate for a room with an area between 250 and 300 square feet.

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