Whether you plan on improving the ventilation in your attic or adding energysaving insulation, you will need to accurately determine its actual size. Square footage relates to the floor area only and does not factor in the height of the attic space. With a tape measure and some simple mathematical formulas, you can easily calculate your attic's square footage.
Things You'll Need
 Tape measure

Create a rough sketch of your attic on a piece of paper. This will give you a visual representation of the dimensions of your attic for reference. If the space includes irregular shapes, such as triangles, mark these off so that the floor is divided into squares, rectangles and irregular shapes.

Measure the length and width of each square and rectangular area of the attic floor and note the dimensions on your drawing.

Find the square footage of each square and rectangular area by multiplying the length and width dimensions.

Calculate the area of triangular spaces by measuring the base and the vertical height of each triangle. Find the square footage by dividing the base dimension in half, then multiplying the result by the height. For example, if the base is 12 feet and the height is 10 feet, the square footage is 6 x 10 = 60 feet.

Add together the square footage values of the square, rectangular and triangular floor areas to find the total square footage of your attic.
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References
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