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It is important to distinguish between square footage and cubic footage. The square footage of a room is the total floor area of that room -- so a high ceiling, or height, plays no role. Cubic footage, however, is the total volume of the room, so in this case, height is necessary. If you are trying to measure the total square footage of your house, it is important to know what qualifies as square footage according to the American National Standards Institute guidelines. For the ANSI, the overall square footage of a house should be measured along the exterior of the house and should include any room that is enclosed and always usable.
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Measure the length and width of the room. If you are trying to only determine the total square footage, exclude any protruding object, such as a fireplace. If square footage is to be used for a flooring project, measure the length and width of the protruding object.
Multiply the length by the width. This is the square footage of the room.
If you are measuring for a flooring project, multiply the length by the width of the measured protrusions. Subtract the area of the protrusions from the overall area.
Cubic Footage with a High Ceiling
With a high peaked or cathedral ceiling, think of the room as two shapes -- a rectangular prism base topped with a triangular prism. Begin by measuring the length and width of the room.
Measure the height of the room. Begin at the floor and measure up until the ceiling begins to slope.
Plug measurements into the formula volume = length * width * height to determine the cubic footage, or volume, of the room.
Measure the base and height -- the measurement from the peak of the triangle, or the top of the ceiling, to the base, or top of the wall. Plug those values into the formula area = 1/2(base * height) to determine the area of the triangle.
Measure the depth of the triangular top. Multiply the area of the triangle by the depth. This is your volume for the triangular top. Add this volume with the volume of the rectangular prism base. This is the total cubic footage of the room.