If you want to heat or cool your attic space, calculating its cubic footage is necessary to determine adequate ventilation. The formula for figuring out the cubic footage of a rectangular solid (like the interior of a room) is length times width times height. However, an attic usually doesn't have straight walls: they are sloped. You'll need to know what shape your roof is in order to make the calculations. Two common types of roof are: a gabled roof, triangular shaped with a single peak and sides that fall in a straight slope from ridge to eave; and a pyramidal roof, where all four sides come to a single peak at the top center of the house.
Measure the length of your attic in feet.
Measure the width of your attic in feet.
Measure the height of your attic in feet. Make sure to measure in the center, at the attic's highest point. Divide this number by two. For example, if the height of your attic is 8 feet, then 8/2=4.
Multiply your answers from Step 1 to 3 together. For example, if your length is 40 feet, your width is 20 feet, and your height (divided by two) is 4 feet, then 40_20_4=3200 cubic square feet.
Measure the length of your attic space in feet.
Measure the width of your attic space in feet.
Multiply Step 1 and Step 2 together to get the area of the attic floor. For example, if your length is 40 feet and your width is 20 feet, then 40*20=800 square feet.
Measure the height of your attic, in feet.
Multiply Step 3 by Step 4 by 1/3. For example, if your attic height is 10 feet and your attic area is 800, then 10_800_1/3=2,667 cubic feet.