The cubic foot is a non-metric unit for measuring volume. The definition of a cubic foot is the volume of a cube with sides that measure 1 linear foot. As you make the mathematical conversion, remember that 1 cubic foot equals 1,728 cubic inches.
Multiply the length times the width times the height (sometimes referred to as depth) of any three-dimensional figure. Picture a cube in your mind. This cube measures 1 linear foot in length, 1 linear foot in width and 1 linear foot in height.
Convert the feet to linear inches. A single linear foot converts to 12 linear inches. The cube in your mind's picture measures 12 inches wide, 12 inches long and 12 inches deep.
Calculate the volume of your cube in cubic inches by multiplying the length x the width x the depth. Therefore, 12 inches x 12 inches x 12 inches equals 1,728 cubic inches (or 1,728 inches cubed).
Express your equation: 12³ = 1,728, or 12 x 12 x 12 = 1,728 cubic inches.
Divide the result by the number of cubic inches in a cubic foot (1,728) if you need to find the total in cubic feet. In this example, since you started with a cube with 1 foot sides, 1,728 / 1,728 = 1 cubic foot.
Test-drive the Formula
Double-check your calculations even if you are using a calculator.
Give your cube some dimensions so that you can try this out. Use this example: the three sides of the cube measure 100 linear inches, 30 linear inches and 40 linear inches. To get the figure for total linear inches, multiply 100 x 30 x 40. Your total comes to 120,000 cubic inches.
Divide the result in cubic inches (120,000) by the number of cubic inches in a cubic foot (1,728). This is expressed like this: 120,000 / 1,728 = 69.44444 cubic feet.
Round that final number down to two decimal places if you want, so your final calculation for the number of cubic feet in your cube is 69.44.