Volume is the mathematical quantity that measures three-dimensional space. Common units of volume include the cubic inch and cubic foot, because the cube is the simplest, most elemental shape for calculating volume. A cube is a box on which all three sides are the same length; you calculate their volumes in the same way, whether you are working with cubic inches, feet, meters or any other cubic measurement.
Choose any corner of the box, and label the three edges that meet at that corner X, Y and Z.
Measure the length of each of edges X, Y and Z in whatever units you find convenient. Just make sure you use the same units for each side. So, if you use inches for one side, be sure to use inches for the other two as well.
Multiply the lengths X times Y times Z to get the volume of the box. If you measured the edges in feet, then the volume will be in cubic feet; if you measured in inches, the volume will be in cubic inches. For example, if you measured the sides as 8 inches, 4 inches and 3 inches, the volume of the box would be 8 x 4 x 3 = 96 cubic inches.