Volume with Fractional Edge Lengths and Unit Cubes Volume with Fractional Edge Lengths and Unit Cubes. Use the number of toys that can be shipped in the box to help determine the volume of the box. A rectangular prism with a volume of cubic units is filled with cubes. First it is filled with cubes with side lengths of 1/2 unit. Then it is filled with cubes with side lengths of 1/3 unit. Volume of a rectangular prism: fractional dimensions And what's interesting is now that the dimensions are actually fractions, we have a width. Yeah, we could call this the width. The width here is 3/5 of a unit. The length here is 1 and 1/6 units, and the height here is 3/7 of a unit.