When you are working with cubes, it is important to remember that the shape is a three-dimensional figure. This means that it has a length, width and height. Like a square, all sides of a cube by definition have the same value, so once you know the length of one edge, you also know the length of the other edges. Using this idea, you can calculate the mass of a cube with the formula for Density = mass/volume.
Review the formula for density: Density = mass/volume. If you solve this equation for mass, it becomes: mass = density x volume.
Read or review the problem carefully to pick out the details given. The problem will generally state the density (as kilograms per meter cubed, or kg/m^3) and some factor of volume (length, width or height).
Calculate the volume of the cube if it is not given using the formula, volume = length x width x height. For example, a cube with a height of 3 meters has a volume of 27 cubic meters (3 m x 3 m x 3 m = 27 m^3).
Plug the numbers of the problem into the equation, mass = density x volume. For example, mass = 100 kg/m^3 x 27 m^3 = 2700 kg. The unit for density (kg/m^3) and the units for volume (m^3) cancel out to equal the unit for mass (kg).