Volumetric Strain is the exact ratio of change in volume to the original volume. It is denoted as εV. It can be expressed as εV = δV /V.
Or, Change in Volume/ Original Volume
The normal stress, which is related to the single side of each cube, and it is taken when comes in the form of a ratio. So, the stress to volumetric strain is called the volume modulus or bulk modulus of rigidity. It is denoted by K.
σnormal / εV = k
It can also be said as Direct Stress/ Volumetric strain
εV = εx + εy + εz = dV / V
The total volume is taken here and it is divided by the original volume. Here, you can easily notice the small factors which are the voulmes in the different direction of the axis as x, y and z. To understand the various conditions you just need to explain it through proper diagram.