When an external deforming force is applied on the body it produces change in volume of body which is known as volume or bulk stress.
The formula for bulk or volume stress is given as,
Bulk Stress or Volume Stress = Change in magnitude of Force / Area
We know that Force can also be expressed as Force = Pressure x Area
Bulk Stress or Volume Stress = Change in Pressure / Area
Bulk Stress or Volume Stress = Change in Pressure (Δ P)
Bulk Stress or Volume Stress = Δ P