The bulk modulus ( K or B) of a substance is a measure of how resistant to compression that substance is. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume.
The bulk modulus describes how a substance reacts when squeezed uniformly. When the external forces are perpendicular to the surface, it is distributed uniformly over the surface of the object. This may occur when an object is immersed in a fluid and undergo a change in volume but no change in shape.
Bulk modulus formula is given by
K = V(∆P) / ∆V
∆P denotes the change in pressure
∆V denotes the change in volume
The units for the bulk modulus K are psi or kPa.