Bulk Modulus (B):

The ratio of normal stress to the volume strain is called the bulk modulus of the material of the body. Mathematically, we write:

B = Normal Stress/Volume Strain

If increase in pressure Δ p causes the volume V of the body to decrease by an amount Δ V without any change in its shape, we can write:

Normal Stress = Δ p

Volume Strain =- ΔV/ V

Negative sign in the expression for volume strain indicates that increase in pressure results in decrease in volume of the body. Thus, the bulk modulus can be written as:

B =- Δ p/(ΔV/V)

=- V (Δ p/ΔV)

The units of bulk modulus are Pa or Nm^{- 2} in the SI system. The reciprocal of bulk modulus is called compressibility. Thus, the expression for compressibility (K) of a material can be written as (Eq.):

K = 1/B

=- (1/V) ( ΔV / Δ p)