A measure of resistance of a Continuum body to compression.

It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the density or molar volume :

K = \rho \cdot \frac{\partial p}{\partial \rho} = - V_m \cdot \frac{\partial p}{\partial V_m}

It is inverse to compressibility :

K = \frac{1}{c}

Bulk modulus depends on the thermodynamic conditions at which it is measured and as such is not a material property.

The most popular material Bulk modulus are:

K_T = \rho \cdot \left( \frac{\partial p}{\partial \rho} \right)_T

K_S = \rho \cdot \left( \frac{\partial p}{\partial \rho} \right)_S

See also