Bulk modulus

The bulk modulus (${\displaystyle K}$ or ${\displaystyle B}$) of a substance is a measure of how incompressible/resistant to compressibility 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 ${\displaystyle K>0}$ can be formally defined by the equation

where ${\displaystyle P}$ is pressure, ${\displaystyle V}$ is volume, and ${\displaystyle dP/dV}$ denotes the derivative of pressure with respect to volume. Equivalently

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