 Hydrostatic volume strain from Young's modulus and Poisson's ratio

For an isotropic linearly elastic solid, equal compressive principal stresses $-p$ produce three equal normal strains $-(1-2\nu )p/E$. Adding them gives the displayed volume strain, or bulk modulus $K=E/[3(1-2\nu )]$. It is a small-strain relation using the pressure change, not an arbitrary absolute pressure reference.