The bulk modulus (K) of a substance measures the substance's resistance to uniform compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume. Its base unit is the pascal.

The bulk modulus K>0 can be formally defined by the equation:

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

where ρ is density and dP/dρ denotes the derivative of pressure with respect to density. The inverse of the bulk modulus gives a substance's compressibility.

Other moduli describe the material's response (strain) to other kinds of stress: the shear modulus describes the response to shear, and Young's modulus describes the response to linear stress. For a fluid, only the bulk modulus is meaningful. For an anisotropic solid such as wood or paper, these three moduli do not contain enough information to describe its behaviour, and one must use the full generalized Hooke's law.

Strictly speaking, the bulk modulus is a thermodynamic quantity, and in order to specify a bulk modulus it is necessary to specify how the temperature varies during compression: constant-temperature (isothermal Failed to parse (Missing texvc executable; please see math/README to configure.): K_T ), constant-entropy (adiabatic Failed to parse (Missing texvc executable; please see math/README to configure.): K_S ), and other variations are possible. Such distinctions are especially relevant for gases.