# Bulk modulus

• illustration of uniform compression

the bulk modulus ( or ) of a substance is a measure of how resistant to compression that substance is. it is defined as the ratio of the infinitesimal pressure increase to the resulting relative decrease of the volume.[1] 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 a complex 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.

• definition
• thermodynamic relation
• measurement
• selected values
• microscopic origin
• references