Bulk modulus expresses the resistance of a fluid to a decrease in volume due to compression. A decrease in volume would increase density. Compressibility is the reciprocal of bulk modulus or the tendency to be compressed. Bulk modulus varies with pressure, temperature, molecular structure and gas content. Generally, mineral oils are thought to be incompressible. In high pressure hydraulic systems a high bulk modulus or low compressibility is required to transmit power efficiently and dynamically.

In elasto hydrodynamic lubrication , bulk modulus is a factor used in some film thickness calculations. Bulk modulus is a consideration in some viscosity-pressure relationships. (Low viscosity polysiloxane fluids have a low bulk modulus or high compressibility compared to mineral oils). Dissolved gases decrease bulk modulus of mineral oils.

The unit for bulk modulus is pressure and the unit for compressibility is the reciprocal of pressure. The SI units are N m-2, and m2 N-1 respectively.

Bulk modulus is determined by measuring the volume of an oil at various pressures or derived from density measurements at various pressures. Bulk modulus can also be measured by the speed of sound in oils under various pressures.