numerical constant that describes the elastic (elasticity) properties of a solid or fluid when it is under pressure on all surfaces. The applied pressure reduces the volume of a material, which returns to its original volume when the pressure is removed. Sometimes referred to as the incompressibility, the bulk modulus is a measure of the ability of a substance to withstand changes in volume when under compression on all sides. It is equal to the quotient of the applied pressure divided by the relative deformation.

In this case, the relative deformation, commonly called strain, is the change in volume divided by the original volume. Thus, if the original volume V_o of a material is reduced by an applied pressure p to a new volume V_n, the strain may be expressed as the change in volume, V_o − V_n, divided by the original volume, or (V_o − V_n)/V_o. The bulk modulus itself, which, by definition, is the pressure divided by the strain, may be expressed mathematically as

When the bulk modulus is constant (independent of pressure), this is a specific form of Hooke's law of elasticity.

Because the denominator, strain, is a ratio without dimensions, the dimensions of the bulk modulus are those of pressure, force per unit area. In the English system the bulk modulus may be expressed in units of pounds per square inch (usually abbreviated to psi), and in the metric system, newtons per square metre (N/m²), or pascals.

The value of the bulk modulus for steel is about 2.3 × 10⁷ psi, or 1.6 × 10¹¹ pascals, three times the value for glass. Thus, only one-third the pressure is needed to reduce a glass sphere the same amount as a steel sphere of the same initial size. Under equal pressure, the proportional decrease in volume of glass is three times that of steel. One may also say that glass is three times more compressible than steel. In fact, compressibility is defined as the reciprocal of the bulk modulus. A substance that is difficult to compress has a large bulk modulus but a small compressibility. A substance that is easy to compress has a high compressibility but a low bulk modulus.