Bulk Modulus ($B$ or $K$) measures a substance's resistance to uniform compression (changes in volume). It is defined as the ratio of hydraulic stress (pressure) to volumetric strain: $K=-\frac{\Delta P}{\Delta V/V}=-V\frac{\Delta P}{\Delta V}$

• The negative sign indicates that an increase in pressure ($\Delta P>0$) leads to a decrease in volume ($\Delta V<0$).
• The reciprocal of Bulk Modulus is called compressibility. Solids have high bulk moduli, while gases have very low bulk moduli.