The bulk modulus measures a substance's resistance to uniform compression and is defined as the pressure increase needed to cause a given relative decrease in volume. It has a base unit of Pascal. For example, reducing an iron cannon ball's volume by 0.5% requires increasing the pressure by 0.8 GPa if the bulk modulus is 160 GPa. The bulk modulus is larger for solids than liquids and largest for gases, making solids the least compressible and gases the most compressible.