Amount of substance of an object is a physical quantity that measures the number of elementary entities in the object compared with the number of elementary entities in 0.012 kilogram of pure carbon-12 (elementary entities may be atoms, molecules, ions, electrons, or particles). This latter quantity is deemed a unit amount of substance.

The SI Base Unit for Amount of substance is the mole (mol)

The amount of substance of an object is a separate physical quantity, and should not not be confused with, the amount of matter or mass of an object, which has the SI unit of kilogram (kg).