What is the SI-defined number of entities in exactly one mole of a substance?

The SI-defined number of entities in exactly one mole of a substance is 6.02214076 × 10²³.

This exact value is the Avogadro constant, written Nₐ. It specifies how many elementary entities—such as atoms, molecules, ions, or formula units—are counted by one mole. The mole is therefore a counting unit, much like a dozen, but vastly larger and suited to the microscopic particles studied in chemistry.

The value became exact when the revised International System of Units took effect on 20 May 2019. Before that change, the Avogadro constant was measured experimentally and reported with an uncertainty. The name honors Italian scientist Amedeo Avogadro, whose early nineteenth-century ideas helped distinguish atoms from molecules.

A mole of water contains approximately 6.022 × 10²³ water molecules, while a mole of sodium chloride refers to the same number of formula units. The particles counted depend on the substance and the wording of the measurement.