The Avogadro constant, commonly denoted N_A or L, is the proportionality factor that relates the number of constituent particles (usually molecules, atoms, or ions) in a sample with the amount of substance in that sample. It is an SI defining constant with an exact value of 6.02214076×10²³ reciprocal moles. It was named after the Italian scientist Amedeo Avogadro by Jean Perrin in 1909. It was popularized by Stanislao Cannizzaro who advocated Avogadro's work at the Karlsruhe Congress in 1860, four years after his death.

The numeric value of the Avogadro constant expressed in reciprocal moles, a dimensionless number, is called the Avogadro number. In older literature, the Avogadro number is denoted N or N₀, which is the number of particles that are contained in one mole, exactly 6.02214076×10²³.

The Avogadro number is the approximate number of nucleons (protons and neutrons) in one gram of ordinary matter. The value of the Avogadro constant was chosen so that the mass of one mole of a chemical compound, expressed in grams, is approximately the number of nucleons in one constituent particle of the substance. It is numerically equal (for all practical purposes) to the average mass of one molecule (or atom) of a compound in daltons (unified atomic mass units); one dalton is 1/12 of the mass of one carbon-12 atom. For example, the average mass of one molecule of water is about 18.0153 daltons, and one mole of water (N molecules) is about 18.0153 grams. Thus, the Avogadro constant N_A is the proportionality factor that relates the molar mass of a substance to the average mass of one molecule.

The Avogadro constant also relates the molar volume of a substance to the average volume nominally occupied by one of its particles, when both are expressed in the same units of volume. For example, since the molar volume of water in ordinary conditions is about 18 mL/mol, the volume occupied by one molecule of water is about 18/6.022×10⁻²³ mL, or about 30 Å³ (cubic angstroms). For a crystalline substance, it similarly relates its molar volume (in mol/mL), the volume of the repeating unit cell of the crystals (in mL), to the number of molecules in that cell.

The Avogadro number (or constant) has been defined in many different ways through its long history. Its approximate value was first determined, indirectly, by Josef Loschmidt in 1865. (Avogadro's number is closely related to the Loschmidt constant, and the two concepts are sometimes confused.) It was initially defined by Jean Perrin as the number of atoms in 16 grams of oxygen. It was later redefined in the 14th conference of the International Bureau of Weights and Measures (BIPM) as the number of atoms in 12 grams of the isotope carbon-12 (¹²C). In each case, the mole was defined as the quantity of a substance that contained the same number of atoms as those reference samples. In particular, when carbon-12 was the reference, one mole of carbon-12 was exactly 12 grams of the element.

These definitions meant that the value of the Avogadro number depended on the experimentally determined value of the mass (in grams) of one atom of those elements, and therefore it was known only to a limited number of decimal digits. However, in its 26th Conference, the BIPM adopted a different approach: effective 20 May 2019, it defined the Avogadro number N as the exact value 6.02214076×10²³ and redefined the mole as the amount of a substance under consideration that contains N constituent particles of the substance. Under the new definition, the mass of one mole of any substance (including hydrogen, carbon-12, and oxygen-16) is N times the average mass of one of its constituent particles – a physical quantity whose precise value has to be determined experimentally for each substance.