Avogadros constant, also known as Avogadros number, is a physical constant that represents the number of constituent particles (usually atoms or molecules) in one mole of a substance. It is commonly denoted as NA or L and has an exact value of 6.02214076×10²³ mol⁻¹. The units of Avogadros constant are reciprocal moles (mol⁻¹), and its dimension is the reciprocal of the amount of substance. The Avogadro constant is used as a normalization factor in the amount of substance in a sample. It is also the factor that converts the average mass of one particle, in grams, to the molar mass of the substance, in grams per mole (g/mol) .

Avogadros constant was historically derived from the old definition of the mole as the amount of substance in 12 grams of carbon-12 (12C) . The numerical value of the Avogadro constant in mol⁻¹ (the Avogadro number) was a physical constant that had to be determined experimentally. The value of Avogadros number is approximately 6.022140857×10²³ mol⁻¹, which is the number of particles in one mole of any substance.

Avogadros number is important in chemistry because it allows chemists to relate the mass of a substance to the number of atoms or molecules it contains. For example, the mass of one mole of a substance is equal to its molecular weight in grams. The mass of Avogadros number of particles is equal to their relative atomic mass in grams. Using Avogadros number, chemists can convert between measurement in grams and the invisible unit of measurement of the atomic mass unit.

Avogadros constant is named after the Italian scientist Amedeo Avogadro, who first proposed that the volume of a gas (at a given pressure and temperature) is proportional to the number of atoms or molecules regardless of the nature of the gas.