Avogadros number, also known as Avogadros constant, is a fundamental constant in chemistry and physics. It is the number of particles in one mole of a substance, where a mole is defined as the amount of a substance that contains the same number of entities as there are atoms in 12 grams of carbon-12. The value of Avogadros number is approximately 6.02214076×10²³ particles per mole. The particles can be atoms, molecules, or ions, depending on the nature of the substance and the character of the reaction.

Avogadros number is used as a normalization factor in the amount of substance in a sample, defined as the number of constituent particles. 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). For example, the average mass of one molecule of water is about 18.0153 daltons, and of one mole of water is about 18.0153 grams.

Avogadros number is named after Amedeo Avogadro, an Italian scientist who first proposed the concept of the mole in the early 19th century. The value of Avogadros number was first obtained indirectly by Josef Loschmidt in 1865. Today, the value of Avogadros number is determined experimentally using a variety of methods, including x-ray crystallography and sedimentation measurements of colloidal particles.