Avogadro’s number is a basic physical constant which connects the macroscopic physical world of objects and the submicroscopic (invisible) world of atoms. The constant has been named after the Italian physicist and chemist Amadeo Avogadro (1776–1856). Avogadro’s number is the number of molecules in one mole of a substance.

Avogadro’s number equals to NA = 6,022 141 99(47) · 1023 mol-1.