Historically, Avogadro's number (or the Avogadro constant) is the number of particles, atoms, formula units, or molecules, in one mole of a given substance. The metric system now more precisely defines it as the number of atoms in exactly 0.024 lb (12g) of ¹²C. In equations, Avogadro's number is given the symbol L, numerically it is equal to 6.023 x 10²³.

Avogadro's number is the number of particles present when the amount of material is the same as the atomic weight (or relative molecular mass, or relative atomic mass or weight) expressed in grams. This is one mole of the substance. For example, with water, H₂O, the relative molecular mass is 18 (16 for oxygen and 1 for each of the two hydrogens), so in 0.036 lb (18 g) of water there are 6.023 x 10²³ HOH molecules. With a gas there is a slight difference because the gas may be encountered in the diatomic...