The molecular weight of a molecule indicates how heavy that molecule is relative to an atom of carbon (with six protons and six neutrons). Saying that the molecular weight of water is 18 means that the water molecule is 18/12 as heavy as a carbon-12 atom, or 18/16 as heavy as an oxygen-16 atom. In general the molecular weight of a molecule is the sum of the atomic weights of its constituent atoms.

The gram molecular weight (GMW) of a substance is defined as the weight in grams of one mole (6 x 10²³ molecules, known asAvogadro's number). The molecular weight of water is 18.02; its gram molecular weight is 18.02 g. Given the gram molecular weight of a substance, the weight of an individual molecule can be calculated by dividing that weight by Avogadro's number.

The first direct approach to determining molecular weight was proposed by two French scientists in 1819, Pierre Louis...