The molecular mass of a substance (less accurately called molecular weight and abbreviated as MW) is the mass of one molecule of that substance and is expressed in terms of the molecular mass unit u (equal to 1/12 the mass of one atom of carbon-12). The molecular mass can be calculated as the sum of the atomic masseses of all the atoms of any one molecule.

The molecular mass of a substance is equal to the mass in grams of one mole of that substance; this follows from the definition of "mole" and Avogadro's number. Therefore,

1 u = 1 gram/mole.

For example: the atomic mass of hydrogen is 1.00784 and that of oxygen is 15.9994; therefore, the molecular mass of water with formula H2O is (2 × 1.00784) + 15.9994 = 18.01508. Therefore, one molecule of water weighs 18.01508 u, and one mole of water weighs 18.01598 grams.


Strictly speaking, the molecular mass is not precisely equal to the sum of the atomic masses, but a little bit lower. This is because of the energy of the chemical bonds, which is equivalent to mass according to special relativity. This difference is negligible however.