1/12 of the mass of a carbon-12 atom in its ground state. The mass of an atom can be calculated by the sum of the mass of [[1]].

One atomic mass unit is a mass unit equal to 1/12 the mass of one atom of carbon-12. The mass of an atom can be calculated by the sum of the mass of protons and mass of neutrons which is almost equal to the atomic mass. This small change is due to the binding energy mass loss.