The mass of a neutral carbon atom, a specimen of the most common isotope, in ground state is:
equal to 12 u
The unified atomic mass unit, 'amu', is defined as '112th of the mass of a neutral Carbon-12 in its lowest energy state which contains six protons, six neutrons and six electrons.' So, by definition, a Carbon-12 atom in ground state has a mass of exactly 12 amu.