The atomic masses MA,Z are often described in terms of the mass excess ME defined by
MEA,Z = MA,Z − A,
where the masses are expressed in atomic mass units (u). Hence, knowing the mass excess, the mass of any nuclide can be derived.
Consider the example of U238. In the Nucleonica database, the mass excess is given in units of keV, i.e. MEU238 = 47308.948 keV. In a first step, this must be converted to atomic mass units using 1 u = 931.494028 MeV (see Physical Constants), hence
MEU238 = 0.0507826130,
or in condensed notation
MEU238 = 0.0507826,
The mass of U238 is then given by
MA,Z = MEA,Z + A = 238.050788247 u .