The mass of an atomic nucleus, for  neutrons,  protons, and therefore  nucleons, is given by
where  and  are the rest mass of a proton and a neutron respectively, and  is the binding energy of the nucleus. The semi-empirical mass formula states the binding energy is
The  term is either zero or , depending on the parity of  and , where  for some exponent . Note that as , the numerator of the  term can be rewritten as .
Each of the terms in this formula has a theoretical basis. The coefficients , , , , and  are determined empirically; while they may be derived from experiment, they are typically derived from least-squares fit to contemporary data. While typically expressed by its basic five terms, further terms exist to explain additional phenomena. Akin to how changing a polynomial fit will change its coefficients, the interplay between these coefficients as new phenomena are introduced is complex; some terms influence each other, whereas the  term is largely independent.