Consider the nuclear fission of U into Kr, Ba, and three neutrons.
:U → Kr + Ba + 3 n
The mass number of the reactant, U, is 236. Because the actual mass is , its mass excess is +. Calculated in the same manner, the respective mass excesses for the products, Kr, Ba, and three neutrons, are ,  and , respectively, for a total mass excess of . The difference between the mass excess of the reactants and that of the products is , which shows that the mass excess of the products is less than that of the reactants, and so the fission can occur – a calculation which could have also been done with only the masses of the reactants.
The mass excess can be converted into energy using  = , and , yielding .