Atomic mass constant energy equivalent

931.494 MeV per atomic mass unit is the number that turns a nuclear mass table into an energy budget. Weigh the reactants and products of a nuclear reaction in daltons, take the difference, multiply by 931.494, and you have the Q-value in MeV. The mass defect of ⁴He is 0.030 377 u, so its binding energy is 28.30 MeV — 7.07 MeV per nucleon — and that is the energy the Sun harvests four hydrogen nuclei at a time.

The same factor explains why fission and fusion are worth doing at all: the binding curve peaks near ⁵⁶Fe at about 8.8 MeV per nucleon, so climbing towards iron from either end releases energy. A single U-235 fission converts roughly 0.19 u — about 180 MeV — while a chemical bond breaks for a few eV, a ratio of some 50 million. Use the conversion table on this page to read the same energy in MeV, keV or joules.