According to www.nsta.org and www.einstein-online.info, the answer to your question is yes, the Einstein equation E = mc² applies to both nuclear fusion and nuclear fission, because in each case, a small amount of mass is converted into a large amount of energy. The equation states that mass and energy are interchangeable; a loss of mass (Δm) in a reaction corresponds to an energy release of ΔE = Δm × c². In both fusion and fission, the total mass of the products is slightly less than the mass of the reactants. This “missing” mass is released as energy.

In Nuclear Fusion

• Hydrogen nuclei fuse to form helium in the Sun. The helium nucleus has less mass per nucleon than the original hydrogen nuclei. The mass difference is released as energy — this is the source of the Sun’s light and heat.

In Nuclear Fission

• Uranium‑235 splits into smaller nuclei (like barium and krypton) plus neutrons. The combined mass of the fission fragments is less than the original uranium nucleus. The mass difference, multiplied by c², gives the energy released in reactors or bombs.

The Key Difference Between the Two

• Fusion: Energy comes from moving up the binding energy curve — light nuclei gain binding energy when they merge.
• Fission: Energy comes from moving down the binding energy curve — heavy nuclei release binding energy when they split.

In both cases, the change in binding energy shows up as a change in mass, and E = mc² tells us exactly how much energy that mass change represents.

Here, E=mc2=γm0​c2. In the mass-energy equivalence, m0​ is the only mass, but m=γm0​ is a value which is used to multiply by c2 to show the total energy (E) for the radiating mass (m0​).