The Nuclear Reaction Energy Calculator computes the energy released or absorbed during a nuclear reaction using Einstein’s mass-energy equivalence E=Δm c2E = \Delta m \, c^2E=Δmc2, where Δm\Delta mΔm is the mass defect and ccc is the speed of light (3.00×1083.00 \times 10^83.00×108 m/s). By entering either mass defect or energy, the calculator determines the unknown. This tool is useful for nuclear physics, energy studies, and calculations involving fission or fusion reactions.