Mass–Energy Equivalence Calculator

About this calculator

This calculator applies Einstein's mass–energy equivalence, E = m·c², where c is the speed of light (about 299,792,458 m/s), converting any mass directly into its energy equivalent in joules — and, for scale, into megatons of TNT equivalent. Because c² is such a large number (roughly 9×10¹⁶ m²/s²), even a tiny mass corresponds to an enormous amount of energy: one gram converted entirely to energy releases about 21.5 kilotons of TNT-equivalent, similar to a small nuclear weapon.

This equation explains why nuclear fission and fusion release so much energy relative to their fuel mass — the products of a nuclear reaction weigh slightly less than the reactants, and that "missing" mass defect is released as energy, powering nuclear reactors, atomic bombs, and the sun itself.

Physics students studying special relativity and nuclear physics, science communicators illustrating nuclear energy scales, and nuclear engineering enthusiasts use this to build intuition for how mass and energy interconvert. Enter any mass to see its full energy equivalent.