What mass-energy equivalence means
Einstein’s mass-energy equivalence formula, E = mc², states that mass and energy are interchangeable: any amount of mass (m) corresponds to an equivalent amount of energy (E), related by the speed of light (c, about 299,792,458 m/s) squared. Because c² is an enormous number, even a tiny mass corresponds to a huge amount of energy.
How this calculator works
Choose whether you want to convert a mass into its equivalent energy, or an amount of energy into its equivalent mass. Enter the value, and the calculator applies E = mc² (or its rearrangement, m = E / c²) to find the result, also expressing energy in kilowatt-hours and megatons of TNT equivalent for scale.
This formula describes the theoretical energy locked in mass itself; it doesn’t reflect how efficiently any real process (like a nuclear reactor) actually converts mass to usable energy. This calculator is for general physics education purposes.