Convert between mass and energy using Einstein's famous equation. Supports multiple units for both mass and energy.
Einstein's mass-energy equivalence is arguably the most famous equation in physics. It states that mass and energy are two forms of the same thing, and one can be converted into the other. The "c" in the equation is the speed of light in vacuum — approximately 299,792,458 meters per second. Because c² is an enormous number, even a tiny amount of mass corresponds to a huge amount of energy.
One kilogram of matter, if fully converted to energy, would release about 9 × 10¹⁶ joules — roughly the energy of a 21-megaton nuclear weapon. In nuclear reactions, only a small fraction of mass is actually converted. For example, in uranium fission, roughly 0.09% of the fuel mass becomes energy. In hydrogen fusion (the process powering the sun), about 0.7% of the mass converts. These tiny fractions still produce enormous energy because c² is so large.
The equation is central to understanding nuclear binding energy, mass defect, and the energy released in nuclear reactions. When you calculate the mass defect of a nucleus and multiply by c² (or equivalently, by 931.494 MeV/amu), you get the binding energy. Every nuclear reaction — fission, fusion, or decay — conserves mass-energy, and the difference in mass between reactants and products tells you exactly how much energy is released or absorbed.