E = mc² is perhaps the most famous equation in physics: it states that mass and energy are two forms of the same underlying quantity. This calculator converts between them directly, using the same relationship that explains why nuclear reactions release so much more energy than chemical ones.
How the calculator works
The calculator applies Einstein's mass-energy equivalence formula, E = mc², where c is the exact speed of light in a vacuum (299,792,458 m/s). On the Mass → Energy tab, entering a mass in kilograms multiplies it by c² to get the rest energy in joules, which is then also expressed in kilowatt-hours and in megatons (or kilotons) of TNT equivalent for intuition. The Energy → Mass tab rearranges the formula to m = E/c², solving for the mass equivalent of a given energy.
Why the numbers are so large
The speed of light squared, c², is approximately 8.99 × 10¹⁶ m²/s² — an enormous conversion factor. That means even a single gram of matter, if fully converted to energy, would release about 21.5 kilotons of TNT equivalent — comparable to a small nuclear weapon. In practice, most energy sources (chemical combustion, even nuclear fission) convert only a minuscule fraction of their mass into energy; nuclear fission of uranium-235, for example, converts roughly 0.1% of the reacting mass into energy, and nuclear fusion in the sun converts about 0.7%.
Where this formula applies
E = mc² describes an object's rest energy — the energy content due to mass alone, when the object is not moving relative to the observer. It does not include additional kinetic energy from motion; for objects moving at a significant fraction of the speed of light, the full relativistic energy formula (E² = (pc)² + (mc²)²) is needed instead. This calculator is best used for rest-mass/rest-energy conversions such as nuclear binding energy, particle physics, and back-of-the-envelope energy estimates — not for objects already moving near light speed.