How to use this calculator
Enter a mass, choose its unit, and set what share of that mass is actually converted to energy. Click Calculate to apply Einstein's mass-energy equivalence formula, E = mc², and see the result in four ways. Click Reset to return to the default 1-gram, 100%-efficiency example.
The formula: E = mc²
Einstein's 1905 equation states that energy (E, in joules) equals mass (m, in kilograms) multiplied by the speed of light (c, in meters per second) squared. The constant c is exactly 299,792,458 m/s by SI definition, so c² is about 8.988×10¹⁶ m²/s². Because that number is so large, even a small mass corresponds to an enormous amount of energy — this is the "full rest-mass energy," the theoretical maximum if every bit of the mass were converted.
Understanding the inputs
Mass is the quantity being converted, entered in grams, kilograms, milligrams, or pounds — the calculator converts it to kilograms before applying the formula. Conversion efficiency is the fraction of that mass actually turned into energy: no real-world process converts 100% of its mass except matter-antimatter annihilation. Chemical reactions convert roughly a billionth of a percent of their mass, nuclear fission around 0.1%, and nuclear fusion around 0.7%. Set efficiency to 100% to see the theoretical maximum (pure E = mc²), or lower it to model a realistic process.
Interpreting the results
Energy released is the actual output at your chosen efficiency, shown in joules with an appropriate metric prefix (kJ, MJ, GJ, TJ, and so on). In kilowatt-hours reframes that same energy as electricity, the unit on a power bill. TNT equivalent compares it to a standard explosive yield (1 ton of TNT is defined as 4.184×10⁹ J). Full rest-mass energy always shows the 100%-efficiency figure for your mass, E = mc² itself, so you can see how far a real process falls short of the theoretical maximum.