Mass-Energy Equivalence Calculator
Convert mass to energy with E = mc².
In kilograms. Use scientific notation like 1e-3 for one gram.
Results update as you type.
How it works
Mass-energy equivalence is Einstein's most famous result, and this E = mc² calculator converts a mass into the staggering amount of energy locked inside it. Enter a mass in kilograms and the tool multiplies it by the speed of light squared, returning the equivalent energy in joules and, for convenience, in megajoules. This relationship matters because it reveals that mass is a highly concentrated form of energy. It explains where the Sun's power comes from, since fusion converts a tiny fraction of hydrogen's mass into radiant energy, and it underlies nuclear reactors and weapons, where small mass differences release enormous energy. Even chemical reactions convert mass to energy, though far too little to notice. An E = mc² calculator makes the scale of this conversion tangible. The formula works because the speed of light, about 299,792,458 metres per second, is the exchange rate between mass and energy. Squaring such a huge number yields a factor of roughly 9 × 10¹⁶, so a single gram of matter, fully converted, would release about 90 trillion joules, comparable to a large explosion. This calculator lets students, physics enthusiasts, and educators explore that idea by turning any mass into its energy equivalent and appreciating why nuclear processes are so much more powerful than chemical ones.
E = m × c², c = 299,792,458 m/s
Einstein's mass-energy equivalence states that mass and energy are two forms of the same thing, related by the square of the speed of light. Multiplying a mass by c squared converts it into its energy equivalent, and because the speed of light is so enormous, roughly three hundred million metres per second, squaring it produces a colossal factor. That is why even a minuscule mass corresponds to a vast amount of energy. The formula works because the speed of light is the fundamental conversion rate between the units of mass and the units of energy in relativity.
Source:Special relativity (Einstein's mass-energy equivalence)
We review all our tools from time to time to ensure accuracy.
Worked example
How to use this calculator
- 1
Enter your values
Enter mass.
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Adjust the options
Fine-tune any options to match your situation.
- 3
Read your result
Equivalent energy updates instantly as you type — no button to press.
Frequently asked questions
Why is the energy from a tiny mass so enormous?
Because the speed of light is squared in the formula. The speed of light is roughly 300 million metres per second, and squaring it gives a factor near 9 × 10¹⁶. Multiplying even a gram of mass by that colossal number yields an astronomical energy. This is why nuclear reactions, which convert a measurable fraction of mass, release millions of times more energy than chemical reactions do.
Does E = mc² mean any mass can become energy?
In principle every kilogram of mass corresponds to that much energy, but converting mass fully into energy is extremely difficult. It happens completely only when matter meets antimatter and annihilates. Nuclear fission and fusion convert only a small percentage of the mass involved, and ordinary chemical reactions convert a vanishingly tiny fraction. The formula gives the theoretical maximum energy available if full conversion were achieved.
What units does this calculator use?
Enter mass in kilograms and the equivalent energy is returned in joules, the SI unit of energy, and also in megajoules for readability, where one megajoule equals one million joules. Because the numbers are so large, results are best read in scientific notation. To enter smaller masses, use scientific notation such as 1e-3 kilograms for one gram or 1e-6 for one milligram.
How does this relate to nuclear power?
In a nuclear reactor or a star, the products of fission or fusion have slightly less mass than the reactants. That missing mass, called the mass defect, is converted into energy exactly as E = mc² predicts. Although the mass difference is a small fraction of the total, multiplying it by the speed of light squared releases the immense energy that powers reactors and the Sun alike.
Is this the complete energy equation from relativity?
E = mc² gives the rest energy of an object, the energy it has purely by virtue of its mass while stationary. The full relativistic energy equation also includes a term for momentum, E² = (mc²)² + (pc)², which matters for objects moving near the speed of light or for massless particles like photons. For an object at rest, that momentum term vanishes and the equation reduces to the familiar E = mc².