Mass-Energy Equivalence
From a single mass, see the rest energy it holds — the huge number behind nuclear power, the shining Sun, and Einstein's most famous equation, E = mc².
One input, one answer
Enter the mass in kilograms and the calculator returns the rest energy (E = mc²) in joules at once.
Use SI units
Enter the mass in kilograms (kg) — then the energy comes back in joules (J), the coherent SI unit of energy.
What is mass-energy equivalence?
Mass in, rest energy out
Mass-energy equivalence is Einstein's insight that mass and energy are two forms of the same thing. Every mass holds an amount of energy fixed by a single quantity, the mass itself, because the conversion factor is constant: the speed of light squared, with c = 299,792,458 m/s in a vacuum. The relationship is energy = mass × c². Once you know the mass, the rest energy is set. Because c² is about 9 × 10¹⁶, even a tiny mass corresponds to an enormous energy — which is why mass is the one input you need for physics education, nuclear science, and astrophysics.
Enter the mass in kilograms to get the rest energy instantly.
One short formula, built from the mass (m) and the fixed speed of light (c).
energy = mass × c²The rest energy is the mass multiplied by the square of the speed of light (m × c²). Because c is constant, the energy grows in direct proportion to the mass: double the mass and you double the energy. The c² factor — about 9 × 10¹⁶ m²/s² — is what turns a few kilograms into an astronomical number of joules.
Suppose you have a mass of 1 kilogram, that is m = 1 kg.
Square the speed of light
299,792,458² = 89,875,517,873,681,760 m²/s² — about 9 × 10¹⁶.
Multiply by the mass
1 × 89,875,517,873,681,760 = 89,875,517,873,681,760 J — about 8.99 × 10¹⁶ joules.
Put it in perspective
That is roughly 21 megatons of TNT — the energy locked in a single kilogram of any matter.
The number you get is the rest energy — the energy equivalent of a mass that is sitting still — and for 1 kg it is about 8.99 × 10¹⁶ joules, roughly 21 megatons of TNT. It is crucial to read this as a theoretical ceiling, not a battery you can drain. In everyday and even nuclear processes, only a tiny fraction of the rest energy is ever released: nuclear fission and fusion convert a small mass defect, a fraction of a percent of the mass, into energy, while the rest stays as mass. This small conversion is still enough to power reactors and make the Sun shine, because c² is so large that even a sliver of mass becomes a vast amount of energy. Complete mass-to-energy conversion happens only when matter meets antimatter. So the figure here explains why nuclear power and stars release so much energy from so little fuel — and reminds you that the rest energy is the upper limit set by E = mc², not the energy you can extract in practice.
The formula is exact for the rest energy of a mass, but a couple of practical points are worth keeping in mind.
Rest energy, not extractable energy
This tool gives the rest energy of a stationary mass — the theoretical maximum from E = mc². Real processes release only a tiny mass defect, so do not read the result as energy you can actually extract. The formula also ignores kinetic energy: a moving mass has more total energy than its rest energy alone. Keep the mass in kilograms so the energy comes back in joules, the coherent SI unit.