E = mc² Mass–Energy Calculator
Convert mass to rest energy (or energy back to mass) in joules, kilowatt-hours, tonnes of TNT and Hiroshima equivalents — E = mc².
How it works
E = m · c² (c = 299,792,458 m/s, so c² ≈ 8.99 × 10¹⁶ J/kg)
Einstein’s relation says mass is a concentrated form of energy: one gram fully converted yields 90 terajoules, about 21 kilotonnes of TNT. Nuclear reactions release only a fraction of a percent of the mass involved — fission about 0.1%, fusion 0.7% — while chemical reactions release so little that the mass change is unmeasurable. The reverse direction shows why everyday energies weigh almost nothing.
Worked example
1 g: 0.001 × (3 × 10⁸)² = 8.99 × 10¹³ J ≈ 25 million kWh ≈ 21.5 kt of TNT.
Frequently asked questions
Does a hot object weigh more?
Yes, by E/c² — a kettle of boiling water is heavier by about 4 × 10⁻¹² kg. Unmeasurable, but real.
Where does the electron-volt version come from?
An electron’s rest mass is 0.511 MeV/c²; a proton’s 938 MeV/c². Particle physicists quote masses in energy units for this reason.
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This calculator is for educational purposes. Double-check important results before acting on them.