Mass-Energy Equivalence Calculator

E = mc² is Einstein's mass-energy equivalence equation from his 1905 Special Theory of Relativity. It states that energy (E) and mass (m) are interchangeable — every mass has an equivalent amount of energy and vice versa. The 'c²' factor (the speed of light squared, roughly 9 × 10¹⁶ m²/s²) shows that even a tiny mass corresponds to an enormous amount of energy. See also our calculate Lorentz Factor.

'c' is the speed of light in a vacuum, equal to approximately 299,792,458 metres per second (about 3 × 10⁸ m/s). In the formula it appears squared (c²), which equals roughly 8.988 × 10¹⁶ J/kg, meaning even 1 kilogram of mass is equivalent to about 89.9 petajoules of energy.

Rest mass energy is the energy an object possesses simply by having mass, even when it is completely stationary (zero kinetic energy). It is given by E = mc². This is distinct from kinetic energy (energy due to motion) and potential energy (energy due to position). Nuclear reactions release a small fraction of this rest mass energy.

In SI units, energy E is measured in Joules (J), mass m in kilograms (kg), and c in metres per second (m/s). The result of mc² therefore has units of kg·m²/s², which equals 1 Joule. For subatomic particles, electron volts (eV), kiloelectron volts (keV), or megaelectron volts (MeV) are more practical units. You might also find our calculate Alfvén Velocity Alfvén Velocity (vA) useful.

One kilogram of mass is equivalent to approximately 8.99 × 10¹⁶ joules of energy — roughly 89.9 petajoules. For perspective, that is about the same as 21.5 megatons of TNT, or roughly the energy released by thousands of nuclear bombs. This illustrates why even tiny mass deficits in nuclear reactions release enormous energy.

A single U-235 fission event typically releases about 200 MeV (megaelectron volts) of energy, corresponding to a mass deficit of roughly 0.19 atomic mass units. Over one kilogram of U-235, fission can release approximately 8.2 × 10¹³ joules — though only a fraction of the total rest mass energy is converted.

No. Einstein's equation showed that the separate laws of conservation of mass and conservation of energy are actually one unified law: mass-energy is always conserved. Energy can change form and mass can convert to energy (and vice versa), but the total mass-energy of a closed system never changes.

Simply rearrange the formula: m = E / c². Divide the energy value (in Joules) by c² (≈ 8.988 × 10¹⁶ J/kg) to get the equivalent mass in kilograms. Use this calculator's 'Mass from Energy' mode to handle the unit conversions automatically.