Mass-Energy Equivalence Calculator
Einstein's famous equation E = mc^2 states that mass and energy are equivalent. The speed of light squared, c^2, is approximately 8.988 x 10^16 joules per kilogram, so even a small mass corresponds to an immense amount of energy. Nuclear power plants convert about 0.1% of fuel mass to energy through fission; matter-antimatter annihilation would convert 100% of mass to energy. This calculator accepts mass in kilograms, grams, or atomic mass units and returns the equivalent rest energy in joules, MeV (megaelectronvolts), and kilotons of TNT equivalent.
Mass-energy equivalence formula
E = m * c^2
Where c = 2.99792458 x 10^8 m/s (exact, per SI definition). 1 atomic mass unit = 1.66053906660 x 10^-27 kg. 1 MeV = 1.602176634 x 10^-13 J. 1 kiloton TNT = 4.184 x 10^12 J.
Reference values
- Electron rest energy: 0.511 MeV (m = 9.109 x 10^-31 kg).
- Proton rest energy: 938.3 MeV (m = 1.673 x 10^-27 kg).
- 1 gram of matter: 8.988 x 10^13 J, about 21.5 kilotons TNT.
- 1 atomic mass unit: 931.49 MeV, the standard conversion used in nuclear physics binding energy calculations.
Mass-energy equivalence: frequently asked questions
What does E = mc^2 mean?
Einstein's equation states that mass and energy are equivalent and interconvertible. A body at rest with mass m contains rest energy E = mc^2. The c^2 factor (about 9 x 10^16 J/kg) means even a tiny mass corresponds to an enormous amount of energy.
How much energy is in 1 gram of matter?
E = 0.001 kg * (2.998 x 10^8 m/s)^2 = 8.99 x 10^13 joules, which equals about 21.5 kilotons of TNT. This illustrates why nuclear reactions, which convert a tiny fraction of mass to energy, release enormous amounts.
Does E = mc^2 apply to nuclear reactions specifically?
E = mc^2 applies to any process involving a change in rest mass. Nuclear fission and fusion release energy because the products have slightly less mass than the reactants. The mass defect, multiplied by c^2, gives the released energy.
What is the mass defect in nuclear physics?
The mass defect is the difference between the mass of a nucleus and the sum of masses of its constituent protons and neutrons. This mass difference is converted to binding energy: the energy required to disassemble the nucleus into its parts.
Can I convert energy back to mass?
Yes. High-energy photon collisions can produce matter-antimatter pairs (pair production). A gamma-ray photon with energy greater than 1.022 MeV (twice the electron rest energy of 0.511 MeV) can produce an electron-positron pair near a nucleus.
Official sources
- NIST CODATA 2018: Speed of Light in Vacuum.
- OpenStax University Physics Vol. 3: Relativistic Energy.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.