☢️ Mass, Energy Calculator (E = mc²)
Convert between mass and energy with E = mc², enter a mass to get its energy, or an energy to get its mass.
Energy E
8.9876e+13 J (5.610e+32 eV)
Mass, energy equivalence: E = m·c², with c = 299,792,458 m/s (exact). Energy also shown in eV. 🔒 Computed in your browser.
How the mass, energy calculator (e = mc²) works
Einstein’s mass, energy equivalence: E = mc², with c = 299,792,458 m/s (exact). A tiny mass corresponds to an enormous energy because c² is huge. Enter mass or energy and the tool gives the other, in joules and electronvolts. E = mc² gives the rest energy contained in a mass; conversely, dividing an energy by c² gives the equivalent mass, which is why particle masses are often quoted in units such as MeV/c².
One gram of mass equals about 9×10¹³ J, the energy released in nuclear reactions comes from converting a small fraction of mass. c is an exact defined constant.
Frequently asked questions
What does E = mc² mean?
Mass and energy are equivalent: a mass m has an energy E = mc², where c is the speed of light. Because c² ≈ 9×10¹⁶, even a small mass holds a vast amount of energy.
How much energy is in 1 gram of mass?
E = mc² = 0.001 × (3×10⁸)² ≈ 9×10¹³ J, roughly the energy of a large nuclear explosion. Enter 0.001 kg to see the exact value.
Where does nuclear energy come from?
A small amount of mass is converted to energy in fission and fusion; the “mass defect” times c² gives the energy released.
What is the value of c?
The speed of light, exactly 299,792,458 m/s by definition of the metre.
Can energy turn into mass?
Yes, in particle physics, sufficiently energetic collisions create particles (mass) from energy, the reverse of E = mc².
Worked example: one kilogram
The rest energy of 1 kg is E = mc² = 1 × (299,792,458)² ≈ 9.0×10¹⁶ J, about 25 billion kilowatt-hours, showing how much energy is locked in even a small mass.
Why is mass measured in MeV/c² in particle physics?
Rearranging E = mc² gives m = E/c², so an energy divided by c² is a mass. Physicists quote masses as energies (an electron is about 0.511 MeV/c²) because it is convenient in reactions where mass and energy interconvert.
Is E = mc² the full energy equation?
It gives the rest energy of a stationary mass. A moving object also has kinetic energy, and the complete relation is E² = (mc²)² + (pc)², which reduces to E = mc² when the momentum p is zero.
What is the mass defect?
The difference between a nucleus’s mass and the total mass of its separate protons and neutrons. That missing mass, times c², is the binding energy released when the nucleus forms, the source of the energy in fission and fusion.