Mass-Energy Equivalence (E = mc²) Calculator
This calculator applies Einstein's famous equation E = mc² to convert between rest mass and its equivalent energy.
Solve for
Mass unit
Energy locked in this mass via E = mc²
- 1
Convert g to kg
1 ÷ 1,000 = 0.001 - 2
c² (speed of light squared)
(2.998 × 10⁸)² = 89,875,517,873,681,760c = 299 792 458 m/s (exact); c² ≈ 8.988 × 10¹⁶ m²/s². - 3
Rest energy E = m × c²
0.001 × 89,875,517,873,681,760 = 89,875,517,873,681.7700
How does this calculator work?
Einstein's E = mc² converts mass into energy. Multiply mass in kilograms by the speed of light squared (about 9 × 10¹⁶) to get rest energy in joules. Just 1 gram equals roughly 9 × 10¹³ joules, about 21 kilotons of TNT. Reverse the formula as m = E / c² to find mass from energy.
Formula
How this is calculated
Einstein's mass-energy equivalence states that the rest energy E of an object equals its mass m multiplied by the square of the speed of light c. Because c is enormous (about 3 × 10⁸ m/s), c² is roughly 9 × 10¹⁶, so even a tiny mass corresponds to a huge amount of energy. Enter a mass and choose its unit; the value is converted to kilograms (1 g = 0.001 kg, 1 mg = 1e-6 kg) before being multiplied by c².
The result is reported in joules and then converted to common units: kilowatt-hours (kWh = E / 3.6×10⁶), electronvolts (eV = E / 1.602176634×10⁻¹⁹), and TNT equivalents (1 ton TNT = 4.184×10⁹ J, 1 megaton = 4.184×10¹⁵ J). In reverse mode, supply an energy in joules and the calculator returns the equivalent mass via m = E / c².
This treats mass as rest mass and gives total rest energy, assuming complete conversion of mass to energy. Real processes (fission, fusion, annihilation) convert only a fraction of the mass, so actual released energy is smaller. Inputs must be finite; negative masses are not physical but will still compute a magnitude.
Frequently asked questions
Because energy scales with the square of the speed of light. c² is about 9 × 10¹⁶ m²/s², so multiplying even 1 gram (0.001 kg) by c² yields roughly 9 × 10¹³ joules — comparable to a large explosion.
Only in principle. E = mc² gives the total rest energy if mass were fully converted. Nuclear reactions convert just a fraction of a percent of mass, so they release far less than the full rest energy.
The exact defined speed of light, c = 299,792,458 m/s, so c² ≈ 8.98755 × 10¹⁶ m²/s².
Also known as
TG we-Calculate Editorial Team. (2026). Mass-Energy Equivalence (E = mc²) Calculator [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/mass-energy-equivalence-calculator
TG we-Calculate Editorial Team. "Mass-Energy Equivalence (E = mc²) Calculator." TG we-Calculate. 2026. https://we-calculate.com/calculator/mass-energy-equivalence-calculator.
TG we-Calculate Editorial Team, "Mass-Energy Equivalence (E = mc²) Calculator," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/mass-energy-equivalence-calculator
@misc{wecalculate_mass_energy_equivalence_calculator, title = {Mass-Energy Equivalence (E = mc²) Calculator}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/mass-energy-equivalence-calculator}}, year = {2026}, note = {TG we-Calculate} }
Did this calculator help you?