E=mc² Calculator

Convert mass to energy or energy to equivalent mass with Einstein's mass-energy equivalence and the exact speed of light.

Mass-energy equivalence

Choose a direction, enter a nonnegative SI value, and calculate using the exact vacuum speed of light.

Mass (kg)

About mass-energy equivalence

Einstein's equation E = mc² expresses the equivalence of mass and energy. It states that a body's rest energy equals its mass multiplied by the speed of light squared. Because the speed of light is approximately three hundred million meters per second, squaring it produces an enormous conversion factor. Even a small mass therefore corresponds to a very large amount of rest energy. This calculator uses the exact defined value of the vacuum speed of light, 299,792,458 meters per second. In mass-to-energy mode it multiplies kilograms by c² and reports joules. In energy-to-mass mode it rearranges the equation as m = E/c² and converts joules into kilograms. Scientific notation keeps very large and very small answers readable without discarding their scale. The energy calculated is total rest energy, not necessarily energy that can be released by an ordinary process. Chemical reactions convert only a minute difference in mass, while nuclear fission and fusion involve larger mass defects but still transform only a fraction of the material's total rest mass. Complete conversion is associated with processes such as matter-antimatter annihilation. A one-kilogram input gives about 8.99 × 10^16 joules, but that does not imply a practical method for extracting all of it. Mass conservation and energy conservation become one combined principle in relativity. A system can gain mass when energy is added and lose mass when energy leaves it. Heating an object, compressing a spring, or charging a battery increases the system's mass by an extraordinarily small amount. Conversely, the energy radiated by a star corresponds to a decrease in its mass. Use SI units directly: kilograms for mass and joules for energy. If your source measurement is in grams, divide by one thousand before entering it. If energy is given in kilowatt-hours, multiply by 3.6 million to obtain joules. The result is appropriate for educational comparisons and ideal mass-energy conversions. Detailed nuclear or particle calculations may also need binding energy, kinetic energy, momentum, and conservation laws beyond the simple rest-energy equation.

E=mc² examples

Input	Equivalent value	Context
1 kg of mass	8.987552e16 J	One kilogram's total rest energy illustrates the huge c-squared conversion factor.
0.001 kg of mass	8.987552e13 J	One gram still corresponds to nearly ninety trillion joules of rest energy.
8.987552e16 J	1 kg	Dividing the energy by c squared recovers the equivalent mass.

How to use the E=mc² calculator

Choose Mass to energy or Energy to mass.
Enter mass in kilograms or energy in joules according to the selected mode.
Select Calculate E=mc² to apply the exact speed-of-light constant.
Read the result in scientific notation and confirm that the displayed unit matches your goal.

E=mc² FAQ

What does c squared represent?

It is the vacuum speed of light multiplied by itself. Its enormous value explains why small masses correspond to large rest energies.

Can all mass be converted into usable energy?

Not in ordinary chemical or nuclear processes. Those processes convert only mass differences, while complete conversion requires exceptional particle interactions.

Why does the calculator use kilograms and joules?

Those are the coherent SI units for the equation. Using grams without conversion would make the answer one thousand times too large.

Does E=mc² apply only to nuclear physics?

No. Mass-energy equivalence applies to every physical system, although its effects are easiest to observe in nuclear and particle processes.

Is this the complete relativistic energy equation?

It is the rest-energy case for an object with zero momentum. Moving particles require the broader energy-momentum relation.