What this calculator does
E=mc2 is Einstein’s statement that mass and energy are the same thing, related by a fixed conversion factor: the speed of light squared. This calculator takes a mass and multiplies it by c², using the exact value of the speed of light, 299,792,458 metres per second, to show exactly how much energy that mass represents if it were fully converted.
The number that comes out is always enormous, because c² itself is an enormous number: roughly 90,000,000,000,000,000 (9 × 10¹⁷) joules per kilogram. A single gram of matter, converted completely to energy, represents roughly the energy released by 21,000 tonnes of TNT, which is why nuclear reactions, which convert only a tiny fraction of their mass to energy, still release so much more energy than ordinary chemical reactions.
The formula
Multiply mass, in kilograms, by the speed of light squared: E = mc², using c = 299,792,458 m/s exactly, as defined by the modern definition of the metre. The result in joules is then converted into several more familiar units of energy.
| Term | Meaning |
|---|---|
| E | Energy, in joules: the amount of energy equivalent to the given mass if fully converted. |
| m | Rest mass, in kilograms: the mass of an object when it is not moving relative to the observer. |
| c | The speed of light in a vacuum, exactly 299,792,458 metres per second by definition. |
The inputs explained
| Field | What to enter |
|---|---|
| Mass (kg) | The rest mass to convert to its energy equivalent, in kilograms (1 gram = 0.001 kg). |
When to use it
Understanding nuclear reactions
Nuclear fission and fusion release energy because a small amount of mass is genuinely converted into energy during the reaction; comparing that tiny mass loss against E=mc² shows why the resulting energy release is so disproportionately large.
Grasping just how much energy mass represents
Converting a small, everyday mass, such as a coin or a paperclip, through E=mc² is a common way to build intuition for why mass-energy conversion barely features in everyday chemistry and mechanics, yet dominates nuclear physics.
Teaching or reviewing special relativity
E=mc² is usually the first and most memorable result students meet from special relativity, and working through actual numbers reinforces just how large the conversion factor between mass and energy really is.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much energy does a given mass represent?
A range of masses, from very small to substantial.
| Mass | Energy | In tonnes of TNT equivalent |
|---|---|---|
| 1 mg | 89,875,517,870 J | 21.48076431 t TNT |
| 10 mg | 898,755,178,700 J | 214.8076431 t TNT |
| 100 mg | 8,987,551,787,000 J | 2,148.076431 t TNT |
| 1 g | 89,875,517,870,000 J | 21,480.76431 t TNT |
| 10 g | 898,755,178,700,000 J | 214,807.6431 t TNT |
| 1 kg | 89,875,517,870,000,000 J | 21,480,764.31 t TNT |
Questions
Why is the energy figure so large for such a small mass?
Because the conversion factor, c², is the speed of light squared: roughly 9 × 10¹⁷ in SI units. Multiplying even a tiny mass by a number that large produces an enormous energy figure, which is the whole point of the equation: mass is an extremely concentrated form of energy.
Does E=mc2 mean any object can release this much energy?
Only if it were fully converted to energy, which does not happen in ordinary circumstances. Chemical reactions, including burning fuel, convert a genuinely negligible fraction of mass to energy; only nuclear reactions and matter-antimatter annihilation convert a measurable, though still small, fraction of mass directly to energy.
What is rest mass, specifically?
Rest mass (or invariant mass) is an object’s mass as measured in a reference frame where it is not moving. It is the mass value used in E=mc² for an object at rest; a moving object’s total energy includes additional kinetic energy on top of this rest-mass energy.
Why does the calculator use 299,792,458 m/s exactly, rather than "the speed of light" as an approximation?
Since 1983, the metre has been defined in terms of the speed of light, making c exactly 299,792,458 m/s by definition rather than a measured, uncertain value. Using the exact figure avoids introducing any unnecessary rounding into the result.
For the extra kinetic energy a moving object carries on top of its rest-mass energy, see the relativistic kinetic energy calculator, if travelling at a meaningful fraction of the speed of light.