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Mass–Energy Equivalence Calculator

Find the energy equivalent of any mass.

Result

89,875,517,873,681,760 J
21.48 Mt TNT

How it is calculated

The tool applies Einstein's relation E = m × c², where m is the mass in kilograms and c = 299,792,458 m/s is the speed of light, so the energy comes out in joules. It then divides by 4.184 × 10¹⁵ J, the energy of one megaton of TNT, to give a familiar comparison. Energy is rounded to a whole joule and megatons to two decimals, so a very small mass can show 0.00 Mt even though its energy in joules is large. Zero is allowed and gives 0; negative masses give no result. Because the value is a floating-point number, only the first 15 or so digits of a very large joule figure are meaningful.

Worked example

Take m = 1 kg. c² = 299,792,458² ≈ 8.98755 × 10¹⁶ m²/s². So E = 1 × 8.98755 × 10¹⁶ = 89,875,517,873,681,760 J, about 9 × 10¹⁶ joules. Dividing by 4.184 × 10¹⁵ J per megaton gives 21.48 Mt TNT. That is the energy that would be released only if one kilogram of matter were converted completely into energy, which real processes never achieve.

Try this example →

Common mistakes

  • Entering grams as if they were kilograms. The field is in kg, so 1 g must be typed as 0.001, which gives 1,000 times less energy than typing 1.
  • Expecting a rounded megaton value for tiny masses. At two decimals, 1 g shows 0.02 Mt (21.5 kilotons), and a few milligrams shows 0.00 Mt, so rely on the joule figure for small masses.
  • Assuming the whole mass of a fuel can be released. In fission or fusion only a small fraction of the mass is converted, so this is the theoretical maximum, not the yield of a reactor or bomb.
  • Typing a decimal comma, such as 1,5, for the mass. Use a decimal point.

About this calculator

This calculator applies Einstein's mass–energy equivalence, E = m·c², where c is the speed of light (about 299,792,458 m/s), converting any mass directly into its energy equivalent in joules — and, for scale, into megatons of TNT equivalent. Because c² is such a large number (roughly 9×10¹⁶ m²/s²), even a tiny mass corresponds to an enormous amount of energy: one gram converted entirely to energy releases about 21.5 kilotons of TNT-equivalent, similar to a small nuclear weapon.

This equation explains why nuclear fission and fusion release so much energy relative to their fuel mass — the products of a nuclear reaction weigh slightly less than the reactants, and that "missing" mass defect is released as energy, powering nuclear reactors, atomic bombs, and the sun itself.

Physics students studying special relativity and nuclear physics, science communicators illustrating nuclear energy scales, and nuclear engineering enthusiasts use this to build intuition for how mass and energy interconvert.

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