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Mass-Energy Calculator

With mass 1 g, fraction actually converted 100, mass-energy comes to 89,875,517,873,682 J — rest energy. It is reached in 5 steps, the last of which is 0.001 * 299792458 ^ 2, and each one is printed on the page with its numbers filled in. The formula is the one published by NIST, not an approximation fitted to it.

The rest energy of a mass from E = mc², in joules, kilowatt-hours and kilotons of TNT, using the defined speed of light.

Formula and sources checked · How we check

Mass 1 g, Fraction actually converted 100

89,875,517,873,682 J

Rest energy for the example below. Editing a field recomputes the calculator below; this figure holds the answer the page was loaded with.

It is written into the HTML rather than drawn by a script, so a search engine reading this page without running JavaScript still finds an answer.

Rest energy
89,875,517,873,682 J
Rest energy
0.001 * 299792458 ^ 289,875,517,873,681.77 J
Energy actually released
89875518 * 100 / 10089,875,517,873,681.77 J
That energy in kilowatt-hours
89875518 / 360000024,965,421.632 kWh
In kilotons of TNT
89875518 / 418400000000021.481 kt
US homes powered for a year
24965422 / 107912,313.541 homes

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Worked example

One gram of anything holds 89,875,517,873,681.8 joules of rest energy — about 24,965,421.6 kilowatt-hours, or 21.4808 kilotons of TNT. That is the scale the equation sets. Nothing releases all of it: fission gets about a thousandth, and burning a gram of petrol gets roughly a ten-billionth.

How to work it out yourself

  1. 1.The mass is the mass converted, not the mass of the fuel. A reactor turns a fraction of a gram of its uranium into energy while consuming kilograms of it.
  2. 2.The efficiency field is what makes the number mean anything. Set it to 0.1% for fission, and the gram of uranium comes down to about 21 tons of TNT rather than 21 kilotons.
  3. 3.c is exact by definition — 299,792,458 metres per second — because the metre is defined from it. The uncertainty in the answer comes entirely from the mass.

The formula

  1. Rest energy0.001 * 299792458 ^ 2
  2. Energy actually released89875518 * 100 / 100
  3. That energy in kilowatt-hours89875518 / 3600000
  4. In kilotons of TNT89875518 / 4184000000000
  5. US homes powered for a year24965422 / 10791

Source: NIST — CODATA value of the speed of light in vacuum, BIPM — the SI Brochure, the metre and the defined value of c, US EIA — a US home bought 10,791 kWh of electricity in 2022

Questions people actually ask

Does E = mc² only apply to nuclear reactions?
No. Every energy release comes with a mass loss, including burning wood. It is just unmeasurably small: a chemical reaction converts about one part in ten billion of the fuel’s mass, which no scale can see.
Where does the lost mass go?
Nowhere — it becomes energy, and if you weighed the sealed system afterwards including all the heat and radiation, the total would be unchanged. The mass "disappears" only when the energy is allowed to leave.
Why is the number so large?
Because c² is about 9 × 10¹⁶ in SI units. The equation is not saying mass is special; it is saying the units of mass and energy are separated by an enormous conversion factor, in the same way that the number of millimetres in a kilometre is large.

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