Free · Relativity calculators 🚀

Rest Mass & Energy Equivalence

Convert rest mass to its energy equivalent or recover mass from energy using the exact speed of light.

Formula & worked example
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How to use this calculator

Enter the known values in the units shown. Results update as you type.
Where results are editable, change one to solve backwards. Lock a value to hold it fixed.
Use the worked example to check the method. Reset restores the starting fields.

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Inputs and results stay in this browser tab. Bookify does not upload or store the values you enter.

Formula and method

One kilogram of rest mass corresponds to 89,875,517,873,681,764 joules of rest energy.

E₀ = mc², with c = 299,792,458 m/s

Worked example

Enter these known values and leave the other values blank.

Rest mass: 1 kg

Rest energy equivalent: 89875517874 MJ

Assumptions and limitations

This is rest energy, not kinetic energy or the energy a particular device can extract.
The speed of light is exact in SI. Electron-mass and atomic-mass-unit selectors use the 2022 CODATA central estimates, whose measurement uncertainties are not propagated.
Earth-mass conversion retains the approximate reference 5.9722 × 10²⁴ kg. Zero mass and zero rest energy are allowed.

Common questions

Does E = mc² describe kinetic energy?

Here it describes rest energy. Relativistic kinetic energy is (γ − 1)mc² and also depends on speed.

References

Bookify updates atomic reference-mass units to 2022 CODATA central estimates; the exact speed-of-light equations are retained. Zero mass and rest energy are accepted.

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