Free · Relativity calculators 🚀

Relativistic Kinetic Energy from Mass & Speed

Calculate relativistic kinetic energy from rest mass and speed, or recover rest mass from nonzero speed and kinetic energy.

Calculator inputs

Loading 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.

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

At 0.6c, γ = 1.25, so one kilogram has kinetic energy of 22,468,879,468,420,441 J.

K = (γ − 1)mc²; γ = 1/√(1 − v²/c²)

Enter these known values and leave the other values blank.

Rest mass must be positive and speed must lie from zero inclusive to c exclusive. Kinetic energy cannot be negative.
The retained solver provides energy and mass solutions; it does not solve an unknown speed. Zero speed gives zero kinetic energy for any mass, so that inverse is not unique.
Atomic reference-mass units use 2022 CODATA central estimates without uncertainty propagation. Speed of light is exact in SI.

It is a low-speed approximation. Compare both results at your speed and use a tolerance suitable for your application.

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