First choose the system of units, either English Units (also called United States customary units, Traditional units or Imperial units) if you are in the USA or Metric if you are in Europe. The rest of the world uses one or the other depending on how manufacturers have decided to market their products.
Then enter two of the three variables (weight, velocity and energy) and hit Calculate. The system will calculate the third variable which you did not enter.
Muzzle Energy Calculator