My favorite Muzzle Energy Calculator was hosted by AirHog, makers of high quality air gun tanks, valves and other accessories. Airhog redesigned their website a while back and have taken the old site offline, which included the calculator. Through the magic of the Internet Archive, I have resurrected and improved their calculator. Hopefully they will not mind, I have emailed them asking for permission. If anyone is interested, my improved javascript code for the calculator is here.
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