According to Pyramid Air Gun Mall, muzzle velocity is calculated by multiplying the grain of the bullet or pellet being fired from the gun by the feet per second the bullet travels after being fired. Divide this number by 450,240 to calculate muzzle velocity. The final number is a constant derived by two times the acceleration of gravity by 7,000, which is the number of grains in a pound.
To give an example of this formula, a 115 grain bullet travelling at 1300 feet per second has a muzzle velocity of 431 foot-pounds. Foot-pounds are the units used for measuring energy in the United States customary and imperial systems of measurement. If you would rather calculate muzzle velocity in joules, you can easily convert between the two. One foot-pound is equal to 1.4 joules. Therefore, in the above example, the bullet with the muzzle velocity of 431 foot-pounds can also be said to have a muzzle velocity of 585 joules.
The numbers required to solve this formula are usually found on the packaging or literature that comes with your gun and bullets. According to Potomac, LTC, bullet grain can be found on the box the bullets come in or through the manufacturer's website.