Initial Velocity is usually referring to muzzle velocity in ballistics. It is the velocity of the projectile as soon as it leaves the barrel. In order to calculate this you will need a chronograph. A Shooting Chronograph is a great investment if you want to get into long range shooting and especially if you handload. If you do not have a chronograph, sometimes the manufacturer of the ammunition lists the velocity on the box. Be aware that this may be different then the actually muzzle velocity of the bullet from your firearm. The velocity is affected by barrel length, temperature, and the firearm.