To calculate vertical velocity for a projectile, use the formula v = -gt + v0, where g is the acceleration due to gravity (-9.8 m/s² in metric or -32.2 ft/s² in English) and v0 is the initial velocity. This equation assumes no air resistance. The vertical velocity decreases over time due to the negative acceleration from gravity. After t seconds, the velocity can be determined by substituting the values into the formula. Understanding this calculation is essential for analyzing projectile motion accurately.