What is the actual diameter of a 9mm bullet?

The actual diameter of a 9mm bullet is approximately 9.01 millimeters, which is often rounded to 9mm for convenience. This measurement refers specifically to the bullet itself, while the cartridge in which it is housed can have different dimensions. The term "9mm" usually refers to the bullet's diameter, but it can also indicate various cartridge types, such as the popular 9mm Parabellum.