The National Institute of Justice has a rating system for body armor (NIJ Standard 0101.03, 0101.04 and the interim 0101.05). As you add layers of a ballistic fiber, such as DuPont Kevlar®, you add protection.

Vests are tested not just for stopping penetration, but also for blunt trauma protection – the blow suffered by the body from the bullet's impact on the vest. Blunt trauma is measured by the dent suffered by a soft clay backstop to the vest – a maximum of 1.7" (44 mm) is allowed.

NOTE: The standard NIJ test rounds are listed below – tested vests stop many other comparable rounds, and lesser threats.