The W1 Penetration Index is an index of penetration. An index is a quantitatively derived term that does not have a real units expression. In other words, this is not a measure of the number of inches that your bullet will penetrate. Rather it should be used to compared one bullet to another. Higher penetration indices will result in more penetration.

The W1 index calculates penetration of your bullet through a very heavy medium (a theoretical medium with a density approximately equal to lead or gold).