Armor penetration

Armor penetration refers to the amount of an opponent's armor level (AL for short) that will be ignored when calculating the damage received by an attack or spell.

It is sometimes thought of as simply an increase in damage dealt, but it is more complicated than this. Guild Wars has a fairly complicated formula for calculating damage, (see Damage) but attacks and skills that have even a seemingly small percentage of armor penetration can cause a significant increase in damage. For instance, if you use an attack that has 25% armor penetration against an opponent with 80AL, that attack would give him an effective 60AL (80 - 25% * 80). This decrease of -20 AL would cause a strike that would normally cause 20 points of damage to do 30 instead (a 50% increase in damage despite only a 25% decrease in armor).

Note: Armor penetration is more effective against higher AL, but still strong against lower AL.

For a detailed chart on how damage, armor level and armor penetration affect each other, see the chart in Damage Calculation