Armor penetration " As of Patch 4 . 0 . 1 , armor penetration has been removed from gear itemization , however it still exists in the game as a passive stat through the mastery system . " Armor penetration ( sometimes abbreviated ArP or called armor pen ) is a value appearing on many talents and abilities that indicates the percentage of armor that will be ignored by an attack . ArP can be found as both Abilities ( example : Sunder Armor ) , Talents like Mace Specialization or as Mastery bonuses . ArP is unique in that the more ArP you have , the greater the increase in damage becomes . Armor penetration is sometimes confused with spell penetration , even though they affect different things . Until recently , this stat has not received much attention from Blizzard , however it was seen in the talent Serrated Blades since patch 1 . 10 . Since itemization introduction of Arena Set 3 , Blizzard has been popularizing the use of this modifier even into . Armor penetration rating . Armor penetration is a rating system that affects a percentage of armor ignored . This means that upgrades scale more cleanly with all bosses but has a side effect of becoming less useful on low armor ones . The conversion rate is about 7 ArP = 1 ArP rating . So if you had 1000 ArP , you now have 142 . 8 ArP rating . In the past , this would have been a 6 % gain on a 15k armor foe and a 12 . 5 % gain on an 8k armor foe ; at level 80 it now gives a 9 . 2 % gain to both targets . Armor penetration rating against WotLK Raid bosses . The following tables provide detailed numbers for the effects of different levels of Armor Penetration Rating ( ArP ) in the case where a level 80 physical damage class attacks a level 83 raid boss with a given level of armor . ArP experiences diminishing returns for target armor lower than 8 , 317 armor . Note : Due to various changes to armor penetration rating , these tables are outdated . The percent reduction and effective armor values are both correct ; however , the armor penetration rating column uses the outdated 12 . 32 : 1 conversion ( as opposed to the current 13 . 99 : 1 conversion ) . Percentage - based ArP . This was Pre - WOTLK : Since some talents operate on percentages such as Mace Specialization , they operate onto the already existing modifiers . Instead of ignoring armor from the base value of the target immediately , it takes into account the effects of Sunder Armor , Expose Armor , Faerie Fire , Curse of Weakness , Sting , and Acid Spit and then does a post operation of the value when it calculates mace specialization and piercing shots . In addition , what this entails for percentage based ArP is that the higher the armor the greater the ArP raw value and the lesser the armor the less ArP raw value . That is to say , percentage based ArP suffers diminishing returns for heavily sundered units in terms of raw ArP value , especially for those with very low armor value . Now everything operates in percentages and stacks additively in two categories : Debuffs and personal Armor Penetration . The two categories stack multiplicatively with each other . Armor penetration benefits . Armor penetration will increase non - Bleed physical damage by a percentage significant for virtually every target , however , for lightly armored targets it is less effective in the damage it provides . From the perspective of a warrior , because armor penetration increases damage , he will get a moderate gain in rage generation as a result of higher armor penetration . Also , armor penetration causes the warrior to increase Execute damage . Opportunistically , from a PvP point of view , a warrior may apply fewer stacks of Sunder Armor , assuming they have a higher armor pen than usual , because they have a head start on armor ignored . Or they may choose to boost the sustained damage further by a significant percentage by applying more stacks . Itemization . Warrior sets : Rogue sets : Hunter sets : Weapons : Trinkets : Enchants : Gems : Countering armor penetration . To effectively neutralize or counter the effectiveness of armor penetration one could simply increase their armor value in combat time with drinking potions such as , changing to Bear Form or Moonkin Form , equipping a Shield , dropping a Stoneskin Totem , casting Inner Fire or Devotion Aura , or speccing in Toughness and Thick Hide . You can also enchant your gear with armor . For example : to counter armor pen . In short , armor penetration is really only countered by more armor . Technical explanation from Ghostcrawler . Quote from : Ghostcrawler " Okay , here is a fairly technical explanation we put together for how armor pen works . " " We didn ’ t want Armor Penetration Rating to be too powerful against low armor targets , like it had been in BC . We also didn ’ t want Armor Penetration Rating to be too powerful against high armor targets . " " So , we decided on a system where there is a cap on how much armor the Armor Penetration Rating can be applied to . So , the first X armor on the target is reduced by the percentage listed in the Armor Penetration Rating tooltip , and all armor past that X is unaffected . Another way of understanding that is we multiply the percentage in the tooltip times the minimum of the two values : the cap , and the amount of armor on the target after all other modifiers . " " Computing the cap is a little tricky unless you are already familiar with how World of Warcraft armor works . There is an armor constant we ’ ll call C . C is derived as follows ( in some pseudocode ) : " " For a level 80 target , C = 15232 . 5 . For a level 83 , C = 16635 . " " The cap for Armor Penetration then is : ( armor + C ) / 3 . " " A level 80 warrior creature has 9729 armor . C = 15232 . 5 . So , the cap is ( 9729 + 15232 . 5 ) / 3 = 8320 . 5 . Let ’ s say a player has 30 % armor penetration from armor penetration rating and no other modifiers that complicate the calculation ( talents , Battle Stance , Sunder Armor , etc . ) . The game chooses the minimum of 8320 . 5 and 9729 , so 8320 . 5 . That is multiplied by 30 % = 2496 . 15 , and so that much armor is ignored . The effective armor on the target is 7232 . 85 ( 9729 - 2496 . 15 ) . From a player point of view , the armor penetration rating didn ’ t ignore the full 30 % , but instead ignored 25 . 66 % . ( 85 . 5 % as effective as expected ) . " " These equations should help you be able to test and verify that Armor Penetration Rating is working correctly and as we designed . The tooltip is not actually inaccurate , as it states : “ Enemy armor reduced by up to 30 . 00 % . ” That " up to " is key . " " Please be sure to test without any other effects which modify the armor calculation ( Battle Stance , Sunder Armor , Mace Specialization , etc . ) as they may involve other systems that add additional complexity to the calculation . " The formula for the Armor Penetration cap can be simplified to " ArPcap = ( 935 / 6 ) * x + y / 3 - 44335 / 6 " , where " x " is the target level and " y " the target armor . Then " effArmor = targetArmor - ArP * min { targetArmor , ArPcap } " as explained above , with the Armor Penetration " ArP " of your character . " ArP " is given , lately changed in 3 . 1 . 2 , by the formula " ArP = min { 1 , ( Sum ArPcontributions ) % } " . Calculating . Calculating the ARP it ' s done in 2 parts : First up - the debuffs . These reduce your target ' s initial armor . For example , if your target has 10k armor and it gets 20 % sunder , its armor would be 8k . The debuffs stack additively , so Faerie Fire will stack with sunder , totaling a 25 % reduction on the target , taking it to 7 . 5k armor . Second , comes the formula GC provided , you grab the armor after the debuff calculation and to that armor you apply the formula ( armor + C ) / 3 . Using the same example as before , lets say the target is level 80 , so C = 15232 . 5 , now calculate ( 7500 + 15232 . 5 ) / 3 = 7577 . 5 . This makes the target ' s armor lower than the result , meaning that each 1 % removes 75 armor and you are hitting a 0 armor target on the 100 % ARP get . The ArP " buffs " ( items , stances and such ) stack additively too , meaning that 90 % ARP on battle stance for a warrior would mean a 100 % ARP final .