Armor penetration
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Armor penetration (sometimes abbreviated ArP) is a number typically appearing on items that indicates the amount of armor ignored by an attack. Some abilities, enchantments, items and spells can contribute bonus armor penetration.
ArP can be found as both abilities (ex. sunder armor), or as passive statistical upgrades found on equipment pieces (ex. Season 3 PvP armor). Armor penetration reduces the amount of armor that your target has against melee attacks.
ArP is unique in that the more ArP you have, the greater the increase in damage becomes.
Examples; 1k penetration is a 5.7% increase on an 8k armor target, 6.8% increase on a 5k armor target, 7.9% increase at 3k armor and a 9.5% increase at 1k armor.
This also illustrates how Armour Penetration has increasing returns with itself. Going from 2k ArP to 2.5k ArP nets the same increase as going from 1.5k ArP to 2k ArP, but going from 1.5k to 2k ArP is a higher increase than going from 1k ArP to 1.5k ArP.
Sometimes confused with "spell penetration".
Armor Penetration Rating
Starting in patch 3.0, in preparation for the expansion 
, all items with Armor Penetration have been modified to a rating system that affects a percentage of armor ignored. This means that gear scales more cleanly with all bosses but has a side effect of become 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 ana 12.5% gain on an 8k armor foe, at level 80 it now gives 9.2% gain to both targets.
| ArP Rating required to ignore 1% armor | |||||||
|---|---|---|---|---|---|---|---|
| Level 60 | Level 70 | Level 80 | |||||
| 4.69 | 7.4 | 15.39 | |||||
Patch changes
- As of Patch 2.4.1, Armor Penetration will now display on the character sheet (Armor Penetration displays in the hit rating section).
