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.

ArP Rating required to ignore 1% armor
Level 60	Level 70	Level 80
3.75	5.92	13.99

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Note: Due to various changes to armor penetration rating, these tables are outdated.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percent reduction and effective armor values are both correct; however, the armor penetration rating column uses the outdated 12.32:1 conversion (as opposed to the current 13.99:1 conversion).

Armor reduction table
Level 80		Level 83 Target Armor
ArP	%AR	1000	2000	3000	4000	5000	6000	7000	8000	8317	9000	10000	20000	30000	40000	50000
0	0.0%	1000	2000	3000	4000	5000	6000	7000	8000	8317	9000	10000	20000	30000	40000	50000
10	0.8%	992	1984	2976	3968	4959	5951	6943	7935	8249	8931	9928	19901	29874	39847	49820
20	1.6%	984	1968	2951	3935	4919	5903	6886	7870	8182	8861	9856	19802	29747	39693	49639
30	2.4%	976	1951	2927	3903	4878	5854	6829	7805	8114	8792	9784	19702	29621	39540	49459
40	3.2%	968	1935	2903	3870	4838	5805	6773	7740	8047	8722	9712	19603	29495	39387	49278
50	4.1%	959	1919	2878	3838	4797	5756	6716	7675	7979	8653	9639	19504	29369	39233	49098
60	4.9%	951	1903	2854	3805	4756	5708	6659	7610	7912	8584	9567	19405	29242	39080	48917
70	5.7%	943	1886	2829	3773	4716	5659	6602	7545	7844	8514	9495	19306	29116	38926	48737
80	6.5%	935	1870	2805	3740	4675	5610	6545	7480	7776	8445	9423	19206	28990	38773	48557
90	7.3%	927	1854	2781	3708	4634	5561	6488	7415	7709	8375	9351	19107	28863	38620	48376
100	8.1%	919	1838	2756	3675	4594	5513	6431	7350	7641	8306	9279	19008	28737	38466	48196
200	16.2%	838	1675	2513	3350	4188	5025	5863	6700	6966	7612	8558	18016	27474	36933	46391
300	24.4%	756	1513	2269	3025	3781	4538	5294	6050	6290	6918	7836	17024	26212	35399	44587
400	32.5%	675	1350	2025	2700	3375	4050	4725	5400	5614	6223	7115	16032	24949	33866	42783
500	40.6%	594	1188	1781	2375	2969	3563	4157	4751	4939	5529	6394	15040	23686	32332	40978
600	48.7%	513	1025	1538	2050	2563	3076	3588	4101	4263	4835	5673	14048	22423	30799	39174
700	56.9%	431	863	1294	1725	2157	2588	3019	3451	3588	4141	4951	13056	21160	29265	37369
800	65.0%	350	700	1050	1400	1751	2101	2451	2801	2912	3447	4230	12064	19898	27731	35565
900	73.1%	269	538	807	1076	1344	1613	1882	2151	2236	2753	3509	11072	18635	26198	33761
1000	81.2%	188	375	563	751	938	1126	1314	1501	1561	2058	2788	10080	17372	24664	31956
1230	99.9%	1	2	2	3	4	5	6	6	7	462	1129	7798	14468	21137	27806
		Total effective target armor after ArP is applied

Damage reduction table
Level 80		Level 83 Target Armor
ArP	%AR	1000	2000	3000	4000	5000	6000	7000	8000	8317	9000	10000	20000	30000	40000	50000
0	0.0%	6.2%	11.6%	16.5%	20.8%	24.7%	28.3%	31.5%	34.4%	35.3%	37.1%	39.6%	56.8%	66.3%	72.4%	76.6%
10	0.8%	6.1%	11.5%	16.3%	20.7%	24.6%	28.1%	31.3%	34.3%	35.1%	37.0%	39.5%	56.6%	66.2%	72.3%	76.6%
20	1.6%	6.1%	11.4%	16.2%	20.5%	24.4%	27.9%	31.1%	34.1%	34.9%	36.8%	39.3%	56.5%	66.1%	72.3%	76.5%
30	2.4%	6.0%	11.4%	16.1%	20.4%	24.3%	27.8%	31.0%	33.9%	34.8%	36.6%	39.1%	56.4%	66.0%	72.2%	76.5%
40	3.2%	6.0%	11.3%	16.0%	20.3%	24.1%	27.6%	30.8%	33.7%	34.6%	36.4%	38.9%	56.3%	65.9%	72.1%	76.4%
50	4.1%	5.9%	11.2%	15.9%	20.1%	23.9%	27.4%	30.6%	33.5%	34.4%	36.2%	38.8%	56.1%	65.8%	72.0%	76.3%
60	4.9%	5.9%	11.1%	15.8%	20.0%	23.8%	27.3%	30.4%	33.3%	34.2%	36.0%	38.6%	56.0%	65.8%	72.0%	76.3%
70	5.7%	5.8%	11.0%	15.7%	19.9%	23.6%	27.1%	30.2%	33.1%	34.0%	35.9%	38.4%	55.9%	65.7%	71.9%	76.2%
80	6.5%	5.8%	10.9%	15.6%	19.7%	23.5%	26.9%	30.1%	32.9%	33.8%	35.7%	38.2%	55.8%	65.6%	71.8%	76.1%
90	7.3%	5.7%	10.8%	15.4%	19.6%	23.3%	26.7%	29.9%	32.7%	33.6%	35.5%	38.0%	55.6%	65.5%	71.7%	76.1%
100	8.1%	5.7%	10.8%	15.3%	19.4%	23.2%	26.6%	29.7%	32.5%	33.4%	35.3%	37.9%	55.5%	65.4%	71.6%	76.0%
200	16.2%	5.2%	9.9%	14.2%	18.0%	21.6%	24.8%	27.8%	30.5%	31.4%	33.3%	36.0%	54.2%	64.3%	70.8%	75.3%
300	24.4%	4.7%	9.0%	13.0%	16.6%	19.9%	23.0%	25.8%	28.4%	29.2%	31.2%	34.0%	52.8%	63.2%	69.9%	74.5%
400	32.5%	4.2%	8.1%	11.7%	15.1%	18.1%	21.0%	23.7%	26.2%	26.9%	29.0%	31.8%	51.3%	62.1%	69.0%	73.7%
500	40.6%	3.8%	7.2%	10.5%	13.5%	16.3%	19.0%	21.4%	23.8%	24.5%	26.6%	29.6%	49.7%	60.9%	68.0%	72.9%
600	48.7%	3.3%	6.3%	9.2%	11.9%	14.4%	16.8%	19.1%	21.2%	21.9%	24.1%	27.1%	48.0%	59.5%	66.9%	72.0%
700	56.9%	2.8%	5.4%	7.8%	10.2%	12.4%	14.5%	16.5%	18.5%	19.1%	21.4%	24.5%	46.2%	58.1%	65.8%	71.0%
800	65.0%	2.2%	4.4%	6.5%	8.4%	10.3%	12.1%	13.9%	15.5%	16.0%	18.5%	21.7%	44.2%	56.6%	64.5%	70.0%
900	73.1%	1.7%	3.4%	5.0%	6.6%	8.1%	9.6%	11.0%	12.4%	12.8%	15.3%	18.7%	42.1%	55.0%	63.2%	68.9%
1000	81.2%	1.2%	2.4%	3.6%	4.7%	5.8%	6.9%	7.9%	9.0%	9.3%	11.9%	15.5%	39.8%	53.3%	61.8%	67.7%
1230	99.9%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	2.9%	6.9%	33.9%	48.7%	58.1%	64.6%
		Total percent damage prevented by the target armor after ArP is applied

Final damage table
Level 80		Level 83 Target Armor
ArP	%AR	1000	2000	3000	4000	5000	6000	7000	8000	8317	9000	10000	20000	30000	40000	50000
0	0.0%	938	884	835	792	753	717	685	656	647	629	604	432	337	276	234
10	0.8%	939	885	837	793	754	719	687	657	649	630	605	434	338	277	234
20	1.6%	939	886	838	795	756	721	689	659	651	632	607	435	339	277	235
30	2.4%	940	886	839	796	757	722	690	661	652	634	609	436	340	278	235
40	3.2%	940	887	840	797	759	724	692	663	654	636	611	437	341	279	236
50	4.1%	941	888	841	799	761	726	694	665	656	638	612	439	342	280	237
60	4.9%	941	889	842	800	762	727	696	667	658	640	614	440	342	280	237
70	5.7%	942	890	843	801	764	729	698	669	660	641	616	441	343	281	238
80	6.5%	942	891	844	803	765	731	699	671	662	643	618	442	344	282	239
90	7.3%	943	892	846	804	767	733	701	673	664	645	620	444	345	283	239
100	8.1%	943	892	847	806	768	734	703	675	666	647	621	445	346	284	240
200	16.2%	948	901	858	820	784	752	722	695	686	667	640	458	357	292	247
300	24.4%	953	910	870	834	801	770	742	716	708	688	660	472	368	301	255
400	32.5%	958	919	883	849	819	790	763	738	731	710	682	487	379	310	263
500	40.6%	962	928	895	865	837	810	786	762	755	734	704	503	391	320	271
600	48.7%	967	937	908	881	856	832	809	788	781	759	729	520	405	331	280
700	56.9%	972	946	922	898	876	855	835	815	809	786	755	538	419	342	290
800	65.0%	978	956	935	916	897	879	861	845	840	815	783	558	434	355	300
900	73.1%	983	966	950	934	919	904	890	876	872	847	813	579	450	368	311
1000	81.2%	988	976	964	953	942	931	921	910	907	881	845	602	467	382	323
1230	99.9%	1000	1000	1000	1000	1000	1000	1000	1000	1000	971	931	661	513	419	354
		Total effective damage from a level 80 white hit of 1000 on a level 83 target after ArP is applied

DPS percentage increase Table
Level 80		Level 83 Target Armor
ArP	%AR	1000	2000	3000	4000	5000	6000	7000	8000	8317	9000	10000	20000	30000	40000	50000
0	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%	0.0%
10	0.8%	0.1%	0.1%	0.1%	0.2%	0.2%	0.2%	0.3%	0.3%	0.3%	0.3%	0.3%	0.3%	0.3%	0.3%	0.3%
20	1.6%	0.1%	0.2%	0.3%	0.3%	0.4%	0.5%	0.5%	0.6%	0.6%	0.6%	0.6%	0.6%	0.6%	0.6%	0.6%
30	2.4%	0.2%	0.3%	0.4%	0.5%	0.6%	0.7%	0.8%	0.8%	0.9%	0.9%	0.9%	0.9%	0.8%	0.8%	0.8%
40	3.2%	0.2%	0.4%	0.5%	0.7%	0.8%	0.9%	1.0%	1.1%	1.2%	1.2%	1.2%	1.1%	1.1%	1.1%	1.1%
50	4.1%	0.3%	0.5%	0.7%	0.9%	1.0%	1.2%	1.3%	1.4%	1.5%	1.5%	1.4%	1.4%	1.4%	1.4%	1.4%
60	4.9%	0.3%	0.6%	0.8%	1.0%	1.2%	1.4%	1.6%	1.7%	1.8%	1.7%	1.7%	1.7%	1.7%	1.7%	1.7%
70	5.7%	0.4%	0.7%	0.9%	1.2%	1.4%	1.6%	1.8%	2.0%	2.0%	2.0%	2.0%	2.0%	2.0%	2.0%	2.0%
80	6.5%	0.4%	0.8%	1.1%	1.4%	1.6%	1.9%	2.1%	2.3%	2.3%	2.3%	2.3%	2.3%	2.3%	2.3%	2.3%
90	7.3%	0.5%	0.9%	1.2%	1.5%	1.8%	2.1%	2.4%	2.6%	2.7%	2.6%	2.6%	2.6%	2.6%	2.6%	2.6%
100	8.1%	0.5%	1.0%	1.4%	1.7%	2.0%	2.3%	2.6%	2.9%	3.0%	2.9%	2.9%	2.9%	2.9%	2.9%	2.8%
200	16.2%	1.0%	1.9%	2.7%	3.5%	4.2%	4.8%	5.4%	5.9%	6.1%	6.1%	6.1%	6.0%	5.9%	5.9%	5.9%
300	24.4%	1.5%	2.9%	4.2%	5.3%	6.4%	7.4%	8.3%	9.2%	9.4%	9.4%	9.4%	9.2%	9.1%	9.1%	9.0%
400	32.5%	2.0%	3.9%	5.6%	7.2%	8.7%	10.1%	11.4%	12.6%	13.0%	12.9%	12.9%	12.7%	12.6%	12.5%	12.4%
500	40.6%	2.6%	4.9%	7.2%	9.2%	11.2%	13.0%	14.7%	16.3%	16.7%	16.7%	16.7%	16.4%	16.2%	16.1%	16.0%
600	48.7%	3.1%	6.0%	8.7%	11.3%	13.7%	16.0%	18.1%	20.2%	20.8%	20.8%	20.7%	20.3%	20.1%	20.0%	19.9%
700	56.9%	3.6%	7.1%	10.3%	13.4%	16.4%	19.1%	21.8%	24.3%	25.1%	25.1%	25.0%	24.5%	24.3%	24.1%	24.0%
800	65.0%	4.2%	8.2%	12.0%	15.6%	19.1%	22.5%	25.7%	28.8%	29.8%	29.7%	29.6%	29.1%	28.8%	28.6%	28.4%
900	73.1%	4.7%	9.3%	13.7%	17.9%	22.1%	26.0%	29.9%	33.6%	34.8%	34.7%	34.6%	33.9%	33.6%	33.3%	33.1%
1000	81.2%	5.3%	10.4%	15.4%	20.3%	25.1%	29.8%	34.4%	38.8%	40.2%	40.1%	40.0%	39.2%	38.7%	38.4%	38.2%
1230	99.9%	6.6%	13.1%	19.7%	26.2%	32.8%	39.3%	45.9%	52.5%	54.5%	54.4%	54.2%	53.0%	52.3%	51.9%	51.6%
		Total percent damage increase due to ArP

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.

Skills and talents

• Deathknight: Blood Gorged.
• Druid: Faerie Fire.
• Hunter: Acid Spit (Worm pet) and Sting (Wasp pet).
• Rogue: Mace Specialization and Serrated Blades. Expose Armor also effectively counts as armor penetration.
• Warrior: Mace Specialization and Battle Stance. Sunder Armor also effectively counts as armor penetration.
• Expose Armor, Sunder Armor, and Acid Spit do not stack.
• Faerie Fire, Curse of Weakness, and Sting do not stack.

Itemization

Warrior sets:

• Heroes' Dreadnaught Battlegear
• Valorous Dreadnaught Battlegear
• Valorous Siegebreaker Battlegear
• Conqueror's Siegebreaker Battlegear
• Conqueror's Wrynn's Battlegear, Conqueror's Hellscream's Battlegear
• Triumphant Wrynn's Battlegear, Triumphant Hellscream's Battlegear
• Triumphant Wrynn's Battlegear (heroic), Triumphant Hellscream's Battlegear (heroic)
• Ymirjar Lord's Battlegear
• Sanctified Ymirjar Lord's Battlegear
• Sanctified Ymirjar Lord's Battlegear (heroic)

Rogue sets:

• Heroes' Bonescythe Battlegear
• Valorous Bonescythe Battlegear
• Valorous Terrorblade Battlegear
• Conqueror's Terrorblade Battlegear
• Conqueror's VanCleef's Battlegear, Conqueror's Garona's Battlegear
• Triumphant VanCleef's Battlegear, Triumphant Garona's Battlegear
• Triumphant VanCleef's Battlegear (heroic), Triumphant Garona's Battlegear (heroic)

Hunter sets:

• Heroes' Cryptstalker Battlegear
• Valorous Cryptstalker Battlegear
• Valorous Scourgestalker Battlegear
• Conqueror's Scourgestalker Battlegear
• Conqueror's Windrunner's Battlegear, Conqueror's Windrunner's Pursuit
• Triumphant Windrunner's Battlegear, Triumphant Windrunner's Pursuit
• Triumphant Windrunner's Battlegear (heroic), Triumphant Windrunner's Pursuit (heroic)

Weapons:

• [Hellion Glaive]
• [Twin's Pact]
• [Edge of Ruin]
• [Dual-blade Butcher]
• [Justicebringer]
• [Stormedge]
• [Marrowstrike]
• [Hoperender]
• [Lotrafen, Spear of the Damned]
• [Archon Glaive],[Hellion Glaive]
• [Ironsoul]
• [Abaddon]
• [Aesir's Edge]
• [Reckoning (sword)],[Edge of Agony]
• [Stormrune Edge]
• [Voldrethar, Dark Blade of Oblivion]

Trinkets:

• [Banner of Victory]
• [Grim Toll]
• [Mjolnir Runestone]
• [Needle-Encrusted Scorpion]

Enchants:

• [Enchant Weapon - Executioner] note: 2x Executioner do not stack.

Gems:

• [Fractured Cardinal Ruby]
• [Puissant Dreadstone]
• [Fractured Dragon's Eye]
• [Fractured Scarlet Ruby]
• [Puissant Twilight Opal]

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 [Indestructible Potion], 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: [Enchant Cloak - Mighty Armor] 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):

If (targetlevel<60)
   C=400+85*targetlevel
Else
   C=400+85*targetlevel+4.5*85*(targetlevel-59);

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.

Patch changes

• Patch 4.0.3 (15-Nov-2010): "We are always looking for ways to improve how player stats work. Our long-term plan is to simplify statistics and provide a more clear understanding of their effects on healing and damage dealing. For example, Spell Power will no longer be its own statistic, and Intellect will provide its benefits instead. We also plan to make Spirit the definitive mana-regeneration statistic, in which case we would remove the mana-per-5-seconds stat from gear. Confusing stats like defense and armor penetration will likely go away. Of course, existing items will be updated to reflect these changes. We're also adding a new stat called Mastery, which ties into a character's existing talents and will grant a unique function for each class. A rogue, for example, might gain an extra combo point through the Mastery stat." (Source)
  
• Patch 4.0.3 (15-Nov-2010): Announced at the BlizzCon 2009 Game Systems Panel, armor penetration will no longer be available on gear. However, it will still be part of the game via the Mastery system. (Source)
              See the source link for an example of how armor penetration will be incorporated into talents in the Cataclysm expansion.
  
• Patch 3.2.2 (2009-09-22): The amount of armor penetration gained per point of this rating has been reduced by 12%.
  
• Patch 3.1.2 (2009-05-19): Capped to 100% (or 1232 armor penetration rating)
  
• Patch 3.1.0 (14-Apr-2009): All classes now receive 25% more benefit from Armor Penetration Rating.
  
• Patch 2.4.1 (2008-04-01): Armor penetration will now display on the character sheet (armor penetration displays in the hit rating section).
  

See also

• Damage reduction

External links

WorldOfWarcraft.com Armor pen nerf in 3.2.2 (archive)

elitistjerks.com [RAID] Boss armor values, post #35

parrydodgespin.com Rogues and Armor Penetration

rehfeld.us/wow Armor Penetration Damage Increase Calculator (accuracy not verified)