Thanks for your patience. We have a lot of things we are looking at in the game right now and can't respond to every issue. We appreciate your not excessively bumping or launching new threads.

Based on preliminary tests, there doesn't seem to be anything amiss with how armor pen is calculated. There is a cap on the amount of armor you can reduce, but it is fairly complicated and depends on things like creature level and "class" (most mobs are warriors).

We'll make a break from tradition and post exactly how the armor penetration number is derived. We tried to examine some of the numbers posted in this thread, but they were complicated by extraneous factors such as not always knowing the target level, the effects of mace spec, and multipliers on say Bloodthirst that we couldn't ascertain.

Give us a bit to get some numbers together and complete our tests and I'll respond to this thread. As a quick and dirty calculation, assume that the cap on a level 83 warrior mob is 8205.75 * your percent of armor penetration.

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Ghostcrawler
Lead Systems Designer

Okay, here is a farily 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 (level<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.

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Ghostcrawler
Lead Systems Designer

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Q u o t e:
Uhh... GC came in and stomped all over my last 5 hours of testing against a paladin in pvp by showing quite a few assumptions I'd thought were safe were wrong.
Ghostcrawler clearly rules me. Not to mention he just saved me hours and hours of testing and theorizing.

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This is honestly one of the reasons we don't do this more often. There is a risk players will stop experimenting and theorycrafting if they think we will eventually just dump all of the answers on them. We like for players to experiment with gear, talents and the like. Having black boxes add depth and a sense of exploration to the game. When everything is known with certainty, you can do things like definitively know the best choice in every situation. Theorycrafting is dead.

However, in this case, the system was both complicated and difficult to test so we figured we'd just open the kimono.

This is not something I could spew off the top of my head. Jimmy the Numbers had to come bail me out. :)

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Ghostcrawler
Lead Systems Designer

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Q u o t e:
Wow, seeing GC's numbers, You guys made WoW more complex then I was lead to believe, time to think harder with my suggestions if I make any.

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It often is complex. That is one reason I advise taking theorycrafted numbers with a grain of salt. They are emphatically NOT useless. But they don't always capture every nuance, variable or situation.

This is also a reason why sneaky bugs can sometimes slip in. That's not intended to be an excuse -- just an explanation for those times when someone says "It's so simple - how could they mess this up?" :)

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Ghostcrawler
Lead Systems Designer