Armor Penetration, Attempted Simplification
As we all know when hovering over your armor value in game, the amount of armor you have reduces your incoming damage by a % based on your armor value. The mobs, players and bosses against whom we dull our blades are no exception. They too have armor, and they too reap it's damage reducing benefits. What this means is that THEIR armor is reducing OUR damage. If we reduce their armor, we reduce that reduction to our damage, and thus we have Armor Penetration Rating. It seems to me that most people are not really sure how it works, so I'm trying to explain it simply, but thoroughly here for people who want to know how it works. Nothing being said is new, and I just referenced a whole bunch of googled sites to pull my information from. I hope it helps.
The assumed armor of a level 80 warrior mob is 9729.
The assumed armor of a level 83 raid boss is 13083.
(rawr has the target armor value set at 10643, and I'm not sure why. There are people much smarter than me working on these programs, and they are probably right. The armor value of a raid boss, regardless of which value is correct, affects only the final numbers that I may reach in my illustrations. And they are just illustrations, they would be equally effective as learning tools if I pulled numbers out of a hat.)
There is a cap to how much armor can be penetrated. To determine the maximum amount of armor that may be penetrated for every mob, the game chooses the the lower number between either the bosses base armor, or the result of their cap formula.
The formula for the armor pen cap is: (armor + C)/3.
C is a constant that is based on the level of the mob. There are tables on the internet telling us the value of c for every level of mob, but if you are interested in how to actually calculate C, it is below.
The formula for C is If (level<60)
C for a level 80 mob is 15232.5.
C for a level 83 mob is 16635.
Let's have an example of how to calculate the armor pen cap.
At level 80:
(9729 + 15232.5)/3 = 8320.5
Now, the game will choose the lower number between the bosses base armor (9729) or the result of the cap formula (8320.5) to determine the maximum amount of armor you may penetrate. Therefore, in this illustration, you may penetrate up to 8320.5 armor.
How does this relate to your gear? Multiply your arP % by the capped armor amount to see how much armor you are penetrating and subtract that amount from the bosses base armor to determine how much armor the boss has remaining. You may then calculate the damage reduction due to armor that the mob has against you if you so choose.
If you have 10% arP, you will penetrate 832.05 armor (8320.5 * .1 = 832.05) and the mob will have 8896.95 armor (9729 - 832.05 = 8896.95) remaining.
If you have 50% arP you will penetrate 4160.25 armor and the mob will have 5568.75 armor remaining.
If you have 100% arP you will penetrate 8320.5 armor and the mob will have 1408.5 armor remaining.
Another big hangup I've noticed is how arP is calculated in relation to the arP debuffs like Sunder Armor and Faerie Fire.
Armor pen is calculated in two steps. The first step deals with debuffs while the second step deals with personal armor penetration amounts. The above formulas and illustrations covered your personal armor penetration, or the second step. Sunder and FF are dealt with in the first step.
To quote Kazeyonoma
Sunder and FF are additive. Sunder removes 20% of the targets armor & FF removes 5% of the targets armor. Therefore, on a boss debuffed with both, it's armor will be reduced by 25%. (20%[sunder] + 5%[FF] = 25%). To calculate this 25% reduction, you need only multiply the mobs base armor by .75 and you get the mobs new, debuffed, armor value. If there is only sunder on the boss you would multiply the base armor by .8. If there is only FF on the mob you would multiply the base armor by .95.
Don't think of Sunder/ FF as Armor Penetration, they aren't, they are a totally different mechanic sorta like... Armor Reduction. ArP determines how much armor you ignore, Armor Reduction determines how much armor the boss actually has.
You take the bosses base armor, calculate the debuffs and then use that final value as the "armor" value in the armor pen cap formula.
For example, the same level 80 mob, now debuffed with Sunder and FF:
9729 * .75 = 7296.75 (debuffed boss armor)
(7296.75 + 15232.5)/3 = 7509.75
The game will again choose the lowest of the numbers between the debuffed base armor and the arP cap formula. This time the maximum amount of armor you may penetrate is 7296.75.
If you have 10% arP, you will penetrate 729.675 armor and the mob will have 6567.075 armor remaining.
If you have 50% arP, you will penetrate 3648.375 armor and the mob will have 3648.375 armor remaining.
If you have 100% arP, you will penetrate 7296.75 armor and the mob will have 0 armor remaining.
Just for reference, I'll calculate a boss level mob with a few different debuffs and personal armor values.
83 mob with no debuffs:
(13083 + 16635)/3 = 9906
If you have 10% arP, you will penetrate 990.6 armor and the mob will have 12092.4 armor remaining.
If you have 50% arP, you will penetrate 4953 armor and the mob will have 8130 armor remaining.
If you have 100% arP, you will penetrate 9906 armor and the mob will have 3177 armor remaining.
83 mob with Sunder and FF:
13083 * .75 = 9812.25
(9812.25 + 16635)/3 = 8815.75
If you have 10% arP, you will penetrate 881.575 armor and the mob will have 8930.675 armor remaining.
If you have 50% arP, you will penetrate 4407.875 armor and the mob will have 5404.375 armor remaining.
If you have 100% arP, you will penetrate 8815.75 armor and the mob will have 996.5 armor remaining.
Finally, just some tips from Tenraiel when gearing your warriors (and probably you other physical dps classes), whether or not to gem arP or str depends quite a bit on the trinket you are using. If you have the Needle Encrusted Scorpion, then you need to get to about 52% arP and after you have reached that amount, gem Str. Same applies for other arP trinkets you may have - the idea is to get to 100% when it procs.
If you are not using a trinket that procs arP, then the closer you get to 100% arP, the better: gem arP. However, if your gear is not good enough that you reach at least 40% arP when gemmed, stay str.
The arP cap for fury (and most classes w/o baked in arP in a talent tree) is 1400.
The arP cap for arms is 1260.
The arP cap for arms with 2pc t9 is 1176.
The arP cap for arms with mace spec is 1050.
The arP cap for arms with mace spec with 2pc t9 is 966.
As a neat trick, if you are not using a trinket that procs arP, you can place executioner on your weapon - an enchant that grants 120 arP rating and slightly outperforms berserking until you get 1280 arP buffed. Subtract 120 from the appropriate arP values above to figure out when executioner becomes inneffective for you silly arms warriors. Do not use executioner if you have a trinket that procs arP.
Use the agi+crit damage meta. Use a nightmare tear in the blue gem slot that allows you to gain the best socket bonus that you can, fill all your reds with str or arP and (almost always) fill the yellows with str/crit. Ignore all the other blue gems slots .... Those OTHER blue gem slots are actually just a way of letting you know that it is a red gem slot. Got it?
Finally, arP is a very complicated stat. If you don't understand why you are gemming arP, then just gem str. Str is ALWAYS a good stat and will not lead you astray.
Rawr, found at www.codeplex.com/Rawr