All About Armor Penetration

Let’s dive in to the meat and potatoes of today’s post. The effectiveness of Armor Penetration. You see this popping up on a lot of gear, such as Season 3 items and a lot of the loot coming out of Zul’Aman. The bottom line is that Armor Penetration is an awesome stat. Let’s look at why, and where the value should be placed.

The first thing we need to know is how Armor calculates damage reduction. The formula for calculating damage reduction is as follows:
Reduction Percentage = Armor/(Armor + 10557.5)
Reduction Percentage = Armor/(Armor + 11960) when a level 73 target attacks you (useful for tanks)
For example, let’s take my current armor, which is 7181. Putting that into the formula, it looks like this:
7151/( 7151 + 10557.5 ) = 7151/17708.5 = 0.404826
Now if we look at myself on the Armory, this number coincides with what Blizzard’s tooltip is displaying, a damage reduction value of 40.48%.

Armor reduction is not a linear thing either. Take a look at this chart borrowed from WoWWiki to get a visual idea of what I mean:
There is a mathematical cap to the effectives of your Armor, usually a problem only high-end Druids have to concern themselves with. :)

OK, have we lost anyone? I hope not. Now we have a quick grasp of how Armor functions, we can start to talk about how Armor Penetration functions as well. Armor Penetration does exactly what it sounds like, it ignores your targets Armor, calculating the mob/person’s damage reduction against your attacks as if they had a lower Armor level due to this stat.

Let’s first take a look at the Armor value of many of the targets we would be dealing with.

So let’s run some hypothetical scenarios real quick so we can get an idea of how this would apply to us in-game. From the research done is this thread, which is reflected above, we know that Attuman the Huntsman has 7500 Armor. So if we plug that into our formula, the damage reduction we get is this:
7500/(7500+10557.5) = 0.41534= 41.53% Damage Reduction
That number also correlates correctly with the information provided by Elitist Jerks. With 41.53% damage reduction, we are capable of doing 58.47% of our possible damage. That is calculated by figuring out that if we attacked a target with zero Armor, our damage would be at 100%, and then subtract the target’s damage reduction. Given that, we can now see what our Armor Penetration will do for us. For instance, let’s say you have 500 Armor Penetration. We can now safely say that Attuman only has 7000 Armor. So let’s crunch that number:

7000/(7000+10557.5) = 0.39869 = 39.869% Damage Reduction
So we now know that we are doing 60.131% of our total possible damage. This is a 2.84% damage increase for us when compared to 0 Armor Penetration. That was figured like this:
60.131/58.47 = 1.0284
As you can see, we are just taking the percentage of the damage we can do after armor reduction with our Armor Penetration and dividing it by the pre-ArP damage. The actual number you will see is 1.028407~.

And not all Armor Penetration amounts are created equal either. Let’s do a comparison between 500 Armor Penetration and 1000 Armor Penetration. With full armor, Attuman has 41.534% damage reduction, allowing us to do 58.47% of our total possible damage.

6500/(6500+10557.5) = 0.38106
With 1000 ArP, Attuman now has a damage reduction of 38.1%, allowing us to do 61.9% of our total damage. This comes as a 5.86% increase in damage when compared to attacking with no Armor Penetration. So if we subtract the damage increase from the first 500 ArP (2.84%) and subtract that from our new number (5.86%).
5.86 – 2.84 = 3.02
So we now see the second 500 Armor Penetration netted us a 3.02% damage increase, while the first 500 only gave us 2.84.

While that gain my seem very slight, remember that you are not the only person with Armor-reducing effects. Sunder Armor, Faerie Fire, Curse of Recklessness all are debuffs that are going to reduce the targets armor. So let’s say Attuman is now debuffed with these spells, including 5 stacks of Sunder Armor. With just those three abilities listed, Attuman is now down to 3490 Armor. This puts him at 24.8% damage reduction, already allowing us to achieve 75.2% of our total possible damage.

I found this calculator that calculates the DPS gain from Armor Penetration. The numbers it is producing seem accurate with all the math we have done thus far.

So hopefully no one’s head has exploded yet and that this is making sense. You can see why the Daggers of Bad Mojo are so highly rated on the Hunter’s desired weapon list, given that together you gain 240 Armor Penetration in conjunction with 162 Attack Power and 82 Agility (after enchanting them of course). :)

Another popular spreadsheet is this one, referencing damage reduction values against other players and their “class value.”

Well, that’s it for now. Hopefully I have all my research right. If you have any questions, comments, corrections, or concerns, leave them right here. Have a great weekend. Especially all of us who get a 3-day one. :D

About Drotara

Drotara (or BehemothDan) considers himself a geek on many levels. A web developer and programmer by trade, he has no shortage of geeky hobbies. When not fulfilling husband and daddy duties, he enjoys WoW, the WoW TCG, Magic: The Gathering, and great board games with friends and family.