Armor penetration or ArP is renowned for being the most confusing, poorly worded, explained, and implemented stat in World of Warcraft. It is partially the reason why Blizzard decided to remove it from the game starting in Cataclysm. Thankfully, on Molten-WoW ArP is now working nearly 100% as intended, and so our lives as hunters are easier.
The reason Armor Penetration is so confusing is the way it’s worded and the simplistic way all humans like thinking. Everyone can assume that 1% Armor Penetration (14 ArP Rating) will give you a 1% damage increase. This is wrong. I realize that it seems like it would be a kind of thing you would say “no you’re wrong! 1% less Armor means 1% moar dps, but let me explain. The reason there is so much confusion as to what ArP does, and how it affects our damage comes from an incorrect understanding of Armor itself.
An incorrect understanding of Armor? But Donorbashed! Armor is simple!
Before we continue, you know how annoying Rogues are with their f**king kidney shot spam, and how pitiful we think Affliction Warlocks are when they are spamming us with so much fear we actually become spam itself?
And then you think to yourself “why doesn’t Blizzard make it so they can’t do that”. Right? Right. (I know I hate that shit too.) Well, they did fix it without you even knowing. They made something called Diminishing Returns. This decreases the effect time after they use the ability on you a few times.
Diminishing Returns, explained as basically as possible is this: The concept that as you acquire more of a stat, the less benefit(effect) it gives you. That means, the more armor you get, the less damage it reduces per point. For those of you who are more familiar (or interested) with the hunter class, the exact formula for Damage Reduction is thought to be something like this.
DR% = Armor / (Armor + 400 + 85 * (AttackerLevel + 4.5 * (AttackerLevel – 59)))
Seeing as how we are assuming that everything we are attacking is level 83 (this is endgame) we can simplify this to something like this:
DR% = Armor / (Armor + 15,232.5)
So let’s say that we are dealing with an endgame boss. No, we’re fighting Marrowgar. Aaaaaaand like every boss in ICC, he has round about 10,000 armor (they actually have 10,643, but I like easy math). That means he has
10,000/(10,000 + 15,232.5).
Or 39.63% reduction.
Now, let’s say we have 140 ArP rating (10% ArP). That means we knock off 10%, or 1,000 of his armor, so now he has 9,000 Armor. Let’s do the formula again.
9,000/(9,000 + 15,232.5) = 37.14% damage reduction.
39.63 – 37.14 = 2.49% damage increase
That means 140 ArP (10%) gives you roughly a 2.49% damage increase. Now! Let’s say we double our ArP, and have 280 rating, or 20%. Marrowgar still has 10,000 but after we apply our ArP, we knock 2,000 armor off and we can do the formula again. What do you think the result will be? 4.98%? Let’s see.
8,000(8,000 + 15,232.5) = 34.43%, or 5.23% increase in damage.
Hopefully this shows how accumulative Armor Pen is. In this example, the decrease in the damage reduction (ie. your increase in physical dps) increases per point a very small amount, but I challenge you to go higher to 20%, or even 40% Armor reduction to see for yourself.
What does this mean for raids?
It means that even when you’re at 100% ArPm you aren’t getting a 100% damage increase. Instead, to find out your damage increase with 100% ArP, find the percent of damage reduction when your target has full Armor (ie. 0% ArP), and that is your dps increase that the ArP cap will give, like so:
10,000(10,000 + 15,232.5) = 39.63%
Now, since 10,000 armor reduces your damage by 39.63%, and the decrease in the damage reduction is equal to your increase in dps, then considering 100% ArP (0 damage reduction, or a 39.63% decrease in damage reduction) the most ArP will increase your dps is by about 40%. Therefore, this also means that ArP is most effective in the end-game raids (VoA, ICC, ToC, RS) because HC and earlier raid raid bosses have progressively less armor, which means less base damage reduction, and less maximum dps increase.
I hope this helps. Thanks for reading!