Let me start by laying out all of the jargon ahead of time. There's two key terms to consider when we're talking about Armor Penetration (or ArP/ArPen, in short form): the Soft Cap (SC) and the Hard Cap (HC). The Soft Cap refers to the idea that you are capable of capping your ArP (the cap is 1,400 currently) when you have some sort of Armor Penetration trinket active, but you're under this cap when the trinkets aren't active. There's two such trinkets in-game: Grim Toll and Mjolnir Runestone. You've probably seen a lot of demand/jealousy over these trinkets, as they are often BIS for many Combat builds.
Grim Toll provides a rogue with 612 ArP while active, and Mjolnir Runestone provides 665 ArP. Because we're trying to reach the Armor Penetration Cap (1,400), this means we'll need either 788 ArP on gear to reach Grim Toll's Soft Cap or 735 ArP on gear if using the Mjolnir Runestone Soft Cap. Otherwise, when the trinket procs, all of the ArP over the cap will be wasted. This is obviously not what you want to do (wasting a portion of a trinket's proc), so you must ensure you are as close to the Soft Cap as possible without exceeding it. A great way to figure out how close you are to the ArP Soft Cap is to look at your passive ArP on your gear. If it's around 400-500, and you're not using any ArP gems, then you could simply swap all of your gems to ArP and reach the Soft Cap without any gear swaps. You can typically gem for between 250 and 400 ArP, depending on the number of sockets in your gear, using Fractured Cardinal Rubies.
Let's say that you weren't fortunate enough to have scored a Grim Toll/Mjolnir Runestone by now, and so you're sitting on alternate trinkets. This is where the Armor Penetration Hard Cap comes into play. The Hard Cap is set at 1,400, which is where your ArP is permanently capped, regardless of if you have a trinket active or not. Using some of the best gear in the game, gemming pure Armor Penetration gems, and focusing solely on ArP, it is still impossible (according to my math) for a rogue to reach the ArP Hard Cap. If you were to spec maces, it would be possible via the bonus from Mace Specialization, but there are no end-game off hand maces and few MH maces worth it.
The real concern is that when Icecrown is released and the item level of gear reaches its highest point, a rogue will be able to hard cap Armor Penetration passively. This results in a significant DPS increase, and will be the gearing objective of the most high end Combat raiders. A well-designed PvE gearset includes between 18-22 sockets, which allow a rogue to gem somewhere around 400 ArP if using all ArP gems. A rogue's "hard cap" set would then have to include 1,000 passive ArP rating: something that I simply can't find on gear currently. My best attempt yielded a fairly high-end set with 899 passive ArP. ICC's release should push this into the realm of possibility.
In short, unless you have a Grim Toll or Mjolnir Runestone, neither of the ArP caps are of any importance to you. If you are lucky enough to be graced with one of these, simply ensure that your ArP is near but not over the Soft Cap for your particular trinket. It's important to note that Sunder/Expose Armor, Faerie Fire, and other ArP debuffs do not affect the ArP Soft/Hard Caps. The ArP caps we talk about are all personal, and they're the same regardless of what buffs or debuffs an opponent may have. Do note that these formulas are based around the idea of fighting a level 83 boss.
The Crit Cap is another one of those jargon terms that you'll hear from time to time without really seeing it explained in detail. Most rogues are talking specifically about the 'White Hit Critical Strike Chance Cap', or WCC for short, which is the point where additional critical strike rating no longer increases your chance to score white swing crits. The reason that this cap exists is the Glancing Blow mechanic, which is implemented to lower melee damage against boss-level mobs. Due to Glancing Blows, a rogue can literally get to the point where they are unable to crit any more often.
Let's first note that neither yellow attacks (special moves) nor poison damage are afflicted by the white crit cap, as they are unable to be glancing blows. They each have their own crit cap, but it's significantly higher and currently unreachable. The crit cap that rogues are currently able to reach only affects white swings. However, white swings make up a huge portion of our a damage, especially as Combat via Prey on the Weak. Let's talk about how this cap is calculated and reached.
Blizzard bases all attacks on a "roll" that takes place when you swing. This roll can be thought of as a /roll 100. Blizzard chooses a random number between 1-100, and the result determines what happens with that attack. If you roll a 1-5, Blizzard converts the attack into a hit. This is due to an undetermined mechanic known as "crit suppression" that most bosses seem to have. It's actually set to 4.8% (as researched by Vulajin), but can be rounded to 5% for this simple example. It just means that 5% of the time, you simply won't crit.
Now we factor in glancing blows, and if you roll a 6-29, you will have a glancing blow. This means that your attack hits for half damage and is unable to crit. That's a total of 29 possible chances for you to "not crit". In addition, we have to factor in a mob's chance to evade your attacks. If you're Expertise capped and Hit capped (white hit capped, or usually 722 hit rating with Precision / Misery), a mob has no chance to avoid you, and so if you roll 30-100, you have a chance of critting the mob. This results in the crit cap for a rogue with capped hit and expertise being 71%, as you are guaranteed to "not" crit 29% of the time.
The complete formula (thanks to Russ for a correction!) is:
Start with 100%
Add a boss' crit suppression of 4.8%
Subtract a boss' dodge and parry, affected by expertise and positioning, usually near 0-2%
Subtract your glancing blow chance of 24%
Subtract your miss, affected by hit, usually between 0-10%
= Your White Crit Cap.
A great WCC calculator is available at Filltheglass.org
. If you're under the White Hit Cap (WHC) and the Expertise Cap (EC) (you can read up on both here
), then this number is even lower, as some of the 1-100 roll will be taken up with misses and dodges. Let's say you're missing 5% hit and 2% dodge from your Hit and Expertise, and so now your crit cap is 64% crit. The question becomes: can you reach the crit cap in any real situation? The answer is 'yes, if you're geared, or very undergeared'.
Take Dark Matter, for example
; it provides 13.33% chance to crit when active
. Considering that 40% crit is not uncommon to see for a Combat rogue while unbuffed, and that there are several critical strike buffs and debuffs (5% from Leader of the Pack
, 3% from Master Poisoner
/Heart of the Crusader
), and Dark Matter's amazing proc, you can easily reach 60%+ critical strike chance in a raiding environment. Blessing of Kings
, Horn of Winter
, and Mark of the Wild
also add to this critical strike chance value. If you're gemming Agility
, this number can get even crazier. However, without the 13% from Dark Matter, it's still pretty rough getting to the crit cap by yourself.
Similar to ArP, there are two White Crit Caps: Soft
. Soft would imply you have a trinket that grants you a significant amount of critical strike rating or Agility (such as the very popular Darkmoon Card: Greatness
), and that you can reach the WCC when this trinket is active. The Hard Cap would imply that you are at your personal WCC even without your trinkets active. The WCC is obviously easier to reach if you have very low hit and expertise. If you're getting close to wasting itemization points on agility/crit due to being at your WCC, you can socket Hit/Expertise to raise your cap and buy you some breathing room. This is one of the reasons that spreadsheets making gearing decisions far easier
: it's almost impossible to do all of this math in your head.
If you're getting close to your WCC
, explore using Expertise or Hit to raise the cap, as this should yield a net DPS boost from the saved critical strike rating. With a low expertise/hit set in ICC, it may be possible to reach the White Hard Crit Cap, so be sure to evaluate your own crit cap before picking up a shiny new trinket or regemming. If you've got a GT/MR, be mindful of the ArP Soft Cap (as you are for hit/expertise). If you don't have a GT/MR or a Dark Matter, then none of this will really apply to you unless you are loaded with ICC gear with a very particular itemization pattern and gemming strategy. For a quick reference chart of any cap-related values, check out Cally's great post at Elitist Jerks