Defense

Defense was an attribute that helped prevent physical damage by reducing the chance to be hit or critically hit. It also increased block, dodge and parry. In Patch 4.0.1 defense was removed from the game as part of Blizzard's effort to "simplify statistics" for Cataclysm. Tanking classes can now become immune to critical hits from mobs by shifting into Defensive Stance (warrior), Blood Presence (death knight), or Bear Form (druid), or by using Righteous Fury (paladin).

Overview
Defense is a skill that helps prevent physical damage by reducing the chance to be hit or critically hit. Additionally, it also increases block, dodge and parry. Mobs receive 0.1% per point of defense. Mobs get an additional benefit from defense of Glancing Blows versus players.

Each point of +Defense skill adds 0.04% to the player's chance to be Missed, to Dodge, and to Parry, before diminishing returns are taken into account, and adds 0.04% to the player's chance to Block with no diminishing returns. This means +25 Defense will grant you an extra 1% Miss, Parry, Dodge, and Block. The formula is constant across all classes. It further decreases the chance of receiving critical hits from any level attacker by 0.04% per point that the target's Defense skill exceeds the attacker's Weapon Skill. Please note that a decrease in your chance to be hit (like the tooltip says) actually increases your chance to be missed. However, all of these chances are based on an opponent of equal level. The rating difference between the attacker's weapon skill and the defender's defense factors into the true chance for miss, parry, dodge, block, crushing blow, glancing blow, and critical hit.

Note that while your tooltip chances to be dodged and parried will take diminishing returns into account, your tooltip decreased chance to be hit (shown while hovering over your Defense on your character sheet) will not. At 540 Defense, your actual chance to be missed by a level 80 mob is 9.288%.

Note that defense continues to be a valuable stat past its "cap," unlike hit rating or expertise, because it still increases chances to dodge, block, parry and be missed. Reaching the soft cap of 540 defense skill (not rating) is considered mandatory for tanking more difficult content, as a single critical hit from a boss can prove fatal.

Critical Hits

 * Quick Facts:
 * Critical Hit immunity for a level 60 player against a raid boss occurs at 440 Defense and requires a defense skill of 140 from gear to achieve.


 * Critical Hit immunity for a level 70 player against a raid boss occurs at 490 Defense and requires a defense skill of 140 (332+ def rating) from gear to achieve.
 * Critical Hit immunity for a level 70 player against a raid boss can also be achieved by a Resilience of 5.6% and requires a resilience rating of 221 to achieve.


 * Critical Hit immunity for a level 80 player in a Heroic Dungeon is 535 Defense, because mobs in a level 80 heroic 5-man are never higher than level 82.
 * Critical Hit immunity for a level 80 player against a Raid Boss occurs at 540 Defense. This requires a defense skill of 140, achieved from a combined 689+ Defense Rating from gear, enchants, gems, etc.
 * Critical Hit immunity for a level 80 player against a Raid Boss can also be achieved by raising Resilience to 5.6%, being a resilience rating of 460.
 * If the combined critical hit reduction from Defense and Resilience together total 5.6% that will also make the player immune to critical hits from Raid Bosses.

See below for a more detailed discussion of Defense.

Critical Hits are strikes by mobs against the player that deal twice the normal damage. The player's Defense and Resilience are both attributes that reduce the chance of an enemy critically striking. The chance to be critically hit is based on the difference between the attacker's weapon skill and the player's defense skill and resilience %.
 * Details

A mob has a weapon skill that is calculated by 5 x Level, and a player has a base defense that is 5 x Level. A player does not have any base resilience. Therefore a level 80 mob has 5x80=400 Weapon Skill. A level 80 player has a base defense of 5x80=400 Defense and no resilience.

When an equal level mob is attacking a player, the mob has a 5% chance to critically strike the player. For every level beyond the player, there is an additional 0.2% chance for the mob to critically strike the player. Therefore a level 83 mob (a raid boss) attacking a level 80 player has a 5% + (3 x 0.2%) = 5.6% chance to critically strike the player. A player in the role of a tank for a raid boss must lower his or her chance of being critically struck by 5.6% to avoid the possibility of taking 2x the damage from a single hit. This is where gear bonuses that augment resilience and defense are important.

Each point of defense beyond the player's base reduces the chance to be critically struck by 0.04%. Therefore, to become immune to critical hits dealt from a mob 3 levels higher, a player must have 5.6% / (0.04% per defense) = 140 additional points in their defense skill to avoid a critical strike. Therefore a level 80 player with a base defense of 400 must have 400 + 140 = 540 Defense to become immune to critical hits through defense alone.

Because a single critical hit from a raid boss can easily remove most, if not all, of its target's HP, becoming crit immune is far more important for tanks than anything else until they are immune; even HP granted from high amounts of Stamina is not enough to mitigate a critical hit.

However, resilience may also be used to lower the chance of being critically struck. There is no base resilience for players. Therefore a level 80 player with no additional defense points, will simply need 5.6% resilience to become immune to critical hits dealt by a raid boss.

Note that given a choice between PvP gear and actual tanking gear, the latter is far better because it provides additional use for tanks such as dodge and parry rating.

For a level 70 player, gear that provides +39.4 resilience rating will yield a 1% resilience. And gear that provides 2.36 defense rating will increase the defense skill by 1. For a level 80 player, gear that provides +82.0 resilience rating will yield a 1% resilience. And gear that provides 4.92 defense rating will increase the defense skill by 1. See below for the formula used to derive defense skill from defense rating. See Resilience for a discussion on the formula to calculate % resilience obtained from resilience rating.

Feral druids have Survival of the Fittest, which reduces the inbound crit rate by 6%, making them naturally immune to melee critical hits in PvE.

Crushing blows
Crushing blows can happen if a mob attacks a player at least 4 levels lower. Because "boss" level mobs are considered to be level 83 they cannot deliver a crushing blow to any level 80 character whose defense skill is fully trained up to his level * 5.

Prior to patch 3.0 crushing blows affected players of at least 3 levels lower than the attacking mob so max-level characters (typically the main tank) could receive a crushing blow from a boss. Because of this change the rest of this section is mostly of historical interest.

Only the player's base defense skill (capped at 5 times the player's level, e.g. 350 for a level 70 player) is taken into account to determine the base chance for a Crushing Blow, and therefore Defense Rating on gear does not directly decrease the chance for a Crushing Blow.

Crushing Blows can be made less likely or entirely avoided by increasing avoidance stats such as Dodge or Parry or the player's Block Rating, as well as using certain skills, to the point where the player's combined Miss, Dodge, Parry, and Block chances against that specific attacker exceed 85%. This is due to how the WoW combat roll system works, cf. Attack Table. In this regard, Defense Rating on gear does help against Crushing Blows.

Refer to the Wiki sections Crushing blow and Uncrushability for an in-depth explanation of this mechanic.

Dazed
If a mob hits a target from behind, and the target takes damage from the hit that isn't Absorbed, there is a chance the player will be Dazed. The base chance for a level 30+ target to be dazed when hit from behind, if the mob's weapon skill is equal to the target's defense skill, is 20%. This chance can get as low as 0% if the target's defense is high enough, or as high as 40% if the target's defense is low enough. If the target is below level 30, it's base chance to be dazed is less; a level 10 target with fully trained defense skill only has a 7% chance to be dazed when attacked from behind by a level 10 mob.

Note that your defense skill is NOT level-capped for purposes of determining your chance to be dazed. It is possible to become undazeable while wearing tank plate gear.

Patch 3.3 made level 1-5 characters immune to being dazed and level 6-10 characters less likely to be dazed.

Changes in patch 2.0
As of WoW 2.0, additional defensive skill does not help avoid crushing blows.

Instead of directly affecting Defense skill, items after patch 2.0 grant a bonus to Defense Rating (DR). Skill bonuses are then calculated as follows:

For a level 60 : 1.5 DR = 1 defense skill For a level 70 : 2.36 DR = 1 defense skill For a level 80 : 4.92 DR = 1 defense skill

It is important to note that for the sake of all indirect effects of Defense Skill (dodge, block, parry, etc), the game only considers Skill as an integer. That is, there is absolutely no difference between 10 Rating (+4.23 Skill) and 11 Rating (+4.65 Skill), because they both yield only 4 integer points of Skill. However, there is a difference between 11 Rating (+4.65 Skill) and 12 Rating (+5.07 Skill); specifically, the difference is exactly 0.04% dodge, block, parry, etc.

At L70 you need 14.75 of defense rating to get 0.25% increases to each of miss/dodge/parry and block for a total increase in avoidance of 1%. At L60 this is 9.375. Both may suffer small inaccuracies due to the above mentioned integer rounding.