Chance to block
Your chance to Block an attack is a percentage that follows this formula:
- Block% = 5% base chance + contribution from Block Rating + contribution from talents + ((Defense skill - attacker's weapon skill) * 0.04)
Your Block Rating (introduced in patch 2.0.1) is provided by items that have a Block Rating bonus. At level 60, you gain 1% to your Block Chance for every 5 points of Block Rating. At level 70, you gain 1% to your Block Chance for every 7.884614944458 points of Block Rating.
For example, a level 60 warrior with a Block Rating of 10, 5 points in the Shield Specialization talent, 325 Defense skill, and who was fighting level 60 mobs would have:
- Block Rating contribution = 10 / 5 = 2%
- Defense contribution = (325 - 300) * 0.04 = 1% (A mob's weapon skill is assumed to be its level * 5)
- 5% base + 2% Block Rating + 5% talents + 1% Defense modifier = 13% Block chance versus level 60 mobs
In combat, you will notice that the percentage of incoming attacks that are Blocked matches the Block percentage shown in your tooltips. Miss chance and Critical chance are unmodified by your Block chance, so you're not "wasting" Blocks on misses nor are you able to Block a Critical. This may seem odd to some folks if they are expecting a "if hit, then check if Parry, then check if Block, then check..." type system. WoW, like many other games, uses a combat results table-based combat scheme (where one roll determines outcome of an attack), so percentages are absolute.
Your parsed Block percentage won't necessarily match your tooltip if you're fighting creatures higher or lower in level to you. See the Defense formula above to understand.
See also: Formulas:Defense, Formulas:Weapon Skill
Damage reduction when a block occurs
When an attack is Blocked, it does not necessarily mean that the target takes no damage. A Block attack-result inflicts the same damage as a normal hit, but reduced by an amount "X" that's based on the Block value and Strength of the target, as follows:
X = [(Shield block value) + ((Strength / 20) - 1)].
Since the warrior tends to have the highest natural Strength score of any class, and warrior gear bestows more Strength bonuses than the gear for other classes, a warrior will tend to block more incoming damage than a paladin or shaman before talents are taken into account.
Notes:
(a) The quantity of damage blocked is calculated after damage mitigation from all other sources (Armor, Defensive Stance, Improved Righteous Fury, Blessing of Sanctuary, etc.). That means if the opponent hits you for 200, mitigated to 100 by Armor, and block according to the formula above is 40, then you will see in your combat window: "... hits you for 60. (40 blocked)".
(b) If the block quantity is greater than the damage received after mitigation, you will then block 100% of the damage: "... attacks. You block."
(c) A Warrior's "Shield Mastery" or a Paladin's "Shield Specialization" talent increases the damage absorbed by (up to) 30%. A Shaman's "Shield Specialization" talent increases the damaged absorbed by (up to) 25%. The formula above becomes:
- X = [(Shield block value * 1.3) + ((Strength / 20) - 1)].
The Shield block value above includes effects listed on items such as the Immovable Object's "Increase the block value of your shield by 27" (tested on live patch 2.0.3).
It is unknown if temporary Buffs such as those in the Burning Crusade are applied before or after shield damage absorbtion talents. An example item is Gnomeregan Auto-Blocker 600.