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Blocking determines your chance to defend against physical melee and ranged attacks delivering physical damage. Unless specifically stated otherwise, it requires a Shield of any type to be equipped in order to work, naturally.

Due to the nature of this mechanic, heroes that are built to take the brunt of attacks, like Warriors, Paladins, Crusaders and the Templar, are most likely to make extensive use of it. Other classes may find themselves using shields more for the defense and/or affixes offered rather than outright blocking attacks.

Diablo I

In Diablo I, the chance to block is based on the hero's Dexterity and is not capped (so players can, in fact, get a 100% chance to block attacks).

Faster Block Rate is naturally given to Warriors and Barbarians, allowing them to quickly recover after a successful block. Other classes must obtain a shield with this affix (like Stormshield) if they wish to block effectively. Otherwise, they run the risk of leaving themselves vulnerable to harm while recovering.

Diablo II

In Diablo II, there is a 75% Cap on the % Chance to Block.

75% Chance of Blocking will stop 75% of physical attacks from hitting but if a player is running, the Total Blocking percentage is reduced to 1/3rd of its original value, with a cap of 25%.[1] If the roll of the dice denies a true block, or if the player can't block at all, then and only then, are Skills such as the Amazon's Avoid and the Assassin's Weapon Block checked for blocking.

You are always eligible for a block check unless you are already in the middle of blocking animation or in the middle of "struck" animation. All other animations, from regular attacks, to Whirlwind, to casting spells, will override the block animation. The blocking will still occur, but no block animation will be played and no block sound will be heard.

Probability of blocking is calculated as follows:

Total Blocking = (Blocking * (Dexterity - 15)) / (Character Level * 2)

Changing the formula around, to calculate dexterity needed for 75% block at a specific character level:

Dexterity = ((150 * Character Level) / Displayed blocking on shield) + 15

Note that displayed blocking will vary between different shields, classes, and is affected by the Paladin's Holy Shield skill.

A character is prevented from doing anything else while blocking. The delay caused by blocking can be shortened with a Faster Block Rate.

Hirelings cannot block, not even an Iron Wolf with a shield.

Diablo III

In Diablo III, the system is simplified: it works against all types of Elemental Damage. Much like it was in the original Diablo, Chance to Block is not capped, although for anyone who is not a Crusader, their blocking chance with a shield may never legitimately exceed 85% (20% base chance, along with a +11% bonus chance from an affix, plus another 54% from legendary items). Another new mechanic to the blocking system is the shield's Block Amount, a value that is also static (though the amount blocked depends on that of the shield itself, with Legendary shields blocking more damage). Each blocked attack deducts a random number within the Block Amount's range from the unsaved damage taken. None of the Nephalem's core attributes have an effect on either blocking values. Some skills, however, may further modify either of these values, or grant a special effect that occurs when an attack is blocked (for instance, Punish, Hold Your Ground, Renewal or Sword and Board). In the case of the Crusader's Shield Bash or Blessed Shield skills, a shield's Block chance also directly increases damage dealt.

Any and all damage that comes from a Damage Over Time source cannot be blocked by shields, making it useful against Block-oriented monsters, such as Primordial Scavengers, but also making the player vulnerable to such damage even if they have a character who is dedicated to making the most out of maxed-out Block chances.

Blood Brother sword utilizes a block mechanic, even if no shield is taken, or, if coupled with a shield, boosts both its own and shield Blocks. It can add up to 20% additional Block chance to the normal cap, potentially bringing it up to 100%.

Some items, like Akarat's Awakening, Justice Lantern, Ivory Tower or Coven's Criterion, add special effects on a successful block, or improve functionality of blocks.

Character Attributes
Diablo I Diablo II Diablo III
DexterityAttack RatingArmor ClassBlock
DexterityAttack RatingDefenseBlock
EnergyManaMana Regeneration