Durability is a property of armor and weapons (excluding missile weapons, Phase Blades and indestructible items) which is gradually reduced by use while equipped: the more points of durability an item has, the longer it will take for it to break and be in need of repair.
- Throwing weapons have hidden durability values, which are reduced when they are ethereal.
- Ethereal items have Durability = [maximum/2] + 1
- Superior items with Increase Maximum Durability (10-15)% have Durability = [ maximum * (100+%) /100]; when ethereal, Durability = [ [maximum/2] * (100+%) /100] + 1
- Magic items that were failed Set items have double maximum Durability, so when ethereal they have Durability = maximum + 1
- Rare items that were failed Unique items have triple maximum Durability, so when ethereal they have Durability = [maximum * 3/2] + 1
- Ethereal rare or Unique items upgraded using the Horadric Cube are set to the maximum Durability of the base Exceptional or Elite item. For example, when ethereal Shaftstop Mesh Armor (Durability [45/2] + 1 or 23) is upgraded to Boneweave, it now has maximum Durability 45.
- Durability has a ceiling of 255: for example, a Heater has maximum Durability 88 but a Heater that was a failed Heater (since there is none) has maximum Durability 255, not 264 (88*3).
- Items bought or gambled from a townsperson have maximum Durability, but those created or crafted using the Horadric Cube, imbued, or dropped in the wilderness do not (unless they are throwing weapons, or ethereal items, which drop with 100% of their reduced durability).
- Missile attacks do not reduce durability.
- Blocking does not reduce shield durability; Weapon Block does not reduce claw durability.
- Items do not lose durability when equipped and used by a mercenary.
- Otherwise, there is 10% chance of equipped armor losing one point of durability whenever the character is struck by a successful melee attack. 
- Chance of armor equipped in each slot being picked to lose durability is weighted as follows:
- Chance of being picked to lose durability is weight/total weight, although empty slots or those which contain broken or indestructible items do not add to total weight: for example, if a shield is not equipped on the left arm, total weight is reduced to 8 and chance of helm losing durability increases from 20% (100 * 2/10) to 25% (100 * 2/8).
- Each successful melee attack has 4% chance of reducing weapon durability by 1.  This includes Blade Shield and Smite (despite the fact it applies shield damage). However, Dragon Talon, Dragon Tail and Dragon Flight do not result in loss of durability for boots or weapon.
- Throwing weapons have 10% chance of reducing weapon durability by 1 with each successful melee attack. However, their hidden durability values apply to each weapon in a stack individually, so when one is reduced to Durability 0, stack size is reduced by one. Thus a throwing weapon's effective maximum Durability when used as a melee weapon is its maximum durability * maximum stack size: for example, each Javelin in a stack has Durability 2 and maximum stack size is 60, resulting in effective maximum Durability 120 (2*60). 
- However, Impale's loss of durability applies directly to javelin stack size, not to durability: where a spear's durability would be reduced by 1, a javelin's stack size would be reduced by 1. 
- Ethereal items lose durability at the same rate as normal items (contrary to what is stated by the Arreat Summit).
When items are reduced to < 21% of their maximum displayed Durability, a silhouette appears in the top right corner of the screen, highlighting them. When items are reduced to 0 durability they are broken: their names are displayed in red text and their equipment slots have a red background when not highlighted, and red now highlights them on the silhouette, all indicating that they are no longer equipped (as can be seen, the Paladin
's broken helm
, body armor
, weapon and shield are no longer visible).
- The more durability an item loses, the more it costs to repair (unless the Horadric Cube is used to repair it). If a character does not have enough gold to repair an item to its maximum durability, it can be partially repaired using gold in inventory (but not that stored in stash) provided it is not broken and it does not also need to be recharged.
- Durability loss normally has no effect on an item's sell value, even when it is broken. However, an ethereal item with > 0 durability will sell for 25% of the value of the same item when it is not ethereal; a broken ethereal item will sell for just 1 gold, but it will not be added to the vendor screen even when there is space for it.
+ Maximum Durability is a hidden value added to the maximum Durability of the items listed below, after any ethereal reduction (it is taken into account by maximum displayed Durability, but it isn't listed among an item's modifiers).