Armor

Armor is a class of items that mitigate physical damage and are equipped in the different armor slots (head, shoulder, back, chest (aka body), wrist, hands, waist, legs, feet, and Off hand for shield). This sort of armor contributes to the Armor attribute, a numerical value that translates into physical damage mitigation in combat. Some non-armor items may grant a Defense attribute bonus.

Basic
Six basic armor types are available for Player characters to equip:
 * Cloth
 * Leather
 * Chainmail
 * Iron
 * Steel
 * Dragon Scale

Special
There are five special types of armor that provide special bonus effects.
 * DSR-Buffs the offensive power of projectile attacks.
 * Magma-Deals damage to melee attackers.
 * Indos-Buffs the offensive power of melee attacks.
 * Elite-Slowly heals the wearer in heavily-wooded areas.
 * Zone-Buffs the offensive power of magic attacks.

Additionally, shields can be equipped by certain classes.

Armor equipment slots
There are nine core slots that a character can equip armor items into and depending on their class; a character can optionally carry a shield in their Off hand.

Damage absorption
Armor augments a character's health to a higher number (if the armor is positive), which represents the amount of physical damage a player can withstand. For instance, a character with 5000 health and 50% armor would be able to absorb 10000 damage before dying.

The formula for determining damage absorption is:

DA = (player health) / (1 - (% damage reduction given by armor in decimal form).

For example, a character with 1000 health and 31.24% damage reduction would be able to absorb 1000 / (1 - .3124) = 1454 (rounded) damage.

A character with 1000 health and 99.98 damage reduction would be able to absorb 1000 / (1 -. 9998) = 500,000 damage.

Damage absorption only applies to physical damage.

Armor damage reduction formula
DR% = (Armor / (Armor + 46257.5)) * 100
 * The formula for a level 90 player (WoW 5.3)

According to the Toshiko UI elements, the formulas for damage reduction are as follows for WoW 2.0.8 in which Attacker = either player or mob.

DR% = Armor / (Armor + 400 + 85 * AttackLevel)
 * Attacker Levels 1 - 59

DR% = Armor / (Armor + 400 + 85 * (AttackerLevel + 4.5 * (AttackerLevel - 59)))
 * Attacker Levels 60+

Simplified, the formula becomes: DR% = Armor / (Armor + (467.5 * AttackerLevel - 22167.5))

Consider the following table which can be derived from the above formula for a lvl 70 tank taking 1000 DPS of "raw" damage:

As can be seen, the effectiveness of adding another 5k of armor is getting lower, there is a "diminishing returns" effect with respect to the DPS reduction. It isn't as pronounced as it may seem looking at the absolute numbers, though. The last line shows that the relative value of 5k armor drops from 33% to 13%, meaning that at the start, one point of AC will be about three times as effective in terms of DPS reduction as near the end. Thus, armor exhibits diminishing returns with respect to the total amount of healing needed to keep a tank alive.

The following is the same formula for a lvl 80 tank against a level 83 mob

However, in terms of absolute time to live with respect to melee attacks, armor has no diminishing return effect. Given a constant melee DPS amount, each additional point of armor (whether it be from 0 to 1 or from 30000 to 30001) will increase the tank's time to live by the same effective amount. 1k additional armor increases time to live by approximately 9.47% (6.01% at level 80??), as shown by the graph. The formula for calculating time to live with respect to melee attacks is:


 * Effective time to live  = 1 / (1 - DR%) * Base time to live

Where DR% is calculated according to the above formula. In this way, armor can be thought of as increasing the effective health of the tank with respect to melee attacks. Since DR caps at 75%, effective time to live caps at 400% of the tanks base time to live (time to live if the tank had no armor).

Prior to hitting the cap, armor is certainly a very desirable stat for tanks, but difficult to find. Most items of similar quality and rarity have similar armor values, and extra armor can be found only very rarely.

Armor caps
The armor caps for various attackers are as follows: