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Armour

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Armour is a rating that measures a character's physical damage mitigation. Every 12 points of Armour blocks up to 1 physical damage per hit. The absolute amount of damage blocked gets closer to the maximum the more damage the hit does; however, the proportion of damage blocked decreases.

Armour rating can be increased by wearing equipment, taking certain passive skills, using spells such as Molten Shell, and drinking a Granite FlaskGranite Flask
Lasts 4 Seconds
Consumes 30 of 60 Charges on use
+3000 to Armour
Requires Level 27Right click to drink. Can only hold charges while in belt. Refills as you kill monsters.
.

Supposedly the most armour monsters have in the game is about 983.[1]

Mechanics

Damage reduction from armour only reduces physical damage taken. Elemental damage and damage-over-time are not affected.

The amount of damage reduction is a function of armour rating and the damage taken. Damage reduction is capped at 90%.

Damage Reduction Factor = Armour / ( Armour + (12 * Damage) )

For example, if you have 1200 armour rating this chart indicates how much damage will be reduced by:

Damage reduction chart.png The vertical axis is damage reduction percent. The bottom axis is original damage.

Equivalently:

Net Damage = Raw Damage2 / (Raw Damage + Armour / 12)
Defense Factor = Raw Damage / Net Damage = 1 + Armour / (12 * Raw Damage) 
= (1 + sqrt(1 + Armour / (Net Damage * 3))) / 2

Armor v Defense.png

Therefore we can conclude:

  • If raw damage is much less than Armour / 12, raw damage has a near-quadratic effect on net damage. Equivalently, in this regime raw damage has a near-linear effect on net DPS if the raw DPS is held constant.
  • If raw damage is much greater than Armour / 12, raw damage has a near-linear effect on net damage. Equivalently, in this regime raw damage has little effect on net DPS if the raw DPS is held constant.
  • Against attacks of a fixed raw damage, armour has a linear effect on the number of attacks it takes to deplete a certain amount of hit points.

Another way of looking at it is in terms of absolute damage reduction:

Absolute damage reduction = Armour / 12 * Raw Damage / (Raw Damage + Armour / 12)

So damage from attacks is reduced by up to 1 per 12 points of Armour, which means that the supposed monster Armour cap of 983 blocks up to 77.5 damage per attack. Note that as raw damage increases, the damage blocked per attack increases, but the proportion of damage blocked decreases.

The derivative of this with respect to Armour is

dAbsolute damage reduction/dArmour = ((Armour / 12) / (Raw Damage + Armour / 12))2

Passives

There are passive skill nodes that grant additional evasion or a combination of bonuses including evasion.

Armour

These passives increase armour rating by a percentage.

In the case of Iron Reflexes, armour is increased by an amount equal to total evasion. In exchange, evasion is reduced to 0.

Name Bonuses
Armour
    • 10% increased Armour
    • 12% increased Armour
    • 15% increased Armour
    • 16% increased Armour
Steel Skin
  • 30% increased Armour
Riddle of Steel
  • 30% increased Armour
Armour Mastery
  • 18% increased Armour
  • 2% increased Movement Speed
  • +10 to Strength
Armour and Evasion
  • 8% increased Evasion Rating and Armour
Leather and Steel
  • 24% increased Evasion Rating and Armour
  • Ignore all Movement Penalties from Armour
Cloth and Chain
  • 24% increased Evasion Rating and Armour
  • +12% to all Elemental Resistances
Armour and Energy Shield
  • 8% increased Armour
  • 5% increased maximum Energy Shield
Body and Soul
  • 20% increased Armour
  • 10% increased maximum Energy Shield
  • 8% to all Elemental Resistances
Faith and Steel
  • 20% increased Armour
  • 10% increased maximum Energy Shield
  • 8% to all Elemental Resistances
Iron Reflexes
  • Converts all Evasion Rating to Armour
  • Dexterity provides no bonus to Evasion Rating

Miscellany

These passives are related to armour, but don't fit into any other category.

Name Bonuses
Acrobatics
  • 30% Chance to Dodge Attacks
  • 50% less Armour and Energy Shield

References

  1. VictorDoom (May 10, 2013). "Unique Items List - 159 of 159". Official Path of Exile Forums. Retrieved June 27, 2013.