Armour
Armour is a form of defence that reduces incoming physical damage from hits. Along with Evasion and Energy shield, Armour is one of the three defence types that can implicitly appear on equipment. Unlike resistance, armour has diminishing returns as it's effectiveness is based on the magnitude of incoming damage (multiplied by 10). As a result armour is most effective in mitigating small hits (i.e. 50% and better reduction for initial damage less then a tenth of the armor value) and least effective in mitigating very large hits (where it's absolute reduction can at most be one tenth of the armor value).
Armour rating can be increased by wearing equipment, taking certain passive skills, using spells such as Molten Shell, and drinking a Granite FlaskGranite FlaskLasts 4 Seconds
Consumes 30 of 60 Charges on use
+3000 to ArmourRequires 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]}
Contents
Mechanics
Damage reduction from armour only reduces physical damage taken from hits. Elemental damage and damageovertime are not affected.
rule of thumb
 To prevent one third of damage, you need armor 5 times the damage (e.g. 500 Armor for 100 damage)
 To prevent half of damage, you need armor 10 times the damage (e.g. 1000 Armor for 100 damage)
 To prevent two thirds of damage, you need armor 20 times the damage (e.g. 2000 Armor for 100 damage)
 To prevent three quarters of damage, you need armor 30 times the damage (e.g. 3000 Armor for 100 damage)
 To prevent 90% of damage, you need armor 90 times the damage (e.g. 9000 Armor for 100 damage)
Formulas
The amount of damage reduction is a function of armour rating and the damage taken. Damage reduction is capped at 90%.
Relative Damage Reduction (Base Formula)
This is the base formula all other formulas are based upon. It is used to calculate the damage reduction factor, i.e. the percentage by which the damage is reduced.
Required Armor
To calculate the armor you need to mitigate a given amount of damage at specific ratio, you can use this formula:
Reverse Damage
You can also calculate the raw damage that would have resulted for a given damage reduction and armour value:
Resulting Damage
To calculate the net damage (i.e. the damage after damage reduction, the following formula can be used:
Absolute Damage Reduction
Another way of looking at it is in terms of absolute damage reduction:
Defense factor
examples
For example, if you have 1200 armour rating this chart indicates how much damage will be reduced by:
The vertical axis is damage reduction percent. The bottom axis is original damage.
Therefore we can conclude:
 If raw damage is much less than Armour / 10, raw damage has a nearquadratic effect on net damage. Equivalently, in this regime raw damage has a nearlinear effect on net DPS if the raw DPS is held constant.
 If raw damage is much greater than Armour / 10, raw damage has a nearlinear 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.
Skills
The Determination aura increases armour of the character and nearby allies. The Molten Shell and Vaal Molten Shell skills temporarily grants player additional armour.
Passive skills
There are passive skills that grant armour bonuses.
Armour rating
These passive skills grant increased armour rating or additional base armour rating. The Iron Reflexes keystone is special in that it converts evasion rating to armour rating.
Name  Bonuses 

Armour 

Armour and Energy Shield 

Armour and Evasion 

Armour and Life Regeneration 

Life and Armour 

Mace Damage and Armour 

Sword Damage and Armour 

Armour Mastery 

Baptism by Light 

Blunt Instrument 

Sanctity 

Cloth and Chain 

Combat Stamina 

Faith and Steel 

Leather and Steel 

Path of the Warrior 

Soul of Steel 

Sentinel 

Steel Skin 

Iron Reflexes 

Armour rating on shields
These passive skills grant increased armour rating from equipped shields.
Name  Bonuses 

Shield Armour 

Plated Defences 

Miscellany
These passive skills are related to armour, but do not fit into any other category.
Name  Bonuses 

Acrobatics 

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