Stat of a hero that provides protection from Physical damage. The more armor a hero has, the more reduction in damage it provides.
Chiron with 159 armor receives an Auto-Attack for 183 physical damage. Cassandra then uses "Proph-Icy" to increase his armor by 44. He now, at a total of 203 armor takes 156 physical damage on a 2nd Auto-Attack.
From this scenario, we can conclude that an increase of 44 armor from an armor value of 159, or + 27.67%, reduces his damage taken from physical attacks by 14.75%. At lower base armor values that addition of armor would be more beneficial.
- ARMOR- DMG Taken(PHY)
As you can see , as armor increases, one receives less and less of a benefit. If this was graphed it would be in the shape of a log curve. This is the idea of how diminishing returns works; as one stat goes up more and more(armor), the other stat is correlated and goes up also, but considerably less and less(PHY dmg taken).
It can be approximated that a difference in 22 armor, creates a difference in rate of reduction of damage taken by about 3.5 damage (Note the difference from 183 to 168, and 168 to 156; 15 and 12 respectively, a difference in 3 in the rate of reduction).
[With this I can predict that at 137 armor, incoming damage would be about 204. At 115 armor ; 226, and so on].