Armor and penetration

Incoming physical damage is multiplied by a factor based on the unit's armor:

Damage multiplier={100100+Armor,2−100100−Armor,if Armor≥0otherwise

Examples:

25 armor → ×0.8 incoming physical damage (20% reduction, +25% effective health). 100 armor → ×0.5 incoming physical damage (50% reduction, +100% effective health). −25 armor → ×1.20 incoming physical damage (20% increase, −16.67% effective health).

Every point of armor requires a unit to take 1% more of its maximum health in physical damage to be killed. This is called effective health.

Example: A unit with 60 armor has 60% more of its maximum health in effective health, so if the unit has 1000 maximum health, it will take 1600 physical damage to kill it.

What this means: by definition, armor does not have diminishing returns, because each point increases the unit's effective health against physical damage by 1% of its current actual health whether the unit has 10 armor or 1000 armor.

: Ignores 35% of targets armor. At 155 armor, you still have 100.75 effective armor, meaning double your effective health. If you have 3000 health, your opponent needs to output 6000 in order to kill you from 100 to 0.

So why do I keep getting killed by Zed and their ADC so quickly?!?

You took the focus of two high damage champs and probably ate both ults and any CC they had, not to mention Zed' Bork. You ate up their most important cool downs, pat yourself on the back, if this is in the middle of a team fight, you've just given your team a chance to kill both Zed and the enemy ADC while everything they have is down. If this happened because you were caught out, well chalk it up to a mistake on your part and don't let it happen again.