Lethality doesn't counter Armor; Armor counters Lethality

TL;DR: The more armor you have the less lethality will affect you. The opposite can be said if you build no armor. Lower armor champions will get destroyed by lethality.

To understand how lethality works, you have to understand how armor works first. Armor becomes less effective as more of it is stacked. The equation for damage reduction for both Magic Resist and Armor is x/(x+100)

This is a table showing damage reduction from each value of armor by increments of 25 armor.

AR = armor DR = damage reduction %^ = percent change from the previous value

AR ------------ DR ---------------- %^ 0 ------------ 0.0% -------------- N/A 25 ---------- 20.0% ----------- 20 50 ---------- 33.3% ----------- 13.3 75 ---------- 42.9% ------------ 9.6 100 ------- 50.0% ------------ 7.1 125 -------- 55.6% ------------ 5.6 150 -------- 60.0% ----------- 4.4 175 --------- 63.6% ----------- 3.6 200 -------- 66.7% ----------- 3.1 225 -------- 69.2% ----------- 2.5 250 -------- 71.4% ------------ 2.2 275 -------- 73.3% ------------- 1.9 300 -------- 75.0% ------------ 1.7 325 -------- 76.5% ------------ 1.5 350 -------- 77.8% ------------ 1.3 375 --------- 78.9% ----------- 1.1 400 -------- 80.0% ----------- 1.1

To keep things simple I will describe how lethality works at level 18 because lethality scales by level. At level 18, 1 lethality is equal to 1 armor penetration which lowers the target's armor by 1. Items with lethality are:

10 15 15 20

With runes and masteries you can get:

Precision - 8.4 Marks - 14.4 Quints - 9.6

The highest lethality possible not stacking items is 82.4.

For the simplicity of not having to make more calculations, I will use 75 lethality as an example to use the chart above.

Most champions have between 75 and 100 base armor so with 9 armor seals many would have right at 100 at level 18. This means that building no armor, a lethality stacking assassin would lower the 100 armor to 25 armor changing 50% damage reduction to 20% damage reduction. This gives the assassin a 60% damage increase against said champion.

If you don't believe me here you go. Attack does 100 damage, 100 armor reduces that to 50 while 25 armor reduces that to 80.

80/50=1.6 If an attack does 50 damage and you get a mastery that increases their damage by 60% you multiply 50 by 1.6 to get 80.

A tank with 300 armor would be reduced to 225. 300 armor gives 75% damage reduction and 225 armor gives 69.2% damage reduction. With the logic above:

30.8/25 = 1.232 They gained 23.2% damage against the tank who already takes less damage than the squishy with 100 armor to begin with who is taking 60% more damage.

**In conclusion, armor not only makes you take less damage from physical attacks as everyone knows, but it also drastically lowers the effectiveness of lethality on top of the damage reduction. **

Edit: A few people have mentioned true damage so I thought I would mention that there is no such thing as doing true damage when reduced below 0 resistance. If someone is reduced to -30 resistance the formula for damage reduction becomes:

2-30/(100-30) = 57% damage increase. This damage is not dealt as true damage but rather as increased physical damage.