Why your damage disappears: armor, magic resist and penetration on Classic

The number on the item is what you deal into nothing. What the target takes depends entirely on what they bought, and you almost never look at what they bought.

Armor filters physical damage, magic resist filters magic, and neither of them gets less useful as it stacks. That surprises people. You would think there are diminishing returns somewhere, and there aren't, because what resistances actually buy is effective health, and effective health keeps climbing for as long as they keep buying.

So the moment your enemy top laner finishes Randuin's Omen on top of the Frozen Heart he already had, your fifth damage item is close to the worst purchase available to you. You are scaling a number that is being divided harder every time he backs.

The practical version, the one that matters while you are standing in the shop with gold burning: look at their build before you look at yours. If two of them have bought armor, that is the game telling you to buy penetration, and it is telling you earlier than it feels comfortable to listen.

The same is true on the magic side and it gets ignored more, because AP players tend to buy Rabadon's Deathcap and assume the problem is solved. Against an Abyssal Scepter and a Banshee's Veil it isn't. Void Staff exists for exactly that game.

Flat penetration takes a fixed slice off the target's resistance. The Brutalizer does it, Youmuu's Ghostblade does it, Sorcerer's Shoes and Haunting Guise do it on the magic side. Percentage penetration takes a share of whatever is standing there, which is what Last Whisper and Void Staff are for.

Against a support who has bought nothing defensive yet, a fixed slice is enormous relative to what they own. That is the whole argument for rushing The Brutalizer on an assassin: in the window where nobody has armor, it is the most damage per gold in the shop.

Against a Cho'Gath three items deep it is a rounding error, and Last Whisper is doing the work instead. Nothing about either item changed between those two games. The target did.

This is also why The Black Cleaver reads badly on paper and plays well: shredding armor helps everyone hitting the same target, not just you, and on a team with two physical damage dealers that adds up in a way the item's own tooltip never shows you.

Vayne's Silver Bolts do not care what the target bought. That is the entire reason she is a problem for anyone stacking health and armor, and the reason building Thornmail against her is a much worse idea than it looks.

Auto attacks go through armor like any other physical damage, which produces a trap that catches good players: doubling your attack speed against a heavily armored target doubles a small number. You feel faster and you are not killing anything sooner.

Madred's Bloodrazor is the classic answer, and it is worth understanding why rather than copying it. Its on-hit damage is magic, so it walks past the armor entirely and asks their magic resist instead, which the enemy tank has usually not bought. It is not a damage item so much as a damage-type switch, and it is dead weight against a team that did buy magic resist.

Master Yi and Tryndamere both live and die on this decision. So does every ADC choosing between another crit item and Last Whisper around the second or third slot.

Everything above is directional. Where exactly flat penetration stops beating percentage penetration for your champion at your level, against that specific build, depends on values that shift every time the community votes something through the Council.

That is what the damage calculator here is for. Put in your build, put in their resistances, and read what comes out instead of trusting a rule someone wrote three patches ago and never revisited. It shows its own assumptions on screen, which you should demand from any tool that hands you a number.