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

Every point of damage you deal is filtered through the target's resistances before it touches their health. Armor filters physical, magic resist filters magic, and true damage skips the whole conversation.

This is why two players with identical items can feel completely different in a fight. The one hitting the support is playing a different game from the one hitting the bruiser who just finished his second armor item, even though their damage numbers on the shop screen are the same.

Here's the part most players have backwards. Resistances don't stop being useful as they stack. Each point of armor is worth slightly more than the last, because what it buys is effective health, and effective health keeps climbing.

The practical consequence is the one that matters at the shop: against a target already deep into resistances, buying more raw damage is the weakest thing you can do. You are multiplying a number that is being divided harder every second. That's the moment to buy penetration instead, and it is almost always earlier than it feels.

Flat penetration removes a fixed slice of resistance. Percentage penetration removes a share of whatever is there.

Against a squishy target in the laning phase, who has very little resistance to begin with, the fixed slice is enormous relative to what they have. Against a tank three items deep, the same fixed slice is a rounding error while the percentage cut is doing real work.

That's the whole rule, and it is why the penetration item that felt useless in your last game felt broken in the one before. Nothing about the item changed. The target did.

Auto attacks go through armor like any other physical damage, which means attack speed and armor interact in a way that catches people out: doubling your attack speed against a heavily armored target doubles a small number.

This is the argument for buying the armor-shredding effect before the third damage item on most carries, and the argument against it when the enemy team simply has no armor to shred. Look at their build, not at yours.

All of the above is directional. The actual crossover point, the game where flat penetration stops beating percentage penetration for your specific champion at your specific level, depends on numbers that move every time the community votes.

That is what the damage calculator on this site is for. Put your build in, put their resistances in, and read the result rather than trusting a rule of thumb someone wrote three patches ago. The tool states its own assumptions on screen, which is more than most will do for you.