Ignoring your enemy's build, ignoring your own ad scalings, ignoring everything not completely necessary to answer the question, and so on, [title]
Also, same question with MagicPen and Ap.
Truly not Gay3 votesArmor pen is more valuable than 1 AD. I'll try to make this as simple as possible
Damage reduction from physical attacks = Armor/(armor + 100)
This means that 100 armor gives you exactly 50% damage reduction with 100/200
As someone builds more armor they get less value from it.
50/150 = 33%
add 50 armor 100/200 = 50% for a gain of 17%
Add 50 again 150/250 = 60% for a gain of 10%
add 50 again 200/300 = 67% for a gain of 7%
Now if you can understand this then I can properly explain armor penetration.
You have a ghostblade for 20 pen against a tank with 200 armor
We know 200 armor = 67% damage reduction from above now with pen 180/280 = 64% damage reduction so you gained 3% damage against them
For an enemy adc: 50/150 = 33% damage reduction after pen 30/130 = 23% damage reduction for a 10% damage increase.
Tattersall1 votesUmm… So… Ad is more efficient against enemies with extremely high armor? Or is pen. always better? I think I get what you're saying about armor penetration, but how does that tie in with ad as an alternative? Or maybe I don't get what you're saying, who knows. Thanks, though.
Truly not Gay2 votesThat depends on how much AD you have in the first place. If you already have a bunch of AD then penetration will give you more damage. If your AD isn't very high to begin with then the 3% damage increase doesn't amount to anything. Anytime you are against a tank, last whisper is the way to go. That will result in about a 30% damage increase depending on how much armor they have.
Tattersall1 votesThanks. This helps (probably).
Truly not Gay2 votesAnother thing you have to consider is the champion you are using. On a champion whose damage comes from base damage on skills like Garen then penetration far outclasses AD. On champions whose damage comes mostly from auto attacks or high ad ratios on abilities like Tryndamere who almost exclusively auto attacks and has a 120% ratio on his E as well as gains AD based on his bonus AD from being at low health with his Q passive.
Tattersall1 votesThis makes sense to me. [slayer-pantheon-thumbs]
Truly not Gay2 votesJust for fun, here is an extreme example. Orianna has 68 armor at level 18 and you have the following build.
Precision mastery for 8.4 Pen marks for 11.5
Black cleaver for 30% Duskblade for 10 Maw for 10 Ghostblade for 20
Total 30% + 60
68*.7 - 60 = -12.4 armor so she has -12.4/(100-12.4) = -14% damage reduction or she takes 14% increased damage.
BLACK REALM GOD3 votesafter a certain amount of armor is added to a build you have to buy %penetration. damage is required regardless.
30% armor penetration from last whisper is only worth it on champs with roughly 150+ armor.
.3 x 100 = 30. 100 - 30 = 70. 70/170 = 41%
100 -20 =80. 80/180 = 44%
if you multiply those number by 100 damage you get 59 and 56 per basic attack. at 150 you get 84 and 88.5
okay. lets increase the defense numbers
300 armor
300/400 = .75
.3 x 300 = 100. 200/300 = .67
300 - (20x3=60) = 240 240/340 = .71
29 and 33 autos for 100 damage and 43.5 and 49.5 for 150
even with fair representation of armor penetration % penetration wins out by a land slide in 300^.
is armor worth it? basically it depends on your champs scaling. if you cant easily reach 300 by late game you're better off just grabbing health or a combo stat item to persuade champions to at least invest a little money into penetration.
damage is worth more than armor after a certain point.
health is worth more than armor after a certain point.
penetration is relative to builds and scaling so you cant determine its value as the value is too situational.
speaking purely at a glance without any outer factors to influence it the only thing you can determine is the value of damage for a basic attack champ. you cant determine the value of damage for physical damage abilities. attack speed will also change the value of armor and damage as well as critical strikes. you have to take scaling into account for all of those.
ability power is even worse because you cant use it without referencing scaling charts specific to each champion.
an important thing to remember is that you cant kill with defense. that automatically makes damage and ability power more cost efficient.
everything else is situational.
ExHentai2 votesNo. There'd be an explanation to go along with this answer, but we are ignoring the enemy's build.
Tattersall1 votesIf the explanation is tied to the enemy's build, then I worded the question poorly. Will you please give me the explanation without having to ignore the enemy's build?
Succubús GF2 votesNot really, because its more like 1 anti resistance > 1 resistance. Also its hard to explain because armor pen items usually come with ad. So basically the more armor pen, the less armor they have. Not sure if that helps but i couldnt quite understand the post
Tattersall1 votesYeah, I guess I worded the question poorly. Sorry.
PhDs Nuts2 votesIt depends on how much armor they have. At low armor values, 1 armor contributes a greater % of the total resistance than at high armor values. If they were to have 20 armor, a 10 armor pen item would nearly double your physical damage output (83.3% physical damage taken increases to 90.9%). At 150 armor, your damage with 10 armor pen would only increase from 40% to 41.7%.
Tattersall1 votesSo what if we compare that to Ad? Only looking at basic attacks, is Ad more efficient late, or does it not scale at all, or what (thanks for your answer btw)?
PhDs Nuts2 votesSo what if we compare that to Ad? Only looking at basic attacks, is Ad more efficient late, or does it not scale at all, or what (thanks for your answer btw)?
That depends on how much AD, crit chance, and armor pen you have, and the armor of the target. Armor pen is generally worthless against high armor. You want % pen from last whisper and black cleaver to reduce armor to lower values, and then armor pen becomes useful again.
Tattersall1 votes…Could you try giving a summary of all of those? Pls? **:**3
PhDs Nuts2 votes…Could you try giving a summary of all of those? Pls? **:**3
In general:
High armor - you want % pen Low armor - you want flat pen You always want high AD in your build (excluding on-hit and hybrids), because doubling AD will always double your damage against the same target. Since items give flat AD, though, you will see diminishing returns as your AD gets higher.
That's part of why the Youmuu/Cleaver build is so popular. You get early armor pen when nobody has built armor, and then the cleaver shreds the armor they build later.
Tattersall1 votesThanks man. I think I'm smart enough to say that this helps. Or it's so complex that I can't comprehend that I don't understand. Probably the former.
The Yetii Rider2 votesSomething you have to understand is that you can't ignore scalings when you talk about this stuff either. AD only contributes to scalings, while armor penetration contributes to the base damage of your abilities and the AD scalings.
So for a champion with lower AD ratios (Talon is a big one, only 180% bonus AD if you use his entire rotation) armor penetration is significantly more impactful.
This goes for AP assassins as well. Pretty much the only true assassin who benefits from AD as much or more than armor penetration is Zed, which is one of the reasons he is balanced on such a thin knife.
Since you're asking about MR too, I will simply say this: 
essentially solves most MR problems. No matter if they're getting MR from base, scaling, runes, masteries, or items, Void will tear it to shreds, and you don't need to stack it up like 
.
Tattersall1 votesI get what you're saying about ad scalings, I just want a more general knowledge of this topic first. I mean, I doubt anything anyone says here would make me build Pen. instead of Ad on, say, Pantheon. Though I do build Cleaver on him, so it'll probably just get more and more nuanced the more I think about it.
The Yetii Rider2 votesDivers and Juggernauts are a different story altogether. Black Cleaver is usually a better item on them because A) unlike flat armor penetration, Black Cleaver is very effective against enemies who stack armor and B) Black Cleaver scales not with how much damage your character deals, but rather how long you can actually stay in combat. A champion like Zed who has to kill someone in a matter of a second and a half wouldn't get as much use out of Black Cleaver as he would out of Youmuu's Ghostblade, but Garen and Darius get great use out of it.
Nasus is an interesting exception to this rule. Of course, Nasus scales incredibly well with how long he can stay in combat, but the bonus AD he gets from Fury of the Sands overrides his need to use Black Cleaver like other juggernauts. That may change with the alterations to his ultimate that are currently being tested.
The Deckowner2 votesIf the enemy has 100 armor, 1 armor pen=enemy effective health against physical damage reduced by 1%.
the lesser enemy armor, the more impactful armor pen.
Orinne2 votesPut another way, armor and MR should be treated more as 1 armor = +1% effective health vs. physical damage, and 1 MR = +1% effective health vs. magic damage. This makes it easier to work with instead of "100 Armor = 50% damage reduction". Health multiplies effective health accordingly (eg. 1000 Health and 50 Armor = 1000 * 1.5) = 1500 effective health vs. physical damage)
On that 1000 health, 50 armor target, if a champion with 100 AD was attacking it, 1 extra AD means the target takes 101 damage. 1 Armor Pen means that 100 AD is now being used against a target with +49% effective health.
In effect, across one auto attack, 1 AD makes the target 1399/1500 effective health, and 1 Ar Pen makes the target 1390/1490. At 10 auto attacks, the target is left at the same effective health amount (490/1500, 490/1490). You'll notice here that AD is a slight bit stronger but it's literally a .003% difference.
EDIT: After running through numbers, AD was apparently very slightly more effective.
Sniper 01 votesNah, ad doesn't do % based damage but armor does % based damage reduction based on the amount with a soft cap I believe.