Damage calculation?

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202halffound

Senior Member

02-04-2012

Often I find that when playing as characters I overestimate/underestimate how much damage my skills will do. I think that this is because I have no idea how damage is calculated in the game.

How does stats such as Ability Power, Magic Resist, and Magic Penetration affect how much damage the skill actually does?


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Almight Aza

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Senior Member

02-04-2012

Say someone has 100 MR, which I think is ~40% resist but assume it's 50%, if you deal 100 magic damage they will receive 50 damage, if you have 20 Mpen it will take their MR down by 20 points and so they will have 30MR lets assume it's 30% reduction and you'll deal 70 damage. Does that answer your question?

Also gotta remember that MR and armour have diminishing returns as in if you add 10 to your current 20 armour, it will give you a larger % resist, and if you have 100 armour and you add 10, you'll get a MUCH smaller amount of resist. The same effect with Mpen and Apen


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Jorick

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Senior Member

02-04-2012

To explain how damage calculation works, you need to understand and put together four things: initial damage (the amount your ability tooltip says you'll do), resistances (armor or magic resist), penetrations (armor or magic penetration), and effective damage (damage done after resistances and penetrations are calculated in). These are the terms I personally use for the damage things, others might call it base damage and end damage or something like that. Anyway...

Initial damage is simple. For physical attacks it's your displayed attack damage. For abilities it's the base damage plus a percentage of your attack damage or ability power (a few champions have abilities that scale off of other things, but AD and AP are the main things), the total calculation of which is shown in parentheses in green (AP) or orange (AD) text after the base damage on the tooltip. Whatever those two numbers add up to is the initial damage of your ability.

Resistances are fairly simple as well. The more armor you have, the more physical damage you resist; the more magic resistance you have, the more magic damage you'll resist. 100 armor/MR = 50% damage reduction, 200 armor/MR = 66% damage reduction, 300 armor/MR = 75% damage reduction... You don't really need to know those numbers since you can check what % damage reduction someone has by clicking on them and hovering your mouse over their armor or MR stat in the info panel in the upper left of your screen. For a better explanation and example of how resistances work, see SomniumProxy's post below mine.

Penetrations are where it gets a little bit complicated. There are two types of penetration: flat and percent. Note that your penetrations are used only for calculating your personal damage to the enemy, they don't permanently reduce the enemy defensive stats and they don't benefit your teammates unless it's an item aura or a debuff put on the enemy that affects their defensive stats. I'll use armor for the examples. Percent armor penetration reduces your target's armor by a percent: if they have 100 armor and you have 10% armor penetration, your damage will be calculated as if they had only 90 armor. Flat armor penetration takes off that flat number: 20 flat armor penetration would make it as if that 100 armor enemy only had 80 armor against your attacks. One thing of note is that flat penetrations are calculated before percent penetrations, which makes the percent penetration take off a little less than it otherwise would; not very important to know unless you plan to do very math heavy theory-crafting for damage output. And another fact you probably don't care about: some items (Black Cleaver) or abilities will say armor 'reduction' rather than penetration, but it means the exact same thing; it just sounds better to say your debuff reduces the enemy's armor than to say it penetrates it.

Now for effective damage, which is just adding it all together. Say you have 250 attack damage with 33% armor penetration and 50 flat armor penetration, the enemy has 200 armor. Based on the order of calculation, first the flat penetration would reduce the enemy's armor to 150. Then the 33% penetration would take off 33%, leaving the enemy with 100 armor against your attack. Since that amount gives 50% damage reduction against physical attacks, your attack would hit the enemy for a grand total of 125 damage.

A sort of short hand version of this all could be made into an equation like so: Initial Damage - ((Armor - Flat Armor Penetration) * Percent Armor Penetration) = Effective Damage.

Kind of a super long explanation, but I like being thorough. I hope it clears things up, even if it does also bore you half to death.


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SomniumProxy

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02-04-2012

Quote:
Originally Posted by Almight Aza View Post
Say someone has 100 MR, which I think is ~40% resist but assume it's 50%, if you deal 100 magic damage they will receive 50 damage, if you have 20 Mpen it will take their MR down by 20 points and so they will have 30MR lets assume it's 30% reduction and you'll deal 70 damage. Does that answer your question?

Also gotta remember that MR and armour have diminishing returns as in if you add 10 to your current 20 armour, it will give you a larger % resist, and if you have 100 armour and you add 10, you'll get a MUCH smaller amount of resist. The same effect with Mpen and Apen
Close enough, but not accurate.

The exact formula for armor and MR is 100 - 100/(100 + value) reduction.

This means that 100 armor is exaactly 50% reduction, 200 is 66%, 300 is 75% etc

While saying that the first 10 armor will give you more % reduction than an other 10 armor when you already have some is true, armor and magic resist don't have diminishing returns in the proper sense of the word. If you take a closer look at the formula, you will see that

Every point of armor increases your effective health by 1% against physical attacks
Which means that every point is equally valuable, in a way. Let me demonstrate this concept.

If you have 1000 hp and 100 armor, this means that you have 100% bonus effective health, thus it will take 2000 damage to kill you. In other words, 50% reduction.

If you have 200 armor, this means 200% effective health, thus it will take 3000 damage to kill you. For 300 armor it's 4000 and so on and so forth.

In other wors, without knowing the exact % reduction, with 57 armor you can still know that it will take exactly 1570 damage to kill you.


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Jorick

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Senior Member

02-04-2012

Quote:
Originally Posted by SomniumProxy View Post
While saying that the first 10 armor will give you more % reduction than an other 10 armor when you already have some is true, armor and magic resist don't have diminishing returns in the proper sense of the word. If you take a closer look at the formula, you will see that

Every point of armor increases your effective health by 1% against physical attacks
Which means that every point is equally valuable, in a way.
Hmm, interesting. I had only ever looked at the final product of resistances, the % damage reduction achieved, not at the formula end of things. Very nicely explained.

Removed my own fallacious statement about resistance devaluation from my above post so as not to be spreading misinformation.


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OmniOmega

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02-04-2012

Each skill has a different "AP ratio". This is how much raw damage for each additional point of AP you get. Ex. You have a skill that gives a ratio of .7, and you buy Zhonya's Hourglass which gives you 100 AP. The additional damage you get from AP is 70.


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SrPink

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Recruiter

03-04-2012

legit post. +1