So I've recently been playing around with the magic resist and armour equations and I had some interesting results. Putting these into practice I have managed to do massive damage against people who decide they don't need early game magic resist.

This guide is for those people just to demonstrate to them why you need to get magic resist.

The equation for armour works exactly the same as the equation for MR.

However the reason I am writing the guide based around MR is that people start off the game with a base of 30. Unless a player knowingly contributes to his magic resist the problem never gets better. With armour though on every champion it gets better as you level. This means for no investment melee damage is reduced against you. Since most armour values are in the 70's or 80's it is a considerable amount of damage.

Furthermore there are a plethora of armour value 100 items, whereas only one 70 MR item, and a hand full of 50 MR items. The MR items also tend to be more expensive making MR more scarce.

So in my mind I feel MR is an integral part of any balanced build, and requires more foresight/planning then armour. Therefore the name MR and you.

The equation looks like this.

%Damage Reduction = Magic resist/(100 + Magic Resist) X 100

So the first thing you should notice is that you can never reach 100 percent magic reduction. You can get very close if you have astronomically high magic resist but you'll never get to 100% damage reduction, which is only fair really.

Magic Penetration and reduction lowers your opponents magic resist. You can only penetrate 40 MR but you can reduce it by any amount.

So let's look at some pertinent magic resist values and their reductions for a quick reference.

30 MR = 30/(100+30) X 100 = 23% Note this is most people's base MR. Tanky DPS aside.

0 MR = 0/100 X 100 = 0 True damage. Figures right?

80 MR = 80/(100+80) X 100 = 44.4% Most people with a banshees veil

100 MR = 100/(100+100) X 100 = 50%

200 MR = 200/(100+200) X 100 = 66.6%

10 MR = 10/(100+10) X 100 = 9% If they get MP boots, or go all out MP like in my build.

Also the modifiers are applied as such.
Magic resist reduction - This has no limit and can reduce the total below 0.
Magic Penetration - This caps at 40, and cannot reduce the total below 0.
Percent reduction - This reduces what's left by your percent reduction.

Thank you Shadow Palm for correcting me on the order of application.

So the big question what does it all mean? well what the numbers should show you is that the lower your magic resist the more bang you get for your buck. Your first 100 MR reduces the incoming magic damage by a whopping 50%. The next 100 only adds 16%, and your fist 30 adds 23%.

So the lower your magic resist the more important it is to buy magic resist the higher the less important. Well your welcome my name is captain obvious. The way I see it though you really should shoot for no more then a 100 MR build unless you are facing magic pen, or just tons of AP, which you shouldn't sacrifice other aspects of your build for more MR then. Also even low returns on your MR runes can be worth the investment as it might free up your item selection. When considering your build MR should be of paramount importance, and you should almost never leave home without it, and have a couple different options available to you.

Thanks to mastajdog's helpful comments I can complete this part of the guide with a lot more accurate numbers.

The equation is basically just what you would get with magic resist but reversed. So at -20 MR you would get 23% increased damage over true damage. You get this by...

% Extra damage = (Negative Magic resistance)/(100+Negative magic resistance)

Again just like with the MR equation we can never reach 1 or greater. So you'll never crit with your spells. Also the lower you drag enemies into the negatives the worse returns you get. Not that it's bad just Soraka being able to bring you down to -180 MR isn't as bad as it could be. Which is why s=the game designers believe her to be balanced. She may not be but that would be I assume their logic.

The reason the equation changes is because if you were to maintain the same equation as you approached -100 MR you could crash the game, do amazing amounts of damage, or prevent over a 100 percent of the damage done.

Defensively:

Well let's look defensively first if your opponent gets some nice magic resist boots quick getting a negatron cloak and sitting on it isn't a bad idea. If you're originally stopping 23% of his damage and he now brings you down to stopping none? the 23% damage increase on his part really. That's almost a death cap AND it works on his base damages. It's too much a boost, and really you're going to have to get one eventually.

If you don't like the idea of putting that much into one item that will sit maybe get merc treads. Remember you don't need a ton of MR when you are operating at low numbers to see a big difference. Also merc treads are just good.

Offensively:

Ah the part I've been waiting for. Offensively it means get it fast get it early. Most people don't rush Magic resist except for maybe mercury treads. If they only have 30 get your magic pen boots, their cheap and help you gank and run away early while other people have tier 1 boots. Now you do true damage and increase your survivability and hopefully force your opponent to adapt to you.

Also this should show you that if you are saying man I wanna do some more magic damage but they have a bunch of magic resist.... I guess I'll get the scepter now. Wrong. At high levels it won't have much of an effect. Get yourself a void staff.

These are the dudes that have above average magic resist at level 18. According to Mobafire which I am assuming is right.

Magic Resist: 52.5


Magic Resist: 43.5

I hope this guide illustrates the importance of magic resist to some of my fellow summoners. I wanted to make sure you saw the potential downfalls of not investing in any until late game, and maybe could give some people an idea of how much magic resist is good, and where the diminishing returns of the equation come into play.