Magic resistance

Magic Resistance (MR) reduces the damage a champion receives from magical attacks by a calculated percentage. Effective Magic Resistance of targets can be decreased with Magic Penetration. You gain Magic Resistance from Runes, Masteries, items, and abilities.

At level 18, Magic Resistance ranges from 30 (most champions) to 54 ( and ) (23 percent and 35 percent damage reduction, respectively).

Damage Reduction
Incoming damage is multiplied by a specific factor based on your Magic Resistance:
 * Damage Multiplier = 100 / (100 + MR) when Magic Resistance ≥ 0

Example: 25 MR &rarr; Damage Multiplier = 100 / (100 + 25) = 100 / 125 = 0.8, so incoming damage is reduced by 20 percent.

It is important to note that the amount of health you have is irrelevant to the effectiveness of Magic Resistance. For example: 25 Magic Resistance always reduces damage by 20 percent.

A graph showing the damage multiplier decrease as Magic Resistance increases.

Effective Health
You can also view Magic Resistance as a sort of "extra health," where each point of Magic Resistance gives your champion 1 percent extra health. For example: if your champion has 100 Magic Resistance, you could calculate the damage multiplier as being 0.5, or that your champion has 100 percent more health. No matter how you view it, the end result is that magical attacks are half as effective against your champion.

This extra health, combined with your normal health, is called effective health. A champion's effective health can be calculated as:
 * Effective Health = Health &times; (100 + MR) / 100 when Magic Resistance ≥ 0

Example: 500 Health and 25 Magic Resistance &rarr; Effective Health = 500 &times; (100 + 25) / 100 = 500 &times; 125 / 100 = 500 &times; 1.25 = 625.

A graph showing effective health increase as Magic Resistance increases. (Effective health represented as a multiplier of base health.)

Negative Magic Resistance
Effects that reduce Magic Resistance (not to be confused with magic penetration) can bring Magic Resistance into negative values, causing magical damage to be more effective. Negative Magic Resistance does not follow the same formula as positive Magic Resistance, however (see below for why). Each point of negative Magic Resistance increases incoming magical damage by 1 percent, as shown in the following formula:
 * Damage Multiplier = 1 &minus; MR / 100 when Magic Resistance ≤ 0

Example: &minus;15 Magic Resistance &rarr; Damage Multiplier = 1 &minus; (&minus;15) / 100 = 1 + 15 / 100 = 1.15, so incoming magic damage is increased by 15 percent.

As a result, the formula for effective health is different as well:
 * Effective Health = 100 / (100 &minus; MR) when Magic Resistance ≤ 0

Reasoning
If the original damage multiplier formula is followed back into negative numbers, you can see that the multiplier begins to increase dramatically, literally approaching infinity when Magic Resistance reaches &minus;100 and being negative when Magic Resistance is less than that. In order to keep this from happening, a different formula is used that does not have such an asymptote; with this new formula having &minus;100 Magic Resistance doubles magical damage instead of making it infinite, and the damage increase is linear beyond that.

Diminishing Returns
Magic Resistance does not have diminishing returns from an effective health perspective; each point of Magic Resistance increases the amount of magical damage required to kill the champion by 1 percent of the champion's health. For example: 100 Magic Resistance reduces magical damage by 50 percent, so 100 percent more damage is required to kill the champion.

This table lists damage reduction and effective health bonuses granted by various Magic Resistance levels. The effective health numbers are based on a champion that has 1000 health. Note that the effective health increase is constant as Magic Resistance increases; increasing Magic Resistance from 0 to 10 gives 100 additional effective health, and increasing from 500 to 510 gives the same 100 additional effective health. No matter how much Magic Resistance is present, the next 10 would provide 100 additional effective health. This can also be seen in the graph linked above (see section: Effective Health, showing a straight line that represents the constant increase.)

Items

 * +24 MR. 400 Gold.
 * +24 MR, +270 Health +18 Armor UNIQUE Aura: +12 Armor / +15 MR / +8 Damage to nearby allied champions. 1925 Gold.
 * +25 MR,  +35 Tenacity UNIQUE Passive: Enhanced Movement 2. 1200 Gold.
 * +30 MR, +40% Attack Speed UNIQUE Passive: Physical Attacks deal 42 bonus magic damage and increase your magic resistance by 5 for 5 seconds (stacks up to 4 times). 2000 Gold.
 * +30 MR, +7.5 Mp5 UNIQUE Passive: Increases your Mana Regen by 1% per 1% Mana you're missing. 890 Gold.
 * +30 MR, +350 Mana +80 Ability Power +7% Movement Speed UNIQUE Passive: 100% chance when an ability is used that your next physical attack deals an additional 100% of your Ability Power in damage. This effect has a 2 second cooldown. 3470 Gold.
 * +30 MR, +35 Damage UNIQUE Passive: If you would take Magic Damage which would leave you at less than 30% of your Maximum Health, you first gain a shield which absorbs 300 Magic Damage for 4 seconds. 60 second cooldown. 1800 Gold.
 * +30 MR, +250 Health UNIQUE Passive: Reduces ability cooldowns by 10% and increases healing and regeneration on yourself by 15%. 1550 Gold.
 * +38 MR, +68 Armor UNIQUE Passive: Revives your Champion upon death, restoring 750 Health and 375 Mana. This effect can only occur once every 5 minutes. 2600 Gold.
 * +48 MR. 740 Gold.
 * +50 MR, +375 Health +375 Mana UNIQUE Passive: Blocks one negative spell every 45 seconds. 2715 Gold.
 * +56 MR, Active: Removes all debuffs from your champion. 90 second cooldown. 1440 Gold.
 * +57 MR, +70 Ability Power UNIQUE Aura: Reduces the Magic Resist of nearby enemy champions by 20. 2650 Gold.
 * +76 MR, +40 Hp5 +8% Movement Speed UNIQUE Passive: Restores 0.35% of your champion's health every second. 2610 Gold.

Champion Abilities

 * lays down a cover of smoke for 8 seconds where she gains 10/20/30/40/50 magic resistance while in.
 * increases her magic resistance by 10/20/30/40/50 for 15 seconds.
 * increases an allied champion's magic resistance by 30/45/60/75/90 for 3.5 seconds.
 * increases his magic resistance by 0.5 per kill with a maximum of 5/10/15/20/25.
 * increases his magic resistance by 20/35/50 + his Dodge Percentage for 5/6.5/8 seconds.
 * increases his magic resistance by 10/20/30/40/50 for 4 seconds.
 * increases his magic resistance by 100/150/200/250/300 for 5 seconds.
 * increases an allied unit's magic resistance by 10/15/20/25/30 for 6 seconds.
 * increases her magic resistance by 30/40/50/60/70 for 3 (+3) seconds.
 * increases an allied champion's magic resistance by 10/15/20/25/30 as long as the ball is attached to it.
 * increases his magic resistance by 50/75/100/125/150 for 6 seconds.
 * increases his magic resistance by 35/50/65 for 25 seconds.
 * increases nearby allied champions' magic resistance by 8/11/13/17/20 while active.
 * increases nearby allied champions' magic resistance by 16.
 * increases his magic resistance by 15/20/25% of enemy's magic resistance, doubles the amount over the next 6 seconds.
 * grants him armor for each nearby enemy champion.
 * increases his magic resistance by 25 (+7/10/13 for each enemy champion hit) for 8 seconds.

Masteries

 * increase your champion's magic resistance by 2/4/6.

Runes

 * and runes may also increase magic resistance.

Ways to reduce Magic Resistance
Note that magic resistance penetration and magic resistance reduction are different. The mechanics listed here only include reduction.

Items

 * +57 MR, +70 Ability Power UNIQUE Aura: Reduces the Magic Resist of nearby enemy champions by 20. 2650 Gold.
 * +50% Attack Speed, +25 Ability Power, UNIQUE: Your physical attacks shred your target doing 20 magic damage and reducing their magic resistance by 6 for 8 seconds. Magic resistance reduction stacks up to 4 times.  1825 Gold.

Champion Abilities

 * reduces attacked target's magic resistance by 15 / 25 / 35 for 4 seconds. The debuff doesn't stack but it refreshes with every attack.
 * reduces a target's magic resistance by 10 / 14 / 18 / 22 / 26 for 5 seconds.
 * reduces nearby enemy champions' magic resistance by 10.
 * reduces the magic resistance of any enemy that passes through it by 15 / 20 / 25 / 30 / 35 for 5 seconds.
 * reduces the magic resistance of the revealed target by 20 / 25 / 30 / 35 / 40% for 12 seconds.
 * reduces the magic resistance of all enemies hit by 8 for 8 seconds. This effect stacks up to 20 times.

Champions MR per level
Most champions do not gain MR as they level, remaining at base 30 MR from the beginning to the end of the game. However, there are some champions who gain MR as they level. The champions who do gain MR per level will have from 43 to 54 base MR at level 18; these champions are: