Magic resistance

Magic resistance (or MR) is a stat that all units have, including minions, monsters, and buildings. Increasing magic resistance reduces the magic damage the unit takes. Each champion begins with some magic resistance which may increase with level. You can gain additional magic resistance from abilities, items, masteries, and runes. Magic resistance stacks additively.

All champions begin with 30 base magic resistance. Few ranged champions gain magic resistance per level, but many melee champions do, with 52.5 being the maximum magic resistance possible without items, runes, masteries, or ally buffs.

Damage reduction
Incoming magic damage is multiplied by a factor based on the unit's magic resistance(same with armor):


 * Damage multiplier100(100MR)&emsp;if MR ≥ 0
 * Damage multiplier2100(100MR) if MR ≤ 0

Examples:
 * 25 magic resistance &rarr; &times;0.8 incoming magic damage (20% reduction).
 * 100 magic resistance &rarr; &times;0.5 incoming magic damage (50% reduction).
 * &minus;25 magic resistance &rarr; &times;1.2 incoming magic damage (20% increase).

Stacking magic resistance
Every point of magic resistance requires a unit to take 1% more of its maximum health in magic damage to be killed. This is called "effective health."
 * Example: A unit with 60 magic resistance has 160% of its maximum health in its effective health, so if the unit has 1000 maximum health, it will take 1600 magic damage to kill it.

What this means: by definition, magic resistance does not have diminishing returns, because each point increases the unit's effective health against magic damage by 1% of its current actual health value whether the unit has 10 magic resistance or 1000 magic resistance.
 * Note: Unlike health, increasing magic resistance also makes healing more effective because it takes more effort to remove the unit's health than it does to restore it.

For a more detailed explanation, see this video.

Magic Resistance as Scaling
These use the champion's magic resistance to increase the magnitude of the ability. It could involve total or bonus magic resistance. By building magic resistance items, you can receive more benefit and power from these abilities.

Champions

 * grants bonus magic resistance to himself equal to 20% of his bonus magic resistance.
 * grants him bonus ability power equal to 50% of his total magic resistance.
 * takes of the target's magic resistance and gains an equal amount of magic resistance. Over 6 seconds, the bonus magic resistance is doubled.

Items

 * active increases its slow duration by a base amount plus 0.5% of the wearer's total magic resistance.

Champion abilities

 * lays down a cover of smoke for 8 seconds where she gains magic resistance while inside.
 * increases her magic resistance by for 5 seconds.
 * increases her magic resistance by.
 * increases an allied champion's magic resistance by for 4 seconds.
 * increases his magic resistance by 20% of bonus magic resistance.
 * increases his magic resistance by every second he remains in combat. This bonus stacks up to 10 times. Graves is considered in combat if he has dealt or received damage in the last 3 seconds.
 * increases his magic resistance by  for 8 seconds.
 * increases his magic resistance by for 4 seconds.
 * increases his magic resistance by.
 * increases her magic resistance by for 3 seconds. The duration of the buff is doubled if any enemies are damaged by the ability.
 * increases his magic resistance by for up to 5 seconds.
 * increases an allied unit's magic resistance by for 6 seconds.
 * increases his magic resistance by for 6 seconds.
 * increases an allied champion's magic resistance by as long as the ball is attached to it.
 * increases his magic resistance by for 6 seconds.
 * Rengar lets out a battle roar, damaging enemies increases his magic resist by for 3 seconds.
 * passively increases her magic resistance by . This bonus is doubled while she is in dragon form.
 * increases his magic resistance by for 25 seconds.
 * Aura: increases nearby allied champions' magic resistance by while active and persists for two seconds after deactivation.
 * Activation: increases hers and one nearby allied champion's magic resistance by an extra for 3 seconds.
 * increases nearby allied champions' magic resistance by 16.
 * increases his magic resistance by of the target enemy's magic resistance, and doubles the amount over the next 6 seconds.
 * increases his magic resistance by for 5 seconds.
 * increases his magic resist by 4% for 5 seconds each time he switches stances. This effect can stack up to 3 times.
 * grants him magic resistance for each nearby enemy champion.
 * increases his magic resistance by for each enemy champion hit for 8 seconds.
 * increases his magic resistance by for each enemy champion hit for 8 seconds.

Masteries

 * increases your champion's magic resist by 1 for each nearby enemy champion.
 * increases your champion's magic resist by.
 * increases bonus magic resist by.

Runes

 * runes and  runes increase magic resistance.

Other

 * Disconnecting gives +1001 magic resistance.

Ways to reduce magic resistance
See magic penetration. Note that magic penetration and magic resistance reduction are different.

Trivia

 * If a summoner leaves the game, their champion gains 1000 armor and magic resistance. This may be to prevent champions like or  from gaining free kills.
 * A level 18  with 1, 5 , 3 points in , 3 points in , , , , , an allied aura, a full page of  runes, and  active will give a total of approximately 1004 magic resist. Switching  for  and having an enemy  with the same setup use  on  and the allied  use  on the enemy , will yield a total of approximately 1297 magic resist. This is the highest possible amount of magic resistance, which is 92.8% reduction.
 * If Garen disconnects from the fountain and has the same setup, he will have 2108 magic resist. This is a 95.5% reduction. Trundle can not have higher than this.