Magic penetration

Magic penetration (or spell penetration) is a champion statistic which allows the magic damage done from their abilities, enhanced attacks, etc. to ignore some or all of a target's magic resistance. All champions have 0 base magic penetration but can increase it with items, runes or abilities for a few champions. This cannot affect enemy magic resistance beyond 0 to deal bonus damage.

Magic resistance reduction is the effect of some items and champion abilities that temporarily reduces the magic resistance of one or more targets. This allows allies to benefit from the reduction of magic resistance. This can reduce magic resistance below 0, resulting in bonus damage (see Magic resistance).

Order of calculations
Magic penetration and magic resistance reduction work exactly like armor penetration and armor reduction.

Magic penetration and magic resistance reduction are applied in the following order:
 * 1) Flat magic resistance reduction
 * 2) Percentage magic resistance reduction
 * 3) Flat magic penetration
 * 4) Percentage magic penetration

Flat magic resistance reduction
Flat magic resistance reduction stacks additively.
 * The target's magic resistance is reduced by a certain amount.
 * If the target has 30 magic resistance and then has its magic resistance reduced by 20, the target will take damage as if it had only 10 magic resistance.
 * 302010
 * Flat magic-resistance reduction can reduce a target's resistance below zero.
 * For example, if an enemy with 10 magic resistance has their magic resistance reduced by 25, the enemy will have &minus;15 magic resistance (causing him or her to take extra damage; see Magic resistance).

Percentage magic resistance reduction
Percent magic resistance reduction stacks multiplicatively. Percentage magic resistance reduction makes a bigger difference on targets with higher magic resistance. For instance, with 40% magic resistance reduction, a target with 200 magic resistance will lose 80 while a target with only 50 magic resistance will lose 20.
 * The target's magic resistance is multiplied by a percentage (100%the listed reduction). Percentage magic resistance reduction is ignored if the target's magic resistance is 0 or less.
 * If the target has 30 magic resistance and then has its magic resistance reduced by 20%, the target will take damage as if it had only 24 magic resistance.
 * 30(1.00.2)300.824

Flat magic penetration
Flat magic penetration stacks additively. Because the target's actual magic resistance value does not change, anything else depending on that value also doesn't change.
 * Some of the target's magic resistance is ignored when dealing magic damage.
 * If the target has 30 magic resistance and you have 10 magic penetration, the target will take damage as if it had only 20 magic resistance (the target's actual magic resistance value will not change, however).
 * 301020
 * Flat magic penetration cannot ignore the target's magic resistance to get below zero.

Percentage magic penetration
Percent magic penetration stacks multiplicatively. Because the target's actual magic resistance value does not change, anything else depending on that value also doesn't change. Percentage magic penetration makes a bigger difference on targets with higher magic resistance. For instance, with 40% magic penetration, a target with 200 magic resistance will be considered as having 80 less while a target with only 50 magic resistance will be considered as having 20 less.
 * A percentage of the target's magic resistance ignored when dealing magic damage.
 * If the target has 30 magic resistance and you have 20% magic penetration, the target will take damage as if it had only 24 magic resistance (the target's actual magic resistance value will not change, however).
 * 30(1.00.2)300.824

Examples
Given 20 flat magic resistance reduction and 30% magic resistance reduction, and the target is affected by 10 flat magic penetration and 40% magic penetration,
 * Target A has 80 magic resistance.
 * The 80 is reduced to 60 by your 20 magic resistance reduction.
 * The 60 is reduced to 42 by your 30% magic resistance reduction.
 * Your magic penetration makes you ignore 10 of the remaining 42 magic resistance, 32 is left.
 * Your magic penetration makes you ignore 40% of the remaining 32 magic resistance, 19.2.
 * Target A takes damage as if it has 19.2 magic resistance.
 * Target B has 18 magic resistance.
 * The 18 is reduced to &minus;2 by your 20 magic resistance reduction.
 * The &minus;2 is not affected by any further calculations because it is less than 0.
 * Target B takes damage as if it has &minus;2 magic resistance.

Champion abilities
Note: Only the magic penetration bonus effect of these abilities is shown here. To read more information on each of these abilities, follow the link on each of them.
 * while active, grants him 10 / 15 / 20 / 25 / 30 % magic penetration.

Masteries

 * : Your magical spells penetrate 10% of the target's magic resistance.

Runes

 * runes provide flat magic penetration. runes provide flat armor penetration and magic penetration.

Neutral buffs

 * Crystal Scar's Aura grants every champion 5% magic penetration while playing on the Dominion game mode.

Champion abilities
Note: Only the magic resistance reduction effects of these abilities are shown here. To read more information on each of these abilities, follow the link to each of them.
 * passively makes his autoattacks reduce the magic resistance of enemies hit by for 4 seconds.
 * reduces the magic resistance and armor of the target by 10 / 14 / 18 / 22 / 26 magic resistance reduction for 5 seconds.
 * passively reduces the magic resistance of nearby enemies by 10.
 * , at rank 2 and above, reduces armor and magic resistance by 1 per hit for 2 seconds (effect stacks up to 50 times).
 * reduces the magic resistance of all targets affected by 15 / 20 / 25 / 30 / 35 for 5 seconds.
 * allows her autoattacks against enemy champions reduces their armor and magic resistance by 3% for 5 seconds (effect stacks up to 5 times).
 * reduces the magic resistance and armor of the target by 5 / 10 / 15 / 20 / 25 for 4 seconds.
 * reduces the magic resistance of all targets by 4 / 5 / 6 / 7 / 8 each second for 5 seconds.
 * reduces the magic resistance of all targets by 20 / 25 / 30 / 35 / 40 % for 12 seconds.
 * reduces the magic resistance of all targets by 12 / 15 / 18 / 21 / 24 magic resistance reduction for 5 seconds.
 * reduces the magic resistance of all targets by 8 / 9 / 10 / 11 / 12 for 8 seconds (effect stacks up to 10 times).
 * reduces the magic resistance and armor of the target by 15 / 20 / 25 % magic resistance reduction up front, it doubles over the next 6 seconds.