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. Note: Magic Penetration 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 reduction are considered on the target champion in the following order:
 * 1) Magic reduction, flat.
 * 2) Magic reduction, percentage.
 * 3) Magic penetration, percentage.
 * 4) Magic penetration, flat.

Flat magic resistance reduction
Flat magic resistance reduction stacks additively.
 * Example: 20 magic resistance reduction reduces the target's magic resistance by 20, so a target with 30 magic resistance will be reduced to 10 magic resistance. (302010) 

Flat magic resistance reduction can reduce a target's magic 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.

Percentage magic resistance reduction
The target's magic resistance is multiplied by a percentage (100%the listed reduction). Percentage magic resistance reduction stacks multiplicatively and is ignored if the target's magic resistance is 0 or less.
 * Example: 20% magic resistance reduction multiplies the target's magic resistance to 80%, so a target with 30 magic resistance will be reduced to 24 magic resist. (3024) 

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.

Percentage magic penetration
The target's magic resistance is treated as being multiplied by a percentage for purposes of damage calculation (100%the listed penetration). Percentage magic penetration stacks multiplicatively and is ignored if the target's magic resistance is 0 or less.
 * Example: 20% magic penetration causes the target to take damage as if its magic resistance was multiplied by 80%, so a target with 30 magic resistance will be treated as though it had 24 for purposes of damage calculation (the target's actual magic resistance value will not change, however).

Because the target's actual magic resistance value does not change, anything else depending on that value also doesn't change (e.g. ).

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.

Flat magic penetration
The target's magic resist is treated as being reduced by an amount for purposes of damage calculation, but cannot be reduced below 0. Flat magic penetration stacks additively.
 * Example: 20 magic penetration causes the target to take damage as if its magic resistance was 20 less, so a target with 30 magic resistance will be treated as though it had 10 for purposes of damage calculation (the target's actual magic resistance value will not change, however).

Because the target's actual magic resistance value does not change, anything else depending on that value also doesn't change.

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 the 20 magic resistance reduction.
 * The 60 is reduced to 42 by the 30% magic resistance reduction.
 * The 42 is considered to be  by the 35% magic resistance penetration.
 * The  is considered to be  by the 10 magic resistance penetration.
 * Target A takes damage as if it has magic resistance.

Graph of % increase in damage of mpen vs no mpen, at up to 300 MR:
 * Target B has 18 magic resistance.
 * The 18 is reduced to &minus;2 by the 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.

Masteries
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.
 * grants % magic penetration.
 * Gain 0.6-3.0 flat + 0.06-0.3 per level Armor and Magic Penetration.

Items

 * reduces the of nearby enemy champions by 10-25 (Scaled with level) (700 range).
 * basic attacks reduce the target's by 5 on hit, stacking up 5 times.

Champion abilities
Percentage reduction:
 * reduces the for 4 seconds.
 * empowers his next auto attack to reduce the for 5 seconds.
 * reduces the for 5 seconds.
 * allows her autoattacks against enemy champions reduces their for 5 seconds. This debuff stacks up to 5 times.
 * reduces the of all targets hit by % for 5 seconds, stacking up to 3 times.
 * reduces the and gains for the same amount. This amount is doubled over the next 4 seconds. The magic resistance reduction lasts for another 4 seconds after the drain completes.

Flat reduction:
 * passively makes his autoattacks reduce the of enemies hit by  for 3 seconds.