A critical strike is a basic attack that deals twice its normal damage. In the real world, it's an attack striking a vulnerable area, while in other MMO and/or RPG settings, it ties into the very common "critical hit" mechanic.
It only applies to basic attacks or certain abilities like critical strike against a building such as a turret or inhibitor (Except , whose fourth shot receives minicrits against all structures).
and that act like a basic attack or and , which are specially stated. It's important to note that you cannot score aMost champions have different attack animations for a critical strike, though some do not; this includes mages and other champions not meant to be built using basic attacks for damage. Some of these champions performing a critical strike (if built being able to perform them) may instead have a soundbite that plays in place of an animation, typically a grunt of effort.
Critical strike is valuable for all champions who primarily deal damage with basic attacks, rather than through abilities. Some champions, such as , have an affinity for critical hits and benefit more from items that improve them. Players can increase the Chance and the Damage of critical strikes with items, abilities, and runes.
Chance
Critical strike chance, as the name states, is the chance of a champion's physical attacks dealing a critical strike. Critical strike chance stacks additively. Runes, items, and all abilities can increase critical strike chance.
When averaged over a large number of autoattacks and at Base critical damage (100% bonus damage), critical strike chance is essentially a damage multiplier where each 1% of critical chance adds 1% bonus damage. Thus at 50% critical strike chance a champion will be doing (on average) 150% damage with autoattacks, not counting Bonus critical strike damage. Because of this, critical strike chance scales well with both attack damage and attack speed. The formula for this damage multiplier is given by:
Damage multiplier = 1 + (Critical chance × (1 + Bonus critical damage))
The maximum critical strike chance you can reach at level 1 through masteries is 20.25%, excluding abilities and items, and is hard capped at 100%.
Critical strike chance has a theoretical gold value of 40 per 1% critical strike chance. This makes the most efficient core item for critical strike chance, as it grants +10% critical chance for the cost of 400 with a 40 per 1% critical chance efficiency. Note that has the same efficiency.
Increasing critical strike chance
Items
Item  Cost  Amount  Availability 

400  10 %  Common  
800  20 %  Common  
3400  20 %  Common  
3400  20 %  Common  
2600  30 %  Common  
2600  30 %  Common  
2600  30 %  Common  
2600  30 %  Common  
1200  20 %  Common 
Temporary Effects
 Unique: When within 1000units of each other, you and your Conduit will generate 1 charge every second, up to 100, with a bonus 4 / 2 charges whenever you or your ally attack, and 8 / 4 on spell cast.
At 100 charge, dealing damage grants you and your Conduit 20% increased AP and +50% flat critical strike chance for 8 seconds.
All current charge is lost if you move further than 1000units apart or hold onto 100 charge for longer than 5 seconds. : Passive:
Champion abilities
Note: Any ability boosting attack speed has an indirect impact on the number of crits.
 100% critical chance on her next second attack. grants her
 100% critical chance every fourth basic attack.
 100% critical chance on its fourth and final shot. has a guaranteed
passively grants him  100% critical chance against targets below 15% of their maximum health. grants him
 100% critical chance for his next basic attack against his closest target. grants him
 100% critical chance for every basic attack striking an enemy unit from behind. This cannot happen on the same target more than once every 3 seconds. passively grants him
 100% critical chance if he hits its target within a 170 range. grants him
 0.35% critical chance per point of Fury he currently has. grants him
 100% critical chance on every third basic attack. passively grants him
 critical chance. grants him twice the amount of
Runes
Damage
Critical strike damage refers to the amount of damage that a champion's critical strike will deal. All champions have 200% critical strike damage by default, except with 110% damage, with 150% (reduced to 112% vs structures) and with 180%. In most cases, critical strike damage stacks additively. Damage modifiers apply to critical strike damage normally.
Modifying Critical Strike Damage
 Champion Abilities
 base critical damage to 140 / 155 / 170 / 185 / 200% from the default of 200%. modifies her
 total critical damage by 25% for basic attacks. multiplicatively reduces his
 base critical damage to 120%  145% (based on level) from the default of 200%. modifies his
 base critical damage to 125 / 137.5 / 150 / 162.5 / 175% from the default of 200%. modifies his
 total critical damage by 10% for basic attacks and 25% for (e.g. 200% to 190% / 175%). multiplicatively reduces his
 Note: base critical damage. Instead, it does the following:
 Only allows critical strikes on autoattacks against targets affected by .
 Removes the randomness of critical strikes. It does so by simultaneously increasing the proc probability to 100% and reducing the extra damage on proc by the same multiplicative factor.
 Adds straight 10% multiplicative bonus damage that is unrelated to critical strikes.
does not modify the
Items
 Unique: Critical strike bonus damage is increased by 50%. : Passive:
Runes
Notes
 The maximum bonus critical strike damage obtainable is 95.51%, as follows:
 For most champions, the maximum critical strike damage available is 200% for a total of amount of 295.51% AD on their critical strikes.
Skills affected by Critical Chance
Certain skills are capable of critical strikes and/or are affected by critical chance.
 Most of them only do bonus 100% AD × (Total critical damage − 100%) if they critically strike.
 : Against champions.
 Life steal. : Magic damage, benifiting from
 : Only applicable to first hit.
 : Only applicable to first hit.
 Life steal. : Magic damage, benifiting from
 Others
 Critical strikes do bonus 250% AD × (Total critical damage − 100%) against minions and monsters. :
 Critical strikes do bonus entire damage portion × (Total critical damage − 100%). :
 Critical strikes do bonus entire damage × (Total critical damage − 100%). :
 critical strike, doing bonus 100% AD × (Total critical damage − 100%). : The shot is a
 Total critical damage − 100%) if it is a critical strike. : The additional shot does bonus entire damage × (
 50% AD × (Total critical damage − 100%) if it is a critical strike. : The additional strike does bonus
 critically strike dealing bonus entire damage × (Total critical damage − 100%). If the first bounce kills it's target, the second bounce will always critically strike. : The second bounce can
 Critical strikes do bonus base damage × (Total critical damage − 100%). :
 Critical strikes do bonus 75% AD × (Total critical damage − 100%). :
 Additions
Miscellaneous
Other skills that are not being affected by critical strikes and chances are displayed as a critical strike due to other reasons.
 Affected by Critical strike damage
 Unaffected by Critical strike damage
 critical strike. : The 2 × damage modifier applied by being chilled by is given out as a
 critical strike. : The triple damage dealt to a target that has been bleeding for more than 2 seconds is given out as a
 critical strike when enemy hits terrain. : The damage is given out as a
 critical strike when the cask ferments after 2 seconds. : The damage is given out as a
 critical strike when acting like a single target spell (hitting only one enemy). : Appears as a
 40% of their maximum health is given out as a critical strike. : The double damage dealt to enemies under
 critical strike. : When the sigil is shattered by Leblanc's next ability against the target, the combined damage of the sigil and the ability is given out as a
 critical strike. : The 40% increased damage dealt to targets is given out as a
 critically strike for a +50% damage bonus due to the passive from if the target is under 15% health. : Will
 critical strike. : The extra damage applied to a target marked with is given out as a
 critical strike. : The doubledamage applied by being knocked into a wall is given out as a
 critical strike. : The extra damage applied when under the effect of is given out as a
 critical strike when used to hexplode a tower. : The damage is given out as a
Trivia
 Similarly in many MMO settings, a number of attack damagebased assassins or secondary assassinbased champions also benefit from high critical strike chance in varying form, as a means to help finish off targets faster as well as to deal more damage in bursts overall.
 Some champions may actually use their critical strike animation for their empoweredbasic attack if they ever activate a singleuse onhit effect, only it does not actually count as an actual critical strike.
 The fastest way to obtain 100% critical strike chance is with 1200:
 Runes: +20.25% Critical Strike Chance
 9 × Marks of Critical Strike Chance (+0.93% × 9 Critical Strike Chance)
 9 × Seals of Critical Strike Chance (+0.42% × 9 Critical Strike Chance)
 9 × Glyphs of Critical Strike Chance (+0.28% × 9 Critical Strike Chance)
 3 × Quintessences of Critical Strike Chance (+1.86% × 3 Critical Strike Chance)
 Items: +30% Critical Strike Chance
 (+10% Critical Strike Chance)
 (+20% Critical Strike Chance)
 Critical Strike Chance Amplification: 100% Critical Strike Chance
 (+ 100% Critical Strike Chance)
 Relevant mathematics:
 100.5% critical chance Critical Strike Chance = (20.25% + 30%) × 2 =
, requiring only  Runes: +20.25% Critical Strike Chance
Champion statistics  

Offensive 

Defensive  
Magical  
Movement 