Like many RPGs, Path of Exile has a Critical Strike mechanic in which a hit has a chance to do more damage than normal. However, PoE's Critical Strike mechanics are a bit more complicated than typical RPGs, so it's worth taking a moment to go through them.
This page is pretty technical. The very brief version is:
- Critical Strikes increase the damage of Hits, not of Damage over Time.
- Increased Critical Strike Chance is a percentage modifier on your base Critical Strike Chance, and is not added to it. Having 100% Increased Critical Strike Chance does not guarantee that you will Critically Strike. Instead, it doubles (i.e., increases by 100%) your base chance to Critically Strike.
Critical Strike Chance
Almost all Skills and other effects in Path of Exile that Hit (in other words, deal damage in a single moment, as opposed to over time) are able to Critically Strike. Skills and other effects that deal damage over time do not Hit and therefore cannot Critically Strike, even if they are applied by a Skill that has a Hit component that does Critically Strike. For example, a Melee Attack may inflict Poison, but the Poison does not and cannot Critically Strike even if the Attack was. (Ailments like Poison do have a separate modifier that increases their damage when caused by Critical Strikes; however, they still do not Critically Strike themselves. See below for more on this.)
Note that Critical Strikes are usually rolled only once per skill use, even if the skill hits more than once. With some exceptions having to do with effects on enemies that modify their chance to be critically struck, this means most skills either strike with all hits or no hits per skill use.
Base Critical Strike Chance
Attacks, which use your weapon's damage, also use your weapon's Critical Strike Chance as their base chance to Critically Strike. For example, if your weapon has a 6.00% Critical Strike chance, any attack made with that weapon has a 6.00% base Critical Strike chance.
Most Spells have their own base Critical Strike Chance listed on their Skill Gem. This varies from Spell to Spell, so some Spells are better-suited for crit-centric builds than others.
If a Skill is not an Attack, and does not list a Critical Strike Chance on its Skill Gem, then one of two things is likely true. Either:
- The Skill deals Damage over Time, which is by nature unable to Critically Strike, or
- The Skill has a 0% base Critical Strike Chance, meaning it cannot normally Critically Strike, but is not actively prevented from doing so. These skills may be able to Critically Strike in certain situations, provided that you have added Critical Strike Chance applied to them (see below).
Modifying Critical Strike Chance
A few effects add additional Critical Strike Chance. Note that this is different from Increased Critical Strike Chance, in the same way that added damage is different from Increased damage (for more on this distinction, see the "More & Increased" page under 'Before You Start'). Any such added effects (usually worded as +X% Critical Strike Chance) are added to the base chance to crit derived from your weapon or the Skill's base Critical Strike Chance, and this happens before any further modifiers are applied below.
The Skill's base Critical Strike Chance (plus any Added modifiers) is then modified by Increased Critical Strike Chance and More Critical Strike Chance (along with their Reduced and Less counterparts). Modifiers here are applied in exactly the same way that damage is modified by Increased or More damage, so see the "More & Increased" page under 'Before You Start' for details).
Note that increases to Critical Strike Chance on weapons are always Local unless noted otherwise and modify the weapon's own base Critical Strike Chance (not your Increased Critical Strike Chance stat); such effects will not apply to Spells since they do not Hit with the weapon, and were applied before the first step (not here) even for Attacks. For more on this, refer back to "Local & Global Modifiers".
Common Misconception: Almost all increases to Critical Strike Chance are Increased Critical Strike Chance, not Added Critical Strike Chance. It is a common source of confusion for new players to think a couple of 50% Increased Critical Strike Chance nodes on the Passive Tree will guarantee Critical Strikes, but that is not true. Similarly, the Controlled Destruction Support (which applies a 100% Reduced Critical Strike Chance to supported Spells) does not at all prevent the Skill from ever Critically Striking.)
For example, suppose we use an Attack with a weapon that has a 6% base Critical Strike Chance. We have a passive effect that gives us +1% Critical Strike Chance, and four nodes that give 50% Increased Critical Strike Chance apiece (for a total of 200%). We start by adding the 1% added chance to our base to get 7%. We then increase this number by 200%, or in other words multiply it by 3, to get our final Critical Strike Chance of 21%.
To elaborate on the misconception above: the best way to think about Increased or Reduced chance is as adding a percentage of the base crit chance of the skill. In other words, 200% Increased Critical Strike Chance adds 200% of the base critical strike chance of the skill, not a flat 200%. This isn't quite accurate when More or Less modifiers are present, but it's a decent way to think about it. Similarly, 100% Reduced Critical Strike Chance subtracts the base crit chance of the skill, which will usually not reduce its critical chance to zero, even not accounting for the crit cap below.)
All of this probably sounds pretty confusing, but everything here is consistent with the normal rules of PoE's modifiers: compute the base amount with base + added amount, combine all Increased and Decreased modifiers and apply them at once, and apply More or Less modifiers individually at the end. It's counter-intuitive mostly because you're probably used to games that simply add flat amounts to Critical Strike Chance.
There are no longer minimum or maximum caps for critical strike chance. You can have anywhere from 0% to 100% Critical Strike Chance.
The Accuracy Check (Attacks Only)
Critical Strikes with Attacks (not Spells or Secondary damage hits) have an extra wrinkle. Even if an Attack both succeeds on its initial Accuracy roll to hit in the first place and succeeds on its chance-to-crit roll, an Attack also has to succeed on a second Accuracy roll to successfully Critical Strike. In effect, an Attack has to "hit twice" in addition to succeeding on its Critical Strike Chance roll to be able to Critically Strike. If the second Accuracy roll fails, the Attack will still hit, but not Critically Strike. (If you are unable to miss, usually through a Your Hits Can't Be Evaded effect, this second roll will always succeed, effectively removing this step from the equation.)
As a result, Accuracy is a very important stat for Attack builds focusing on Critical Strike.
Spells and Secondary hits, which do not interact with Accuracy in any way to begin with, do not have this secondary check.
Critical Strike Multiplier
When a Hit deals a Critical Strike, it will do extra damage. The amount of extra damage is determined by your Critical Strike Multiplier. By default, this multiplier is 150% for players (130% for monsters, but you'll rarely care about this number), but it can be modified by equipment or passives that gives +X% Critical Strike Multiplier.
All modifiers to Critical Strike Multiplier that I'm aware of are simply added to it. If you have +35% Critical Strike Multiplier, your total Critical Strike Multiplier is 185%, meaning your Critical Strikes deal 85% more damage than they normally would.
The Critical Strike Multiplier is effectively a More multiplier (not an Increased one) and is applied separately from other modifiers to damage.
Ailments are special effects that can be applied by damaging Hits, and scale based on the damage of the Hit. The full mechanics of Ailments are complex and are described separately on their own page, but Ailments have a special interaction with Critical Strikes that is important to note here.
- Critical Strikes with some damage types have a 100% chance to apply certain Ailments. Critical strikes with Fire damage have a 100% Chance to Ignite; critical strikes with Lightning damage have a 100% Chance to Shock, and critical strikes with Cold damage have a 100% Chance to Freeze.
- If a Critical Strike applies one of the damaging Ailments (Ignite, Bleed, or Poison), the Ailment will do 50% More damage than it normally does. This is not, strictly speaking, causing the Ailment to critically strike in its own right (which Damage over Time cannot do), but is applied as a simple modifier to its damage. Critical Strike Multiplier does not, by default, affect this bonus, although the keystone passive Perfect Agony (located at just right of center on the passive tree) allows a portion of your Critical Strike Multiplier to affect Ailments.