Chance to Hit (EU2012)

In XCOM: Enemy Unknown, the chance that any given shot will land on your target is based on the sum total of a number of variables. Some work for you, some work against you, but all are additive: if you're told that shiny new rifle gives you a +10 Aim, then indeed your final chance to hit simply goes up by 10% when you're using it. Understanding which bonuses and penalties may apply to a given position will allow you to determine whether it's worth moving a unit there, whereas weighing up the chance to hit a target against the amount of damage you might deal to it is useful when considering taking a defensive action instead.

Attack (or Aim) is the base statistic each unit has which accounts for the bulk of its accuracy in battle. For XCOM units, this goes up at a set rate based on rank and class, whereas for aliens it's determined by their race and the difficulty level.

Defence, on the other hand, is the statistic which lowers each unit's chance to be hit. Your units may improve their base defence stat by donning certain forms of armour (keeping in mind that most only grant additional hitpoints, lessoning the effects of damage, rather then your chance to avoid it altogether). Some alien races have an innate defensive bonus which is further increased on higher difficulties.

In addition, units may source additional aim/defence bonuses from soldier perks, terrain factors and equipment, cover being one of the most important examples.

The chance to score a Critical Hit is calculated in much the same way, though the percentage displayed is dependant on the shot hitting in the first place. If you're shown an 80% chance to hit with a 25% chance of a critical, then your real chance to score a critical hit on the target is 20% (a 20% chance of missing, a 60% chance to score a regular hit, and a 20% (25% of 80) chance of getting a critical).

Just before OK'ing a shot on your target, you may select "More Info" within the game's HUD to see a list of which modifiers would apply to it.

Solder Stats

A typical XCOM rookie starts off with a flat 65 Aim stat. Generally, as their rank improves, they gain additional points at a static rate (that is to say, a Lieutenant Sniper will always have an Aim stat of 84). Second Wave options can instead be used to mix up the abilities of your ranks - "Not Created Equally" has them start with an aim stat of somewhere from 55 to 80, whereas "Hidden Potential" randomises their gains.

The DLC unit Shaojie Zhang has an extra +5 Aim relative to other non-Second Wave affected heavies. Heroes may also exceed the norm (Ken Levine in particular clocks in with a massive score of 200, nearly double the usual potential of his sniper class!).

In addition to their base stats, each class may choose from a variety of perks which can potentially improve accuracy, decrease it in exchange for another bonus, raise their defence, or affect the aim/defence values of other units in play:

Assaults

The Assault class sports moderate accuracy, but their shotguns grant huge bonuses at near range - and they're very good at closing the distance. At distances further then ten tiles, their pistols (or perhaps rifles, if equipped) make for more reliable damage, albeit somewhat reduced in power. They can acquire abilities that improve their defence when up in the alien's faces.

Squaddie	Corporal	Sergeant	Lieutenant	Captain	Major	Colonel	Total Gains
70 (+5)	73 (+3)	76 (+3)	79 (+3)	82 (+3)	85 (+3)	89 (+4)	+ 24

Related abilities:

• Flush: Reduced damage for a +30% aim (also forces the target to move).
• Rapid Fire: Two shots, but at a -15% aim penalty to each.
• Tactical Sense: A +5% defensive bonus for every enemy unit that the assault can see (up to 20%).
• Aggression: A +10% crit chance bonus for every enemy unit that the assault can see (up to 30%).
• Close & Personal: Up to a +30% crit chance bonus depending on the distance to the target (what distances for what bonuses?)

Heavies

Heavies have very poor aim compared to other classes, but make up for it with a number of abilities which ignore the stat, and at close range their accuracy improves somewhat. Their rockets always have at least a 90% chance to strike their targets, and having them open up on your enemies first can often result in them destroying alien cover or granting other accuracy benefits to their squad-mates (whether the heavies themselves manage to hit or not).

Squaddie	Corporal	Sergeant	Lieutenant	Captain	Major	Colonel	Total Gains
67 (+2)	68 (+1)	69 (+1)	70 (+1)	71 (+1)	73 (+2)	75 (+2)	+10

Related abilities:

• Holo-Targeting: +10% aim to all other allied fire on the target until next turn.
• Suppression: -30% to the target's aim until next turn or until the heavy is damaged.

Snipers

Snipers gain no accuracy bonuses from their high-powered rifles, and in fact take a penalty if they try to use them at close range (pistols have a better chance to hit at distances less then ten tiles). However, their innate accuracy stats are through the roof, and optional abilities at higher ranks may push them even higher.

Squaddie	Corporal	Sergeant	Lieutenant	Captain	Major	Colonel	Total Gains
75 (+10)	78 (+3)	81 (+3)	84 (+3)	88 (+4)	95 (+7)	105 (+10)	+ 40

Related abilities:

• Damn Good Ground: A passive +10% aim and defence while above the target (even if flying by way of Archangel Armour).
• Executioner: A passive +10% aim if the target has less then 50% health.
• Opportunist: Removes the aim penalty (and crit restriction) from Overwatch shots.
• Snap Shot: Allows firing as a second action, but at a -20% aim penalty. (Due to a bug, taking this perk applies this penalty to extra shots granted by Double Tap as well.)
• Disabling Shot: Shot empties the target's weapon, but at a -10% aim penalty.
• Low Profile: The sniper receives a high cover bonus of +40% while in low cover, rather then the usual +20%.
• Headshot: A +30% chance to crit.

Supports

Supports are very "mid-range" in terms of accuracy, and their potential damage output isn't anything to brag about in comparison to other classes. It should be noted that regardless of range, the rifles and pistols available to them will typically each have the same chance to hit as the other (excluding any weapon-specific bonuses, such as that available to the Light Plasma Rifle or Foundry-upgraded pistols). Their smoke grenades can grant defence bonuses to units they cover.

Squaddie	Corporal	Sergeant	Lieutenant	Captain	Major	Colonel	Total Gains
70 (+5)	73 (+3)	76 (+3)	79 (+3)	82 (+3)	85 (+3)	90 (+5)	+ 25

Aim-related abilities:

• Combat Drugs: A passive bonus granting a +20% aim and +10% crit chance bonus to units firing from within the support's smoke clouds.
• Smoke Grenade: Creates a smoke cloud granting +20% defence to all units (friend or foe) within it.
• Rifle Suppression: -30% to target's aim until next turn.
• Dense Smoke: A passive bonus causing the support's smoke clouds to grant +40% defence instead of the regular 20%.

Alien Stats

Aliens are, for the most part, about on par with lower-ranked XCOM soldiers, though on difficulty levels above normal they receive a +10 aim/crit bonus (and an extra +10 aim for Outsiders and Muton Elites on Impossible).

Many aliens have innate defensive bonuses, which XCOM solders require certain forms of armour to acquire.

It's worth noting that the Light Plasma Rifle seen earlier on in the game (especially favoured by Thin Men) confers an additional +10 bonus to its wielder (though it's been reported that this doesn't apply to the aliens - so maybe this is only worth noting on the basis that it doesn't work?). Those aliens which use melee attacks (the Chryssalid, the Zombie, and the Muton Berserker) never miss.

Name	Aim	Defence	Name	Aim	Defence
Sectoid	65	0	Outsider	70	0
Thin Man	65	0	Floater	50	0
Chryssalid	Melee	10	Muton	70	10
Zombie	Melee	0	Drone	60	10
Cyberdisc	70	10	Sectoid Commander	85	20
Muton Berserker	Melee	20	Sectopod	80	30
Heavy Floater	70	10	Muton Elite	80	20
Ethereal	n/a	40	Uber Ethereal	n/a	40

Weapon Range

An aim bonus may be granted based on the distance to the target, depending on the type of weapon used. For the purposes of determining this bonus, guns always fall into one of three different categories: shotguns, sniper rifles, and "everything else".

Most - from the humble Pistol through to the legendary Heavy Plasma - are treated the same in terms of accuracy. Have two soldiers at the same distance from your target, each'll get the same range bonus as the other. Such weapons may only receive range bonuses - if you're too far away to receive one (ten tiles or more), then there's no penalty.

The rifles used by Snipers, on the other hand, are the reverse: being closer to your target inflicts a penalty, and moving further away simply lessons this (again, a distance of ten tiles or more is where range ceases to have any bearing on accuracy).

The only weapon type that has access to a range bonus in addition to a penalty is the shotgun class used by Assaults. A distance of less then ten tiles sees their accuracy spike up at a much faster rate then other weapons; but a distance further then ten tiles inflicts a penalty at the exact same rate (though oddly enough, no such penalty applies to their damage - if they happen to hit at range, they'll be just as devastating as a point-blank blast to the face).

The following formulas describe accuracy drop-off. Again, note that if the standard weapon bonus drops below 0, it's ignored, if the sniper bonus rises above 0 it's ignored, and the shotgun formula always applies. x and y describe the distance in tiles along each axis between the target and the shooter (i.e. Pythagoras is used to get the proper distance on any angle). Final results are rounded down.

Standard Guns	Shotgun Class	Sniper Class

In addition, a very slight fractional penalty may be applied depending on the direction the shot is coming from. For example, a soldier with a rifle sitting an exact two tiles away from an alien may have a 33% range bonus, but another soldier with the same weapon and at the same distance might only get a 32% bonus if he's on a 90° angle (it appears this is because the base bonus drops from 42 to 41.6 in such a case).

A list of pre-calculated values for some ranges have been recorded here.

Note: The partial brackets used in the shotgun formula represent a floor operation, or to put it another way, "round this value down".

Note 2: These formulas may not be 100% accurate.

Other Factors

(Stub, will detail potential cover/height bonuses then link off to Cover for info about flanking. Also talk about hunkering down and overwatch (note - what sort of penalties does Covering Fire get?).)

Psionic Abilities

(Stub, briefly cover abilities that can affect firing accuracy, then link off to Psionic_(EU2012) for more info)

Critical Hits

(Stub, will need to link off to Damage for info about, well, damage)