Damage

Overview

Damage in XCOM comes in two major ways, and then there are some oddballs:

• Firearm or gun damage, commonly known as "weapons", and
• Explosive damage

A 2 x 2 table of the most important types of damage is as follows. Percents mean, percent of weapon strength as shown in UFOpaedia:

           Firearms   Explosives
Units       0%-200%    50%-150%
Terrain    25%- 75%      50%

Unit Armor is always subtracted from calculated damage, before any damage is applied. Units take damage and "remember" it, but terrain is not affected at all unless and until damage is equal to or greater than its armor strength, in which case it is destroyed.

So, the UFOpaedia shows average weapon strength versus units. But they only impart half their listed UFOpaedia strength versus terrain - a fixed half, for explosives, and half on average, for firearms.

Some units also have modifiers versus particular weapon classes, as shown below.

Some of the less common (but no less fun!) forms of damage are:

• Stun damage, provided by the Stun Rod and Stun Bomb and even smoke. Oddly, the Stun Rod is the lone melee weapon in XCOM, and the Small Launcher's stun bomb follows the rules of explosives for its blast pattern and tile "explosive" strength, but gives 0-200% stun damage at a given tile; see Small Launcher for more clarification.
• Incendiary damage is a complex but ultimately fairly weak type of damage.
• Objects dropped on the ground can only be destroyed by explosive damage, at particular fixed strengths as shown here .

Formula

 Damage to Unit = (Weapon Damage × Damage Modifier) - Armour 

If the bullet does negative or zero damage, then the bullet is ignored (for damage purposes, but you still get Firing Accuracy experience if it did hit). If the bullet does damage, then there's a chance that the armour plate that it struck will deteriorate by a few points.

That's the basic idea. Let's expand on the variables a bit:

Weapon Damage

Weapon damage will be a random number between 0 and two times the damage level of the weapon listed in the ufopaedia. In reality, all of the damages listed in the ufopaedia are the averages for the weapons. Note that the doubling of the damage is done before the modifiers are applied.

Damage Modifier

All units in the game will have various modifier levels for all the ammo types in the game. The modifier is simply a percentage multiplier that changes the weapon damage, for better or for worse. For more information, see the next section for specifics.

Armour

The armour section the bullet hit. This can be the front, rear or the left or right side armour for ballistic projectiles. Under armour can only be hit when the target is at or standing right next to the very centre of an explosion from an area-effect weapon, otherwise it's treated as directional damage. To put it bluntly, armour is pure damage reduction.

Damage Modifiers

Everything's 100% except the following:

Vulnerable to...

Incendiary:
    Reaper             170%

High-Explosive:
    Silacoid           130%

Laser:
    Sectopod           150%

Melee:
    Unarmoured Soldier 120%
    Civilian           120%
    Sectoid            120%
    Celatid            120%
    Floater            120%

Celatid Acid Spit:
    Unarmoured Soldier 160%
    Civilian           160%
    Sectoid            160%
    Celatid            160%     I find it amusing that Celatids
    Floater            160%     are vulnerable to their own spit -- Danial
    Personal Armour    110%

Resistant to...

Armour-Piercing:
    Cyberdisc       80%
    Muton           60%
    Zombie          60%

Incendiary:
    Chryssalid      80%
    Ethereal        70%
    Snakeman        70%
    Tanks           40%
    Personal Armour  0%
    Power Suit       0%
    Flying Suit      0%
    Silacoid         0%

High-Explosive:
    Sectopod        80%
    Zombie          80%
    Tanks           70%
    Cyberdisc       60%

Laser:
    Zombie          70%

Plasma:
    Sectopod        80%
    Zombie          70%

Stun:
    Personal Armour 90%
    Chryssalid      90%
    Power Suit      80%
    Flying Suit     80%
    Ethereal        80%
    Zombie           0%

Melee:
    Tanks           90%

Celatid Acid Spit:
    Tanks           40%

Susceptible to...

                    1     2     3     4     5     6     7     8     9    10    11     12     13
Armor Piercing     100   100   100   100   100   100    60   100   100   100   100    80     60
Incendiary         100     0     0    40    70    70   100     0   80    170   100   100    100
High-Explosive     100   100   100    75   100   100   100   130   100   100    80    60     80
Laser              100   100   100   100   100   100   100   100   100   100   150   100     70
Plasma             100   100   100   100   100   100   100   100   100   100    80   100     70
Stun               100    90    80     0   100    80   100   100    90   100   100   100      0
Melee              120   100   100    90   100   100   100   100   100   100   100   100    100
Acid Spit          160   110   100    40   100   100   100   100   100   100   100   100    100

Column       Unit Type(s)
   1         Soldier, Civilian, Sectoid, Celatid, Floater
   2         Personal Armor
   3         Power Suit, Flying Suit
   4         Tanks, Hovertanks
   5         Snakemen
   6         Ethereal
   7         Muton
   8         Silacoid
   9         Chryssalid
  10         Reaper
  11         Sectopod
  12         Cyberdisc
  13         Zombie

To summarize re: the all important X-COM armor: All types have normal (100%) susceptibility to AP, HE, Laser, and Plasma damage. Personal Armor has 90% susceptibility to stun, while Power/Flying Suits have 80% to stun. Finally, Personal, Power and Flying are completely resistant to Incendiary.

Example

Adapted from Zombie's kindly example:

Scenario: Beginner level Muton mission. Your soldier is carrying a normal Pistol and you want to know what is the chance that a Muton Soldier's front armor will absorb a direct hit without lowering health.

Vitals: 
Muton Soldier front armor = 10
Muton Soldier health = 125
Muton susceptibility to Armor Piercing ammo = 60%
Pistol listed power = 26 (average)

Calculation: 

Max damage for a Pistol against a Muton is lowered due to susceptibility:

 Modified max damage = INT(26 * 2 * 60 / 100)
                     = INT(52 * 60 / 100) 
                     = INT(3120 / 100)
                     = INT(31.2)
                     = 31

where 2 is the max modifier and 60/100 is the damage modifier

The range of values the Pistol can deal is 0 to 31 points of damage (or a range of 32 different values). The equation's steps are shown because XCOM's math sometimes leads to slightly unexpected values in subsequent steps.

The Muton Soldier has a front armor rating of 10. It will absorb up to this much damage without changing health. In other words, it will negate the first 10 points of damage a Pistol can dish out (0-10; 11 possible values). Since the Pistol's range is 32, the probability that a direct hit will be blocked by Muton armor is simply 11 / 32 = 34.4%.

Okay, say you want to know the probability of doing damage to a Muton's health. Weapon range of the pistol is still 32, but instead of absorption we want penetration. 10 points of damage is absorbed, so we need at least 11 to penetrate. 32 - 11 = 21 values which will breach. 21 / 32 = 65.6%.

21 is also the maximum number of damage points which might be deducted from the Muton's health. Assuming the Muton was initially uninjured, in the worst case (for him!) he will have an ending health of 125 - 21 = 104.

On average, there will be 15.5 damage dealt (0-31). Then armor (10) reduces this to an actual (penetrating) average damage of 7.67.

Armor vs. Health Damage

When there is penetrating damage (i.e., damage is >0 and not all blocked by armor), Health and Armor Damage have a very simple relationship:

 Health Damage = 10 * Armor Damage, minus 1 to 10

Or the other way around:

 Armor Damage = INT( Health Damage / 10 ) + 1

In table form:

Armor  Health Damage
Damage   Min  Max
   0      0    0
   1      1    9
   2     10   19
   3     20   29
   4     30   39
   5     40   49
   6     50   59
   7     60   69
   8     70   79
   9     80   89
       etc.

Of course, this assumes armor is not zeroed out by the hit. Also note that you will always have armor damage if you have health damage and vice-versa, if there was damage penetration.

You will see this relationship if you watch damage from individual hits. However, the relationship is muddied if you only look at a unit after a number of hits (or, of course, if armor is broken through). The most extreme example would be if a unit were hit with e.g. five hits of 1 damage point each. It would have 5 health damage and 5 armor damage, which would appear contrary to the above table.

This relationship has been been tested for a wide variety of explosions and some light weapons vs. Mutons, and some hits on soldiers. It has not otherwise been widely tested, but is presumed true unless/until found otherwise.

As stated, the above is for damage that penetrates. It is possible for hits (guns or explosions) to do zero damage. This happens on the rare occasion that a weapon damage roll is actually zero, or, much more commonly, when armor has blocked all the damage. You still get an experience point toward Firing Accuracy if this happens. You only have to hit the target(s) to get Firing experience; there doesn't have to be damage.

See Also

• Weapons
• Explosions
• Firing Accuracy
• Health
• Medi-Kit
• Experience