Damage

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Overview

Damage in the world of X-COM follows a series of rules that define how damage is dealt and taken. This article will explain the basics of how damage is done.

Definitions

For the purpose of this article, damage will be differentiated into two main categories. Firearm and Explosive. There is also

Firearm

This damage type covers most damage types as per the table below.

Enemy Unknown/UFO Defense	Terror From The Deep
Armour Piercing (AP) Laser Plasma Acid Spit (Silacoid) Stun (Melee) Stun (Area Effect)	AP Gauss Sonic Drill Electric Shock (Deep One) Freeze (melee) Freeze (area effect)

Explosive

This damage type covers all weapons that use the High Explosive damage type.

Incendiary

This damage type covers Incendiary/Phosphor damage. It follows its own complex damage model and isn't covered in great detail here. Refer to its article for more details.

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The damage categories are further subdivided into three target categories, units, terrain, and object.

Units

Cover all actors in the field from X-COM units to aliens to tanks.

Terrain

Cover all floors, ceilings, walls and any other fixtures that are contained in a tile such as light poles, post boxes, tables, 'middle' walls, oil drums, etc.

Object

Covers all items that can be picked up off the ground and manipulated by units, including weapons and bodies.

Damage Ranges

When dealing damage, the game will perform a random dice roll to determine what percentage of the weapon damage is to be dealt to the target. This range will vary depending on the weapon category and target category.

Target \ Damage	Firearms	Explosive
Units	0 - 200%	50% - 150%
Terrain	25% - 75%	50%
Objects	n/a*	100%#

^* Objects are only damaged by high explosives.

^# Objects technically use the Unit damage ranges to determine how much damage to compare against.

Effects of Armour

For damage vs. units, armor reduces the overall calculated damage before any damage is taken off the unit's health.

Terrain and Objects do not have hit-points and rely solely on their fixed armour levels. If damage exceeds their armour, they are destroyed, otherwise they survive.

When terrain is destroyed, it is replaced by the next 'damaged' tile in its sequence. When all available tiles are destroyed, only regular earth will remain or nothing in the case tiles above ground.

Objects are destroyed completely when their armour is exceeded by damage. See Object Destruction for the specifics.

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, as shown below.

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.

(edit: In TFTD, the listed weapon damage in the UFOpaedia is still the average damage caused, but the difference from UFO is that random weapon damage is rolled twice and averaged, which produces values which are more often closer to the average)

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

A damage modifier is a percentage value that alters base damage when calculating damage. Other ways to interpret this include damage multiplier, damage susceptibility, damage resistance, damage vulnerability, etc.

Every unit in the game is assigned an armour class, with each class containing a table of modifiers that determine how well it absorbs the various damage types.

The damage percentages start at 100% for unaltered damage. Values over 100% indicate more damage will be taken, while values under 100% mean less damage is taken.

Susceptible to...

Ammo Type	Damage Modifier Category
	T+I	Hu+	PA1	PS	Tnk	Snk	Eth	Mut	Sil	Chr	Rea	Sec	Cyb	Zom
Armour Piercing	100	100	100	100	100	100	100	60	100	100	100	100	80	60
Incendiary	100	100	80	0	40	70	70	100	0	80	170	100	100	100
High-Explosive	100	100	100	100	75	100	100	100	130	100	100	80	60	80
Laser	100	100	100	100	100	100	100	100	100	100	100	150	100	70
Plasma	100	100	100	100	100	100	100	100	100	100	100	80	100	70
Stun	100	100	90	80	1002	100	80	100	100	90	100	100	100	0
Melee	100	120	100	100	90	100	100	100	100	100	100	100	100	100
Acid Spit	100	160	110	100	40	100	100	100	100	100	100	100	100	100

¹ Due to a bug in the unit spawn routine, Personal Armor gets assigned the modifiers of a Power Suit.
² Tanks cannot be stunned.

Column	Modifier Index	Unit Type(s)
T+I	00 / x00	Terrain and items.
Hu+	01 / x01	Unarmored Soldiers, Civilians, Sectoids, Celatids and Floaters.
PA	02 / x02	Personal Armor.
PS	03 / x03	Power and Flying Suits.
Tnk	04 / x04	Tanks and Hovertanks.
Snk	05 / x05	Snakemen.
Eth	06 / x06	Ethereals.
Mut	07 / x07	Mutons.
Sil	08 / x08	Silacoids.
Chr	09 / x09	Chryssalids.
Rea	10 / x0A	Reapers.
Sec	11 / x0B	Sectopods.
Cyb	12 / x0C	Cyberdiscs.
Zom	13 / x0D	Zombies.

To summarize re: the all important X-COM armor:

• AP, HE, Laser, and Plasma: All types of X-COM armor have full/normal (100%) susceptibility to these types of damage
• Stun: All armour has a 20% resistance to stun damage. (Personal Armour should only have a 10% resistance, but receives the modifiers of a Power Suit due to a bug.)
• Incendiary: All armour is immune to fire damage. (Personal Armour should only have a 20% resistance, but is bugged.)

See Also

• GEOSCAPE.EXE
• Alien Life Forms
• Weapons
• Damage Modifiers (TFTD)

For another way to view the same info above, see this.

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 (200% of UFOpaedia listed power) and 60/100 is the damage modifier (60%)

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 has a range of 32 possible values, 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. The pistol still has 32 values, 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 7.22. (It's not simply 15.5-10=5.5 because initial damage that's less than ten becomes zero penetrating damage, not negative penetrating damage. The removal of these negative values causes the expected average to rise, as it were.)

If you need precision while testing, note that the game applies truncation after making its damage roll. It does not first determine the maximum (including damage modifiers), and then roll randomly/evenly from 0 up to the maximum, per se. To wit: if you hack a pistol to an average damage strength of 1, there would not be an overall roll of 0 to a maximum of 1 (with average damage of 0.5) as might be implied from the Muton-versus-AP example above. Instead, there is a roll of 0, 1, or 2, to which the .6 modifier is applied. Only a roll of 2 results in non-zero damage (INT(2x60/100)= INT(1.2)= 1), and the Muton only takes an average of 0.33 damage, not 0.5. Usually this effect is insignificant relative to the large damage values found in the game, but it could matter in testing situations involving very low damage strengths. It also means you need to take into account all possible values versus individual truncation effects, if you want an ultra-precise listing of e.g. how often armor is breached.

Penetrating Damage vs Armor Damage, Health Damage and Fatal Wounds

Penetrating Damage is simply the final, modified damage (modified by all susceptibility and vulnerability factors), minus the current armor level on the relevant facing of the target.

When there is Penetrating Damage (i.e., damage is >0 and not all blocked by armor), Health and Armor Damage occurs, i.e. Health and Armor levels are reduced.

Health Damage - the reduction in the target's current Health - is simply equal to Penetrating Damage. Health is allowed to go to negative values (negative values are only really relevant to Stun damage - otherwise, dead is dead).

Health Damage, in turn, dictates the probability of suffering Fatal Wounds (normally just for XCom soldiers):

• Between 1 to 10 points of Health Damage, the chance of Fatal Wounds increases more or less linearly from 9% to 90%
• There is 100% chance of Fatal Wounds if taking 11 or more points of Health Damage
• Whenever a target affected by Fatal Wounds, there is always an equal chance of 1, 2 or 3 Fatal Wounds, regardless of Health damage sustained.

For more information on Health Damage versus Fatal Wounds, see Fatal_Wounds#Probability_of_Fatal_Wounds.

Armor Damage - the amount that the target's current Armor level is reduced by, on the impacted facing - is equal to one tenth of Penetrating Damage, rounded up (to a maximum of the current Armor Level, since it can't go below zero). This means that an opponent who is almost invulnerable to your weapons, will slowly become more vulnerable, with every successful penetrating hit.

Note that Stun/Freeze damage, and Fire damage, do not reduce Armour level, and do not cause Fatal Wounds.

Relationship Between Health and Armor Damage

For situations where Penetrating Damage is less than the current Armor level, Health and Armor Damage have a very simple relationship to each other:

 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. Stated another way, you will never have armor damage if you don't have health damage (i.e., when there is insufficient damage to breach the armor).

You will see this health/armor 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 penetrating damage point each. The unit 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 damage fails to penetrate. 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