Damage
Overview
Damage in XCOM comes in two major ways, and then there are some oddballs:
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 unaffected unless the damage is equal to or greater than its armor strength, at which point it will be breached or destroyed. (Some terrain objects will initially turn into a "damaged" object which itself can usually be destroyed. See Destroying Terrain for more details.)
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.
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, 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.
- 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%
Tanks 0%
Melee:
Tanks 90%
Celatid Acid Spit:
Tanks 40%
Susceptible to...
<trstyle="background: #c0c0c0; color:white">| Ammo Type | Damage Modifier Category | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| T+I | Hu+ | PA | 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 | 0 |
| Incendiary | 100 | 100 | 0 | 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 | 100 | 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 |
| 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: Personal Armor has 90% susceptibility to stun, while Power/Flying Suits have 80% to stun. (Less than 100% is better.)
- Incendiary: Personal, Power, and Flying Armor are completely resistant to Incendiary.
For another way to view the same info above, see this.
See also: TFTD Damage Modifiers
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.
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. 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.