Star Wars: Knights of the Old Republic/Combat

Attack
Attacks with weapons cannot normally hit and subsequently damage an opponent unless an attack roll is equal to or greater than that opponent's defense. An attack roll is the sum of attack bonuses and a d20 roll. According to Feedback, a minimum d20 roll (1) is an automatic miss while a maximum d20 roll (20) is an automatic hit, so chance to hit has a minimum of 5% and a maximum of 95% regardless of whether or not the total attack roll would equal or exceed defense. Maximum 95% chance to hit is achieved when the sum of attack bonuses + 2 (the lowest d20 roll that is not an automatic miss) is equal to or greater than an opponent's defense.

According to Feedback, melee on ranged bonus of +10 is added to attack roll when a melee attacker attacks a ranged opponent. Conversely, Close Proximity Ranged Bonus of +10 is added to attack roll when a ranged attacker attacks any opponent in close proximity.

Base
All characters have a base attack bonus which increases as level increases. Progression varies according to class:

Attribute modifier
Strength modifier is added to attack roll when using melee weapons, or lightsabers when equal to or greater than Dexterity modifier.

Dexterity modifier is added to attack roll when using ranged weapons, or lightsabers when greater than Strength modifier.

Dual Wield Penalty
Using two weapons or a double-bladed weapon normally incurs an attack penalty, which depends on whether or not the attacker has any Two-Weapon Fighting feat and a balanced weapon (which adds a Small Offhand Bonus of +2 to the attacks of the main hand):

Feats
Numerous feats apply a bonus or penalty to the attack roll:

Effect bonus
Various weapons add an attack bonus to the attack roll, while the dark side power Slow applies a penalty of -2.

Critical threat
As well as resulting in an automatic hit, a maximum d20 roll of 20 results in an automatic critical threat. A critical threat is a second attack roll which can result in a critical hit applying a second damage roll when equal to or greater than an opponent's defense.

Some weapons have a higher critical threat range, increasing the chance of a roll within that range:

Lightsabers and Short Lightsabers all have base critical threat range 19-20 (10%), while Double-Bladed Lightsabers only have 20-20 (5%). The Keen bonus can increase base critical threat range, as can the Critical Strike and Sniper Shot feats. According to Feedback, any Keen bonus is additive with feat bonuses, resulting in the following:

Defense
Characters can only be hit and subsequently damaged by attacks with weapons when an attack roll is equal to or greater than defense. An attack roll is the sum of attack bonuses and a d20 roll. According to the Feedback, a minimum d20 roll (1) is an automatic miss while a maximum d20 roll (20) is an automatic hit, so chance to hit has a minimum of 5% and a maximum of 95% regardless of whether or not the total attack roll would equal or exceed defense. Minimum 5% chance to hit is achieved when defense is greater than an opponent's attack roll + 19 (the highest d20 roll that is not an automatic hit).

All characters have base defense 10, to which their Dexterity modifier is added. Maximum Dexterity bonus to defense is restricted when wearing armor: for example, an armor displaying Max Dexterity Bonus: +5 would restrict defense added by the Dexterity modifier to +5, even when it was greater than +5.

However, although the Descriptions for robes display Max Dexterity Bonus as well, according to Feedback this restriction does not actually apply. Even though all robes display Max Dexterity Bonus: +8, Dexterity modifiers greater than +8 are still added to defense in their entirety.

Many opponents also have a Natural bonus and an Armor bonus. From comparing Defense Bonus displayed in an armor's Description and the output of Feedback, this bonus is often split between Armor and feats and effects. The Defense Bonus of robes has no Armor bonus.

Feats and effects
Feedback sums bonuses and penalties from all of the following under feats and effects:

Feats and effects are restricted to a maximum defense bonus of +10. For example, a Jedi wearing a Jedi Master Robe (+3) and using Master Speed (+4) and Force Armor (+6) only receives a defense bonus of +10, not +13.

Feat
One set of feats is separate to feats and effects in Feedback:

Class
Some classes receive a defense bonus from feats they gain at certain levels:

Class bonuses are restricted to a maximum defense bonus of +10. This maximum can only be achieved by a main player character at level 18 with any Jedi class who was a level 6 Scoundrel, but it cannot normally be exceeded (since even a level 12 Scoundrel could only become a level 8 Jedi at most).

Debilitated bonus
Debilitated bonus results in -4 defense whenever a target is debilitated by any of the following:

Being debilitated also results in Dexterity modifier not being added to defense. Characters can avoid being debilitated with a successful save, Immunity: Mind-Affecting or Force Immunity.

Critical hit
Critical hits occur when the d20 roll of a successful attack is within the weapon's critical threat range and a second attack or threat roll is also successful. This results in a second damage roll being added to the first, which includes all but the following:
 * Constant damage item bonuses (for example, an Opila crystal's Damage Bonus: 3, Energy)
 * Sneak Attack damage

For example, the damage of a critical hit from a dark side Scoundrel Jedi Guardian's Master Force Jump Sneak Attack using a Lightsaber with the Sigil (Damage Bonus: 1-6, Energy) and Opila (Damage Bonus: 3, Energy and Massive Criticals: 2-12) crystals, Strength modifier +8, Weapon Specialization: Lightsaber and dark side mastery would be as follows:

Due to the need for two successful rolls, the Critical Strike and Sniper Shot feats are extremely dependent on chance to hit to maximize their damage. However, the second attack or threat roll has no minimum or maximum restriction so 0% and 100% chance are possible.

Taking chance to hit and critical threat range into account, the multipliers that critical hits apply to damage rolls are as follows:

For example, the total damage of a dark side Jedi Guardian's Attack using a Lightsaber (2-16) with the Sigil (1-6) crystal, Strength modifier +8, Weapon Specialization: Lightsaber (+2) and Dark Side mastery (1-8) is 14-40, or 27 on average: with Master Critical Strike quadrupling critical threat range from 19-20 (10%) to 13-20 (40%), 50% chance to hit results in average damage being multiplied by just 1.1 (0.5 * 0.4 * 0.5 = +0.1), to 29.7; 75% results in it being multiplied by 1.225, to 33.075; and 95% with 100% on the second roll results in it being multiplied by 1.38, to 37.26.

Damage Resistance
Damage is first reduced or negated by any shields, then the Force power Energy Resistance or Improved Energy Resistance. Since the latter resists the first 15 points of cold, electrical, energy, fire and sonic damage, and successful saves normally halve the damage of the grenades and mines that often apply those types of damage, the following bonuses become less useful as the game progresses.

Damage Immunity
Damage Immunity is normally excessive since a combination of high saves, Energy Resistance or Improved Energy Resistance, and Damage Resistance can already significantly reduce damage, or even negate it entirely.

Damage Immunity: 100% vs. Electrical may be useful against the Dark Side Force powers Shock, Force Lightning and Force Storm (although it does not apply to the Force point damage of the latter), and combining it with Immunity: Mind-Affecting removes the need for Will saves entirely.

Vitality

 * The red bar represents a character's health, measured in vitality points. Vitality points (VP) decrease when a character takes damage from an attack or other effect. If the total reaches zero, the character falls unconscious and is unable to move or fight until healed. If the entire party falls unconscious, the game will end. To continue, load a previously saved game or begin from the last AUTO SAVE point. As long as one character in the party remains conscious to guard the others, the game will continue.

Constitution modifier affects Vitality points per level, resulting in total Vitality from the table above being adjusted by modifier * level. The Toughness and Improved Toughness feats both add 1 vitality point per level, while Master Toughness and Wookie Toughness both add 2 vitality points per level.

Regeneration
NOTE: Unlike Force points, vitality points do not regenerate and must be healed through the use of Force powers, medpacs and other items.

While this is normally true, the Regeneration bonus allows it to do so. However, any bonus only applies once every six seconds (every two combat rounds):

Maximum Regeneration: +6 for Canderous, or just +1 per second. HK-47's Regeneration bonuses are cumulative, so after his fourth repair Regeneration: +2 is applied.