A character's Defense represents how hard it is for opponents to land a solid, damaging blow on the character. It's the attack roll result that an opponent needs to achieve to hit the character. The average, unarmored civilian has a Defense of 10. A character's Defense is equal to:
10 + Dexterity modifier + class bonus + equipment bonus + size modifier
Dexterity Modifier
If a character's Dexterity is high, he or she is particularly adept at dodging blows or gunfire. If a character's Dexterity is low, he or she is particularly inept at it. Characters apply their Dexterity modifier to Defense.
Sometimes a character can't use his or her Dexterity bonus. If a character can't react to a blow, that character can't use his or her Dexterity bonus to Defense.
Class Bonus
A character's class and level grant the character an innate bonus to Defense. This bonus applies in all situations, even when the character is flat-footed or when the character would lose his or her Dexterity bonus for some other reason.
Equipment Bonus
If a character wears armor, it provides a bonus to his or her Defense. This bonus represents the armor's ability to protect the character from blows.
Armor provides a minimum bonus to anyone who wears it, but a character who is proficient in the use of a certain type of armor receives a larger bonus to Defense.
Sometimes a character can't use his or her equipment bonus to Defense. If an attack will damage the character just by touching him or her, that character can't add his or her equipment bonus (see Touch Attacks, below).
Size Modifier
The bigger an opponent is, the easier it is to hit in combat. The smaller it is, the harder it is to hit. Since this same modifier applies to attack rolls a creature doesn't have a hard time attacking another creature of the same size. Size modifiers are shown on Table: Size Modifiers.
Other Modifiers
Other factors can add to a character's Defense.
Feats: Some feats give a bonus to a character's Defense.
Natural Armor: Some creatures have natural armor, which usually consists of scales, fur, or layers of thick muscle.
Dodge Bonuses: Some other Defense bonuses represent actively avoiding blows. These bonuses are called dodge bo-nus-es. Any situation that denies a character his or her Dexterity bonus also denies his or her dodge bonuses. Unlike most sorts of bonuses, dodge bonuses stack with each other.
Magical Effects: Some campaigns may include magic. Some magical effects offer enhancement bonuses to armor (making it more effective) or deflection bonuses that ward off attacks.
Touch Attacks
Some attacks disregard armor. In these cases, the attacker makes a touch attack roll (either a ranged touch attack roll or a melee touch attack roll). The attacker makes his or her attack roll as normal, but a character's Defense does not include any equipment bonus or armor bonus. All other modifiers, such as class bonus, Dexterity modifier, and size modifier, apply normally.