SRD:Attack Roll

Attack Roll
An attack roll represents your attempt to strike your opponent on your turn in a round. When you make an attack roll, you roll a d20 and add your attack bonus. (Other modifiers may also apply to this roll.) If your result equals or beats the target’s Armor Class, you hit and deal damage.

Automatic Misses and Hits
A natural 1 (the d20 comes up 1) on an attack roll is always a miss. A natural 20 (the d20 comes up 20) is always a hit. A natural 20 is also a threat—a possible critical hit.

Attack Bonus
Your attack bonus with a melee weapon is:


 * Base attack bonus + Strength modifier + size modifier

With a ranged weapon, your attack bonus is:


 * Base attack bonus + Dexterity modifier + size modifier + range penalty



Base Attack Bonus
A base attack bonus is an attack roll bonus derived from character class and level or creature type and Hit Dice (or combinations thereof). Base attack bonuses increase at different rates for different character classes and creature types. A second attack is gained when a base attack bonus reaches +6, a third with a base attack bonus of +11 or higher, and a fourth with a base attack bonus of +16 or higher. Base attack bonuses gained from different sources, such as when a character is a multiclass character, stack.

A character’s base attack bonus does not grant any more attacks after reaching +16, cannot be less than +0, and does not increase due to class levels after character level reaches 20th.

A minimum base attack bonus is required for certain feats.