5e SRD:Attack Rolls
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When you make an attack, your attack roll determines whether the attack hits or misses. To make an attack roll, roll a d20 and add the appropriate modifiers. If the total of the roll plus modifiers equals or exceeds the target's Armor Class (AC), the attack hits. The AC of a character is determined at character creation, whereas the AC of a monster is in its stat block.
Modifiers to the Roll
When a character makes an attack roll, the two most common modifiers to the roll are an ability modifier and the character's proficiency bonus. When a monster makes an attack roll, it uses whatever modifier is provided in its stat block.
Ability Modifier. The ability modifier used for a melee weapon attack is Strength, and the ability modifier used for a ranged weapon attack is Dexterity. Weapons that have the finesse or thrown property break this rule.
Some spells also require an attack roll. The ability modifier used for a spell attack depends on the spellcasting ability of the spellcaster.
Proficiency Bonus. You add your proficiency bonus to your attack roll when you attack using a weapon with which you have proficiency, as well as when you attack with a spell.
Rolling 1 or 20
Sometimes fate blesses or curses a combatant, causing the novice to hit and the veteran to miss.
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target's AC. This is called a critical hit.
If the d20 roll for an attack is a 1, the attack misses regardless of any modifiers or the target's AC.