Critical Hits

When you make an attack roll and get a natural 20 (the d20 shows 20),
you hit regardless of your target’s Armor Class, and you have scored a
threat. The hit might be a critical hit (or “crit”). To find out if it’s a critical
hit, you immediately make a critical roll—another attack roll with all the
same modifiers as the attack roll you just made. If the critical roll also
results in a hit against the target’s AC, your original hit is a critical hit.
(The critical roll just needs to hit to give you a crit. It doesn’t need to
come up 20 again.) If the critical roll is a miss, then your hit is just a
regular hit.
A critical hit means that you roll your damage more than once, with all
your usual bonuses, and add the rolls together. Unless otherwise
specified, the threat range for a critical hit on an attack roll is 20, and the
multiplier is ×2.
Exception: Extra damage over and above a weapon’s normal damage,
such as that dealt by a sneak attack or the special ability of a flaming
sword, is not multiplied when you score a critical hit.
Increased Threat Range: Sometimes your threat range is greater than
20. That is, you can score a threat on a lower number. Longswords, for
instance, give you a thread on a natural attack roll of 19 or 20. In such
cases, a roll of lower than 20 is not an automatic hit. Any attack roll that
doesn’t result in a hit is not a threat.
Increased Critical Multiplier: Some weapons, such as battleaxes and
arrows, deal better than double damage on a critical hit. See Table 7–5:
Weapons (page 116) and the Critical section of Weapon Qualities (page
114).
Spells and Critical Hits: A spell that requires an attack roll, such as
shocking grasp or Melf’s acid arrow, can score a critical hit. A spell attack
that requires no attack roll, such as lightning bolt, cannot score a critical
hit.