According to PHB p. 194 you always score a critical hit on any attack roll.
Does this rule also apply to magical attacks, e.g. the inflict wounds spell?
Role-playing Games Stack Exchange is a question and answer site for gamemasters and players of tabletop, paper-and-pencil role-playing games. It only takes a minute to sign up.Sign up to join this community
Yes! Attack rolls for spells are attack rolls, per the page you cite (PHB p. 194 ), so spell attacks can score a critical hit.
Whether you're striking with a melee weapon, firing a weapon at range, or making an attack roll as part of a spell, an attack has a simple structure.
This is explained explicitly in the Sage Advice Compendium:
Can spell attacks score critical hits?
A spell attack can definitely score a critical hit. The rule on critical hits applies to attack rolls of any sort.
For a more detailed explanation, take for example, the spell fire bolt:
You hurl a mote of fire at a creature or object within range. Make a ranged spell attack against the target. On a hit, the target takes 1d10 fire damage.
Since this spell calls for an attack, all the rules for Making an Attack apply:
When you make an attack, your attack roll determines whether the attack hits or misses.
Some spells also require an attack roll. The ability modifier used for a spell attack depends on the spellcasting ability of the spellcaster, as explained in chapter 10.
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, which is explained later in this chapter.
When you score a critical hit, you get to roll extra dice for the attack's damage against the target. Roll all of the attack's damage dice twice and add them together.
So say our 1st level Wizard cast fire bolt at an enemy and rolls a 20 on the d20 roll. This would be a critical hit, meaning the attack automatically hits, regardless of AC, and the damage would be 2d10, instead of the 1d10 for a regular hit.
This is discussed at length in this question: If you roll a 1 on a saving throw against a damage spell, do you take extra damage? Korvin Starmast writes:
There is no specific rule in this edition for a d20 result of 20 or 1 on a saving throw -- except the specific rule for the death saving throw when a PC is at 0 HP. (See below; Basic Rules p. 76). Without a specific rule like that for fireball, the general rule for saving throws is how it works.
A missed save is a missed save. The damage for a save is per the spell description. Fireball's spell description does not call for boosted damage due to a given result on the saving throw. Damage is whatever comes up on the damage dice rolled; damage is halved if the save is made.
Finally, Hellsaint's answer to this question goes more in depth about why saving throw spells don't get critical failures: Should rolling a 1 on a spell saving throw double that spell's damage?