Critical Hit: double the dice, then add modifiers

SRD 5.2.1 · Damage

At the table

1. A 20 on the attack’s d20 scores a Critical Hit and hits regardless of AC or modifiers.
2. Roll all of the attack’s damage dice twice and add those dice together.
3. Add the relevant modifiers afterward; read any additional feature separately.

Example decision

An attack normally deals 1d8 + 3 damage. It scores a Critical Hit, and no other feature changes its damage. What expression is rolled?

2d8 + 3.

The d8 is a damage die, so it is rolled twice. The fixed +3 is a modifier and is added after the dice; the critical rule does not double it. If the attack includes additional damage dice, the rule says all of the attack’s damage dice, so read those features when forming the complete expression.

Watch for this mistake

A natural 20 on an ability check is not an attack Critical Hit under this entry. Do not transfer the attack-specific damage procedure to every d20 roll.

Ask the table

Which parts of the attack’s damage are dice, and which are fixed modifiers?

Source rule

If you roll a 20 on the d20 for an attack roll, you score a Critical Hit, and the attack hits regardless of any modifiers or the target’s AC. A Critical Hit lets you roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers. See also “Playing the Game” (“Damage and Healing”).

Source: SRD 5.2.1, 179. Online guide.