Exact model
A natural 20 critical success on a d20 attack roll
- Probability:
- 5%
- Odds:
- 1 to 19
- dBan
- −12.8 dBan
One of the fair d20’s 20 equally likely results is a natural 20, which the 2024 D&D Basic Rules treat as a critical hit on an attack roll.
Calculation
p = 1 natural 20 / 20 results = 1/20
Assumptions
The d20 is fair and rolled once for an attack. Modifiers, rerolls, advantage, and disadvantage are excluded. Critical outcomes for other checks or other role-playing systems can follow different rules.