The D20 System
The d20 is the most iconic die in tabletop RPGs, forming the core mechanic of D&D, Pathfinder, and similar systems. Most important checks use d20 + modifiers vs a Difficulty Class (DC) or Armor Class (AC). Each number 1-20 has an equal 5% chance.
The flat distribution creates high variance, making each roll feel impactful. Unlike dice-pool systems with bell curves, d20 systems have equal probability for every outcome, meaning luck plays a significant role alongside character skill.
Core Mechanics
- Attack Roll: d20 + attack mod vs AC
- Saving Throw: d20 + save mod vs DC
- Ability Check: d20 + ability mod + proficiency vs DC
- Natural 20: Auto-hit, critical on attacks
Advantage & Disadvantage
D&D 5e's advantage/disadvantage: roll 2d20, take higher (advantage) or lower (disadvantage). Advantage adds ~+3.33 to average; disadvantage subtracts ~3.33. They do not stack.
Probability Table
| DC | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 5 | 80% | 96% | 64% |
| 10 | 55% | 80% | 30% |
| 15 | 30% | 51% | 9% |
| 20 | 5% | 9.75% | 0.25% |
Frequently Asked Questions
What is a natural 20?
The die shows 20 before modifiers. On attacks it is always a hit and critical hit (double damage dice). Many groups treat nat 20 ability checks as automatic successes.
Do advantage/disadvantage stack?
No. Any number of advantage sources gives advantage; one source of each cancels to a normal roll.
Odds of consecutive nat 20s?
(1/20)×(1/20) = 1/400 = 0.25%. Rare but memorable!