Dice Mechanics Explained: D20, Advantage, and Why Your Dice Are (Probably) Fair
D&D players swear by their lucky D20. But is there actually such a thing as a 'loaded' die? Here's the math behind dice fairness and the probability of rolling a natural 20.
The Math Behind Your D20
A fair D20 has an equal 5% chance of landing on any face. That means:
- Chance of rolling a natural 20: 5% (1 in 20)
- Chance of rolling a natural 1: 5% (1 in 20)
- Chance of rolling 10 or higher: 55% (11 in 20)
- Chance of rolling 15 or higher: 30% (6 in 20)
Advantage vs. Disadvantage
| Mechanic | Average Roll | Chance of 15+ | Chance of Nat 20 |
|---|---|---|---|
| Normal | 10.5 | 30% | 5% |
| Advantage | 13.8 | 51% | 9.75% |
| Disadvantage | 7.2 | 9% | 0.25% |
With advantage, your chance of rolling 15+ nearly doubles. With disadvantage, a natural 20 is a 1-in-400 event.
Common Dice Types
D4 (4 sides) — Damage for daggers, small weapons
D6 (6 sides) — Standard die, damage for shortswords
D8 (8 sides) — Damage for longswords (one-handed)
D10 (10 sides) — Percentile dice, damage for versatile weapons
D12 (12 sides) — Damage for greataxes
D20 (20 sides) — Attack rolls, saving throws, ability checks
Step-by-Step: Roll Virtual Dice
- Open the dice roller
- Select your die type (D4, D6, D8, D10, D12, D20)
- Click to roll — the result appears with color coding (green for max, red for min)
- Roll history is tracked below
Try it now: Roll Virtual Dice →
D4 through D20. History tracking. Mobile friendly.
Frequently Asked Questions
What is a D20 in D&D?
A D20 is a 20-sided die used in Dungeons & Dragons for attack rolls, saving throws, and ability checks. A natural 20 (rolling exactly 20) is a critical hit — an automatic success with bonus damage.
What is 'advantage' in D&D?
Advantage means rolling two D20s and taking the higher result. Statistically, advantage increases your average roll from 10.5 to 13.8 — equivalent to a +3.3 bonus. It's one of the most powerful mechanics in the game.
Are dice truly random?
Casino dice are precision-machined to within 0.0005 inches for fairness. Cheap plastic dice can have air bubbles or uneven edges that bias results. A 'fair' die should show each face within 2-3% of the expected 16.7% for a D6.