D20 Dice Roller
Twenty triangular faces on an icosahedron, and the die everyone pictures when someone says polyhedral dice. It is the emblem on the shirts, the keychains and the shop signs. The reason is simple: in most modern role-playing games almost every meaningful action starts with a d20 hitting the table.
Press Roll to throw the dice.
Press Space to roll again. Type notation like 2d6+3 or 4d6dl1 straight into the box.
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A d20 has 20 faces numbered 1–20. On a fair die every face is equally likely, so any single result comes up 5% of the time. Rolls run from 1 to 20, the average is 10.5, and the standard deviation is 5.77. The shape it is built from is the icosahedron.
- Minimum
- 1
- Maximum
- 20
- Mean
- 10.5
- Median
- 10.5
- Std deviation
- 5.77
- Chance per face
- 5%
d20 probability
One die is flat. There is no curve and no middle to aim for, because each face has exactly one way to come up out of 20 possible results, so nothing is more likely than anything else. Add a second die and that changes immediately: totals start piling up in the middle. The dice probability guide works through why.
| Total | Ways | Probability | This or lower |
|---|---|---|---|
| 1 | 1 | 5% | 5% |
| 2 | 1 | 5% | 10% |
| 3 | 1 | 5% | 15% |
| 4 | 1 | 5% | 20% |
| 5 | 1 | 5% | 25% |
| 6 | 1 | 5% | 30% |
| 7 | 1 | 5% | 35% |
| 8 | 1 | 5% | 40% |
| 9 | 1 | 5% | 45% |
| 10 | 1 | 5% | 50% |
| 11 | 1 | 5% | 55% |
| 12 | 1 | 5% | 60% |
| 13 | 1 | 5% | 65% |
| 14 | 1 | 5% | 70% |
| 15 | 1 | 5% | 75% |
| 16 | 1 | 5% | 80% |
| 17 | 1 | 5% | 85% |
| 18 | 1 | 5% | 90% |
| 19 | 1 | 5% | 95% |
| 20 | 1 | 5% | 100% |
When you'd roll a d20
You roll a d20 when the question is whether you succeed, not how much. Add your bonus, compare against a target number, and the result decides it. Because the spread is flat, a skilled character can still miss badly and a weak one can still land something impossible, which is most of the drama at the table. Twenty numbered faces, one moment of held breath.
Common uses
- attack rolls in D&D 5e
- saving throws and ability checks
- advantage and disadvantage rolls
- Contested rolls where two characters each roll and compare totals
- Initiative at the top of combat, before anyone acts
- Death saving throws and other pass-or-fail checks with no modifier attached
d20 questions
What is a natural 20?
It means the die itself shows 20 before any bonuses are added. On an attack roll that is a critical hit, which normally means rolling the damage dice twice. A natural 1 is the opposite: an automatic miss regardless of how high your bonus is.
How does advantage work?
You roll two d20s and use the higher one. Disadvantage is the same roll but you take the lower. Only one die counts, so the bonus applies once. It is a quick way of showing favourable or awkward circumstances without stacking extra arithmetic on the roll.
What is the d20 System?
It is the ruleset built around resolving actions with a single twenty-sided die, published for third edition D&D and released under the Open Game License in 2000. That licence let other publishers build compatible games, which is why so many titles share the same core roll.
Are d20s fair?
A well made one is. Cheap moulded dice can carry uneven flashing or air bubbles from casting, which nudges the balance. You can float a die in salt water and spin it: if the same faces keep surfacing, the die has a bias worth retiring.