What Is a d20? Odds, Uses and History of the 20-Sided Die
A d20 is a 20-sided die shaped like an icosahedron, numbered 1 to 20. Each number has a 5% chance to come up, and the average roll is 10.5. In D&D 5e you roll it for attack rolls, ability checks and saving throws, adding a modifier and trying to meet or beat a target number.
What is a d20 in D&D?
The d20 is the twenty-sided die that drives Dungeons & Dragons. Almost every time your character tries something risky, such as swinging a sword, picking a lock or resisting a spell, you roll a d20, add a bonus and see whether the total is high enough. If you don't have one handy, you can roll a fair one in your browser with the online d20 roller.
The name follows the standard dice notation: "d" for die, "20" for the number of sides. You'll see it written as d20 or 1d20 in rulebooks, and 2d20 when you roll two at once.
What shape is a d20?
A d20 is an icosahedron, a solid made of 20 identical equilateral triangles. It has 12 corners (vertices) and 30 edges, and five triangles meet at every corner. It is one of the five Platonic solids, the only shapes in which every face is the same regular polygon and every corner looks the same. That symmetry is what lets each face have an equal chance of landing on top.
On most d20s the numbers on opposite faces add up to 21: the 20 sits opposite the 1, the 19 opposite the 2, and so on. This spreads high and low numbers around the die, so a small flaw in the shape is less likely to favor one end of the range.
What are the odds on a d20?
On a fair d20, every number from 1 to 20 has a 1 in 20 chance, which is 5%. The average roll is 10.5. Because each number is equally likely, the d20 has a flat, or uniform, distribution: needing a 15 or higher is exactly 30%, no matter how many times you have rolled before.
| You need (on the die) | Chance of success | Rough odds |
|---|---|---|
| 2 or higher | 95% | 19 in 20 |
| 5 or higher | 80% | 4 in 5 |
| 10 or higher | 55% | 11 in 20 |
| 11 or higher | 50% | 1 in 2 |
| 15 or higher | 30% | 3 in 10 |
| 20 only | 5% | 1 in 20 |
A quick rule: each +1 to your modifier adds 5 percentage points to your chance of success, until you hit the floor or ceiling. Over many rolls the numbers even out but streaks are normal. In 20 rolls, the chance of seeing at least one natural 20 is only about 64.15%. For more on that, see the odds of a natural 20.
When do you roll a d20 in 5e?
Fifth edition uses the d20 for three kinds of rolls. The 2024 rules group them under the name "D20 Tests".
- Attack rolls: roll d20 + your attack bonus. If the total equals or beats the target's Armor Class, you hit.
- Ability checks: roll d20 + the ability modifier (plus your proficiency bonus if you are proficient in the skill or tool). The DM sets a Difficulty Class, often 10 for easy, 15 for medium and 20 for hard. Initiative is a Dexterity check.
- Saving throws: roll d20 + the ability modifier (plus proficiency if your class grants it) to resist a spell, trap or poison, against the DC of the effect.
Damage and healing are never rolled with the d20; they use the smaller dice listed in our guide to D&D dice names.
Advantage and disadvantage
When something helps you, you have advantage: roll two d20s and use the higher number. When something hinders you, you have disadvantage and use the lower. Advantage raises your chance of a natural 20 from 5% to 9.75%, and disadvantage lowers it to 0.25%. Read advantage and disadvantage in 5e for the full odds.
What do a natural 1 and a natural 20 do?
A "natural" roll is the number on the die before any modifiers. The rules treat the extremes specially, but only in some cases:
| Roll type | Natural 20 | Natural 1 |
|---|---|---|
| Attack roll | Always hits; critical hit (roll the damage dice twice) | Always misses |
| Ability check | No automatic success in the official rules | No automatic failure in the official rules |
| Saving throw | No automatic success in the official rules | No automatic failure in the official rules |
| Death saving throw | You regain 1 hit point | Counts as two failures |
This works the same way in the 2014 and 2024 rules. Many tables add a house rule that a natural 20 on a skill check is a special success, but it isn't part of the core rules. The critical hits guide covers how crit damage is calculated.
Where did the d20 come from?
Ancient twenty-sided dice
The icosahedron is not a modern invention. Twenty-sided dice from ancient Egypt, dating to the Ptolemaic and Roman periods, are held in museum collections, some with Greek letters on their faces instead of numbers. Their exact use is not known.
The d20 in early role-playing games
Before role-playing games, most tabletop wargames used ordinary six-sided dice. The original Dungeons & Dragons, published in 1974, made polyhedral dice central to play, using the d20 for attacks and saving throws. Many early gaming d20s were numbered 0 to 9 twice, so players colored in one set of numbers to tell 1–10 from 11–20, or used them as a d10.
The d20 becomes the core mechanic
Early editions mixed many systems: percentile rolls for some skills, d6 rolls for others, and attack tables (later the THAC0 number) that worked in the opposite direction from today. Third edition, released in 2000, unified nearly everything into one rule: roll a d20, add modifiers, and meet or beat a target number. Fifth edition kept that core idea, and it is why the d20 is now the symbol of the hobby.
Is a d20 roller as fair as a real d20?
A well-made physical d20 is close to fair, but cheap dice can have air bubbles or uneven faces that tilt the odds slightly. An online roller has no physical flaws. Ours uses the browser's crypto.getRandomValues to pick each number, so every result from 1 to 20 is equally likely. Use whichever your table prefers.
FAQ
What are the odds of rolling a 20 on a d20?
Exactly 1 in 20, or 5%, on a fair die. With advantage, where you roll two d20s and keep the higher, the chance of at least one 20 rises to 9.75%.
What does d20 mean?
The d stands for die and 20 is the number of sides. So 1d20 means roll one twenty-sided die, and 2d20 means roll two of them.
What happens when you roll a natural 20 in D&D?
On an attack roll, a natural 20 always hits and is a critical hit, so you roll the damage dice twice. On ability checks and saving throws it has no automatic effect in the official rules, though many tables house-rule one.
What shape is a d20?
A d20 is an icosahedron: a solid with 20 identical equilateral triangle faces, 12 corners and 30 edges. It is one of the five Platonic solids.
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Contains content from the System Reference Document 5.1 and 5.2 by Wizards of the Coast LLC, used under the Creative Commons Attribution 4.0 International License.