About the twenty-sided die
The d20 is an icosahedron: twenty equilateral triangles, five meeting at each corner — the Platonic solid with the most faces. Opposite faces add up to 21, each number has a flat 5% chance, and the average roll is 10.5. It is the die that decides almost everything in Dungeons & Dragons and the many games built on it.
Natural 20s and natural 1s
A natural 20 means the die itself shows 20, before any modifier. In D&D 5th edition a natural 20 on an attack roll is a critical hit — it always hits, and you roll the damage dice twice — while a natural 1 always misses. Officially that applies to attack rolls only, but many tables extend it to ability checks and saving throws. This roller flags natural 20s and 1s on any single d20, including advantage and disadvantage rolls.
Advantage and disadvantage
With advantage you roll two d20s and keep the higher (2d20kh1, or type adv);
with disadvantage you keep the lower (2d20kl1, or dis). Advantage is worth
about +3.3 on average, but its real effect depends on the number you need — it helps most around a target of 11:
| Target (DC) | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 2 | 95% | 99.8% | 90.3% |
| 5 | 80% | 96% | 64% |
| 8 | 65% | 87.8% | 42.3% |
| 10 | 55% | 79.8% | 30.3% |
| 11 | 50% | 75% | 25% |
| 12 | 45% | 69.8% | 20.3% |
| 14 | 35% | 57.7% | 12.2% |
| 15 | 30% | 51% | 9% |
| 16 | 25% | 43.8% | 6.25% |
| 18 | 15% | 27.8% | 2.25% |
| 19 | 10% | 19% | 1% |
| 20 | 5% | 9.75% | 0.25% |
A natural 20 goes from 5% to 9.75% with advantage and falls to 0.25% with disadvantage (1 in 400 — both dice must show 20).
Rolling with a modifier
Most d20 rolls add a bonus: your ability modifier plus proficiency. Use the modifier buttons or type, for example, 1d20+5. To find your chance of success against a Difficulty Class, subtract your modifier from the DC and read the table above — needing a 15 with +5 is the same as needing a 10 on the die: 55%.
An ancient shape
Twenty-sided dice are far older than role-playing games: icosahedral dice survive from Ptolemaic and Roman Egypt, some with Greek letters on their faces. The modern gaming d20 arrived with Dungeons & Dragons in 1974.
Is my physical d20 fair?
Cheap resin dice can hide air bubbles or uneven edges that favour some faces. A common home test is to float the die in very salty water and spin it a few times: if the same number keeps floating to the top, it is weighted. A virtual d20 has no such problem — every face here is exactly 5%.