Every face has a 1 in 20 chance — 5% each. Average roll: 10.5. With advantage the average rises to about 13.8.
The d20 is the die the whole hobby is named around. In Dungeons & Dragons and every game built on the d20 System it decides attack rolls, ability checks and saving throws — which is to say it decides whether anything happens at all.
It is an icosahedron: twenty equilateral triangles, the largest of the Platonic solids, and the closest thing to a sphere in the set. That is why it rolls the longest, and why it is the die most likely to end up on the floor.
A natural 20 means the die itself shows 20, before any modifiers. On an attack roll it automatically hits regardless of the target’s armour, and it scores a critical, which means you roll the attack’s damage dice twice. A natural 1 on an attack automatically misses no matter how large your bonus.
What surprises a lot of players is how narrow that rule is. By the rules as written, ability checks and saving throws have no automatic success or failure — a natural 20 on a Persuasion check is just 20 plus your modifier, and against a DC of 25 you still fail. Plenty of tables house-rule otherwise, which is fine, but it is a house rule rather than the printed one. The 2024 revision keeps criticals tied to attack rolls in the same way.
Advantage means rolling two d20s and taking the higher; disadvantage means taking the lower. Averaged across the whole range advantage is worth roughly +3.3 to your result — but that single number hides the interesting part.
The benefit is not constant. It is largest in the middle of the range and smallest at the edges. If you need an 11 or better, advantage takes you from 50% to 75% — a full 25 percentage points, worth more than any bonus you could realistically stack. If you need a 2, you were already succeeding 95% of the time and advantage barely moves you. The practical lesson is that advantage is most valuable on the rolls that are genuine coin flips, and nearly wasted on the ones you were going to make anyway.
A d20 is perfectly flat: a 3 is exactly as likely as a 17. That means the die contributes a spread of nineteen points to every roll, and against that, character bonuses look small. A +5 modifier — a serious investment for a mid-level character — shifts your success chance by 25 percentage points, which is a lot, and still leaves the die firmly in charge.
Compare a system like GURPS, which rolls 3d6. The same sort of range exists but results pile up around 10 and 11, so an expert almost always outperforms an amateur and upsets are genuinely rare. Neither approach is better; they are choices about how often the story should surprise you. The d20 is built to surprise you often.
It means the die shows 20 before modifiers. On an attack roll that is an automatic hit and a critical, so you roll the damage dice twice. On ability checks and saving throws the rules as written give it no special effect — many tables add one as a house rule, but a natural 20 does not automatically succeed at everything.
About +3.3 on average, but the real answer depends on the target number. On a roll you would make half the time, advantage takes you from 50% to 75% — a 25 point swing. On a roll you were already making 95% of the time it is nearly worthless. Advantage is at its best on genuine coin flips.
Yes. Set the dice count to 2 and roll both at once, then take the higher result for advantage or the lower for disadvantage. You can also type 2d20kh1 into the notation box and have the keep-highest done for you.
Seven dice come in the set and new players are told to buy them before anyone explains what they do. This is what each one is for.
The maths behind why some rolls feel reliable and others feel like coin flips — with the numbers you can actually use at the table.
A fair question with a real answer. Here is how digital rolls are generated, where they can go wrong, and why physical dice are not the gold standard people assume.
Last updated: August 20, 2026
