Up to 100 at a time.
A flat 5% on every face
A d20 has twenty faces, numbered 1 to 20, and each one comes up with probability 1/20, which is 5%. Nothing is weighted toward the middle, so a 20 is exactly as likely as a 10 and the average roll is 10.5. That flat shape is why a single d20 feels swingy next to dice that are added together.
Many tabletop systems give the two ends of the die special meaning, usually a critical hit on a natural 20 and an automatic miss on a natural 1. However your table rules them, each has a 5% chance on any one roll, and a natural 20 does not become more likely after a run of low rolls.
Advantage and disadvantage
Rolling with advantage means rolling two d20s and keeping the higher. Disadvantage keeps the lower. Set “How many dice” to 2 above and the result shows the highest and lowest next to the total, so either one can be read off directly.
With advantage, the chance that at least one of the two dice shows a 20 is 1 − (19/20)² = 39/400, or 9.75%. That is close to double the single-roll 5% but short of 10%, because the 1 roll in 400 where both dice land on 20 only counts once. Disadvantage mirrors it: a natural 1 becomes a 9.75% event, and a natural 20 needs both dice, which happens 0.25% of the time. The average kept result moves from 10.5 to 13.825 with advantage and down to 7.175 with disadvantage.
| Need at least | One d20 | Advantage | Disadvantage |
|---|---|---|---|
| 5 | 80% | 96% | 64% |
| 10 | 55% | 79.75% | 30.25% |
| 11 | 50% | 75% | 25% |
| 15 | 30% | 51% | 9% |
| 18 | 15% | 27.75% | 2.25% |
| 20 | 5% | 9.75% | 0.25% |
The table shows where advantage matters. Needing 11 or more, a straight roll succeeds half the time, advantage lifts that to 75%, and disadvantage drops it to 25%: a swing of 25 points either way. Near the edges the effect shrinks. A target of 20 only goes from 5% to 9.75%, and a target of 5, already an 80% roll, rises to 96%.
More dice: 2d6 roller · d100 percentile roller · Dice roller with any number of sides
Where the randomness comes from
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