Roll D20
One twenty-sided die. Every face from 1 to 20 carries exactly the same 5% chance, which is why the d20 became the standard die for checks in tabletop roleplaying games.
1d20 1
Press roll.
Lowest possible 1, highest 20, average 10.5.
Every face is 5%
A d20 is flat. Each number from 1 to 20 comes up one time in twenty, which is 5% exactly. That is the whole appeal: you can do the maths in your head. A natural 20 is 5%. So is a natural 1.
The chance of meeting or beating a target number is simply five times the number of faces that qualify.
| Need | Chance | With advantage |
|---|---|---|
| 10 or more | 55% | 79.75% |
| 15 or more | 30% | 51% |
| 20 | 5% | 9.75% |
Advantage and disadvantage
Rolling twice and keeping the higher raises the average from 10.5 to 13.825. Keeping the lower drops it to 7.175. Advantage is worth roughly plus three, and it is worth most in the middle of the range, where it adds 21 percentage points to a target of 15.
No die is ever due
In twenty rolls the chance of seeing at least one 20 is 64%. That leaves 36% of twenty-roll stretches with no 20 at all, which is common rather than cursed. Repeats arrive quickly too: across just five rolls, the odds that some number shows up twice are 41.9%.
One more thing worth knowing. The average roll of 10.5 is a number the die cannot show, so ten faces sit below average and ten above, with nothing in the middle.
For the same flat spread without dice, use a random number from 1 to 20. For a flatter, smaller die see the d12, or set any count and any sides in the dice roller.