D20 roller
Rolls appear here
A twenty-sided die: each face has a one in twenty chance, five percent each. Type standard dice notation for anything else, 2d6+3, 1d100, 4d8.
How to roll a d20
The two numbers that carry the drama are 1 and 20, each with a five percent chance, and it is worth knowing that a natural 20 is not automatically a success outside attack rolls in most rules, that is a common table convention rather than what the rulebook says. On advantage and disadvantage: rolling twice and taking the higher is worth roughly a five-point bonus in the middle of the range and much less at the extremes, because the improvement depends on how likely you were to succeed already.
Questions
One in twenty, five percent, on every roll. Previous rolls have no effect.
Type it into the notation box: 1d20+5 rolls the die and adds five.
Roll 2d20 and take the higher of the two. It is worth about five points in the middle of the range, less at the extremes.
Yes. The draw uses rejection sampling over a cryptographic source, so no face is favoured, which is more than can be said for most physical dice.
Any standard notation works: 4d6, 1d100, 3d8+2.