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Roll any number of dice with standard RPG dice types (d4, d6, d8, d10, d12, d20, d100). Add a modifier for tabletop games.
Dice Notation:
NdS+M = Roll N dice with S sides, add modifier M
Example: 2d6+3 = Roll 2 six-sided dice, add 3
Each die result is uniformly random from 1 to S.
A d20 is a 20-sided die, the most iconic die in tabletop role-playing games like Dungeons and Dragons. It is used for attack rolls, saving throws, and ability checks.
A standard RPG dice set contains 7 dice: d4, d6, d8, d10, d10 (percentile), d12, and d20. The two d10s are rolled together to generate numbers from 1 to 100 (d100).
A modifier is a fixed number added to or subtracted from a dice roll. In D&D, your ability scores provide modifiers. For example, a Strength of 16 gives a +3 modifier to strength-based rolls.
The average (expected value) of a single die is (sides + 1) / 2. For a d6: (6+1)/2 = 3.5. For 2d6: 7. For a d20: 10.5. The average roll increases linearly with more dice.
A critical hit occurs when you roll a natural 20 on an attack roll d20. The attack automatically hits and you roll the damage dice twice, adding both results (plus modifiers).
The probability is 6/20 = 30%. There are 6 favorable outcomes (15, 16, 17, 18, 19, 20) out of 20 equally likely results.