RPG Dice Probability Calculator
Calculate average rolls, min/max values, and success probability for any combination of RPG dice with modifiers and a target number.
Results
What is it?
The RPG Dice Probability Calculator models the uniform distribution of dice rolls. For multiple dice (e.g. 2d6), the distribution becomes bell-shaped � middle values are more likely. The success probability assumes a uniform distribution across the min-to-max range, which is an approximation for multiple dice but exact for a single die.
How to use
Select the number of dice and their type (d4 through d100). Enter any flat modifier (ability score modifier, proficiency bonus, etc.) and the target number you need to meet or beat. The calculator shows average roll, success probability, and the possible range.
Example scenario
D&D 5e: Rolling 1d20 + 5 (modifier) against DC 15: average roll = 15.5, minimum = 6, maximum = 25, success probability = 55%. Rolling 2d6 for damage against an armour class you need 14 to hit: average = 7, range 2-12.
Pro tip
In D&D 5e, Advantage (roll 2d20 take highest) raises effective average from 10.5 to 13.8. Disadvantage drops it to 7.2. Each +1 bonus on a d20 adds exactly 5% success chance against any fixed DC. Proficiency bonus adds +2 to +6 depending on level � always apply it to proficient skill checks.