About this tool
The D20 Advantage Calculator gives the probability that a twenty-sided die roll plus your modifier meets or beats a target — an armour class to hit, or a difficulty class to pass a check or saving throw. It handles Normal, Advantage and Disadvantage and runs entirely in your browser.
The base single-roll chance is p = clamp((21 − (target − modifier)) / 20, 0.05, 0.95): you succeed on any face at or above (target − modifier), and the clamp encodes the rule that a natural 1 always fails and a natural 20 always succeeds, so the chance never leaves the 5%–95% range. Advantage rolls two dice and keeps the best, giving 1 − (1 − p)²; Disadvantage keeps the worst, giving p². The expected d20 face is 10.5 normally, 13.825 with advantage and 7.175 with disadvantage, and the natural-20 crit chance is 5%, 9.75% or 0.25% respectively.
Enter the target number, your total modifier and the roll mode to see the hit chance, the raw die face you need, the expected roll and the crit chance update instantly. For a target of 11 over your modifier the normal chance is 50%, rising to 75% with advantage.