Advantage Disadvantage Roller

Keeping an extreme from two d20s

Advantage rolls two independent d20 values and keeps the higher; disadvantage keeps the lower. The result is therefore an order statistic, not an ordinary single roll. For a target t, advantage succeeds with probability 1 - ((t - 1)/20)² because both dice must be below t to fail. Disadvantage succeeds only when both dice meet t.

A target-15 comparison

At target 15, a normal d20 succeeds on six faces for 30%. With advantage, failure requires both dice to be 14 or lower, so success is 1 - (14/20)² = 51%. With disadvantage, both dice must be 15 or higher, giving (6/20)² = 9%. These results explain why keeping one of two faces has a much larger effect near a meaningful threshold than a small flat modifier.

Read both faces when exceptions matter

If the two rolls are 4 and 17, advantage keeps 17 while disadvantage keeps 4. Some game systems attach special consequences to a natural extreme before or after the kept result is chosen. This tool supplies the two independent faces and the kept value; the governing rules decide whether an unkept 20, an unkept 1, or a tie condition has any further effect.

Not a replacement for every modifier

Advantage and disadvantage are distinct from rolling extra dice and adding them, from rerolling once, or from adding a fixed bonus. They also may cancel under a game's own rule instead of stacking. Use this result only where the rule explicitly asks for two d20s with one extreme retained.

Why target location matters

The benefit of retaining the higher face depends on the target. At a target of 2, a normal d20 already succeeds 95% of the time, so advantage has little room to help. At a target of 20, normal success is 5% and advantage is 9.75%. Use the actual threshold when judging the impact of the mechanic. A verbal difficulty label cannot substitute for that numeric threshold.

Questions about this dice rule

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