TABLETOP & RPG
Advantage Calculator
Calculate success odds for normal, advantage, and disadvantage rolls against any target number.
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Advantage and disadvantage guide
Advantage and disadvantage are the most iconic mechanic in tabletop RPGs — roll twice, keep the better or worse. But the odds are surprisingly unintuitive: advantage at a DC 15 is worth roughly +5 on the die, yet at DC 20 it is barely +2. This calculator shows the real success chances for normal, advantage, and disadvantage against any target number.
The math behind rolling twice
With advantage, you fail only if both dice fail. So the chance of success is 1 − (failure chance)². Against a DC of 15 on a d20, that is 1 − (14/20)² = 51% — versus 30% on a single roll. Disadvantage is the opposite: you succeed only if both dice succeed, so (6/20)² = 9%.
The equivalent flat bonus varies with the DC. Advantage is worth about +5 at mid-range DCs but shrinks toward the extremes — which is why it feels great on hard-but-not-impossible checks and nearly useless on trivial ones.
Design implications
If you are converting a tabletop system to digital, decide whether "roll twice" is worth the extra time vs a flat bonus. Advantage is more dramatic (players love rolling two dice), but a flat +5 is easier to compute and balance. Many digital adaptations quietly convert advantage to a flat bonus for speed.
Tip: For game feel, advantage is a "player moment" — the double roll is the fun. Keep it even if the math says a flat bonus is equivalent. Disadvantage, by contrast, reads as pure punishment; use it sparingly.
Frequently asked questions
How much is advantage worth on a d20?
Roughly +3 to +5 depending on the target number — most valuable at mid-range DCs (11–15), least valuable at the extremes.
How do I calculate advantage odds?
Success chance = 1 − ((DC−1)/20)². Disadvantage = ((21−DC)/20)². This calculator does both instantly.
Does advantage stack?
In most systems, no — it is a binary state. If multiple sources grant advantage, they just set it once. Some systems allow "double advantage" (roll 3, keep best); the math extends the same way: 1 − (failure)³.
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