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D&D Roll Odds Calculator

Find your odds to hit a DC with advantage or disadvantage.

Result

30.00%
51.00%
9.00%

How it is calculated

The tool gives your chance of meeting or beating a Difficulty Class (DC) on a twenty-sided die. For a single roll, the faces that succeed are DC to 20, so p = (21 − DC) / 20. With advantage you roll twice and keep the higher result, so you only fail if both dice fail: 1 − (1 − p)². With disadvantage you keep the lower die, so both must succeed: p². The DC is cut to a whole number and any DC below 1 is treated as 1. A DC above 20 shows 0% for all three, since a bare d20 cannot reach it. The tool does not add your modifier: subtract your total bonus from the DC you are facing and enter that number instead.

Worked example

You need a 15 or higher on a d20, so DC = 15. The successful faces are 15, 16, 17, 18, 19 and 20, which is 6 out of 20, so p = 0.30 and a normal roll succeeds 30.00% of the time. With advantage you fail only when both dice fail: 0.70 × 0.70 = 0.49, so the chance of success is 1 − 0.49 = 51.00%. With disadvantage both dice must succeed: 0.30 × 0.30 = 0.09, or 9.00%. Advantage raises your chance from 30% to 51%.

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Common mistakes

  • Entering the DC without subtracting your modifier. With a +5 bonus against DC 20, enter 15, because you only need to roll a 15.
  • Assuming advantage adds a flat +5. Its effect is largest in the middle of the range (from 30% to 51% here) and shrinks towards very easy or very hard checks.
  • Treating advantage and disadvantage as equal and opposite. At 30% advantage gives 51% but disadvantage gives 9%, a drop of 21 points against a gain of 21, yet at 95% advantage adds only about 5 points while disadvantage removes about 10.
  • Expecting a natural 20 to always succeed. This tool only counts faces, and a DC above 20 shows 0%.

About this calculator

This calculator finds the probability of meeting or beating a Difficulty Class (DC) when rolling a d20 in Dungeons & Dragons 5th edition, under three roll types: a normal single roll, rolling with advantage (roll two d20s, keep the higher), and rolling with disadvantage (roll two d20s, keep the lower). Advantage and disadvantage don't shift the odds linearly — they skew the probability distribution toward high or low results, with the biggest swing happening for DCs near the middle of the range.

Players and Dungeon Masters use it to understand how big an edge advantage really gives (often described loosely as "+5", though the actual bonus varies by DC) and to balance encounters and skill checks fairly.

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Comments (1)

  • Editorial team

    Advantage is worth the most near the middle of the range. Needing 11 or higher (DC 11, 50%), advantage lifts you to 75%, while at DC 3 (90%) it only reaches 99%. Disadvantage at DC 11 drops you to 25%. Subtract your modifier from the DC before entering it. Because the tool only counts faces of one d20, it cannot show rerolls or extra dice. Which rule or edge case from your table would you like to see covered?

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