D&D Roll Odds Calculator

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.

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%.