D&D Advantage Math
Exact chances to roll at least a value on a d20—normal, advantage, or disadvantage. For planning checks, not every table rule.
Example: flat DC 15 ability check with +0 modifier → you need 15+ on the d20.
- Normal d20: 30.0%
- Advantage: 51.0%
- Disadvantage: 9.0%
Full table — P(roll ≥ k) on the d20
| Need ≥ | Normal | Adv | Dis |
|---|---|---|---|
| 1 | 100.0% | 100.0% | 100.0% |
| 2 | 95.0% | 99.8% | 90.3% |
| 3 | 90.0% | 99.0% | 81.0% |
| 4 | 85.0% | 97.8% | 72.2% |
| 5 | 80.0% | 96.0% | 64.0% |
| 6 | 75.0% | 93.8% | 56.3% |
| 7 | 70.0% | 91.0% | 49.0% |
| 8 | 65.0% | 87.8% | 42.3% |
| 9 | 60.0% | 84.0% | 36.0% |
| 10 | 55.0% | 79.8% | 30.3% |
| 11 | 50.0% | 75.0% | 25.0% |
| 12 | 45.0% | 69.8% | 20.3% |
| 13 | 40.0% | 64.0% | 16.0% |
| 14 | 35.0% | 57.7% | 12.2% |
| 15 | 30.0% | 51.0% | 9.0% |
| 16 | 25.0% | 43.8% | 6.3% |
| 17 | 20.0% | 36.0% | 4.0% |
| 18 | 15.0% | 27.8% | 2.3% |
| 19 | 10.0% | 19.0% | 1.0% |
| 20 | 5.0% | 9.8% | 0.3% |
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Related on the blog
- Advantage, Stat Rolls, and Table Odds — Without the Hype
What advantage actually does to your d20 odds, how 4d6-drop-low spreads ability scores, and where our free tools fit—exact math where we can, clear limits everywhere else.
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