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d20 odds cheat sheet: what a DC really means
Chance a d20 roll plus modifier meets a target number. The first column is the number you need on the die itself (after adding your bonus). Advantage rolls two dice and keeps the higher; disadvantage keeps the lower. Computed directly: P(success) = (21 - needed) / 20.
| Need on die | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 5 | 80% | 96% | 64% |
| 7 | 70% | 91% | 49% |
| 9 | 60% | 84% | 36% |
| 11 | 50% | 75% | 25% |
| 13 | 40% | 64% | 16% |
| 15 | 30% | 51% | 9% |
| 17 | 20% | 36% | 4% |
| 19 | 10% | 19% | 1% |
Reading it as a GM
- Need 11 is a coin flip (50%). Advantage turns it into 75%.
- Advantage is worth the most near the middle (about +5 on the die) and the least at the extremes.
- A DC that needs a 15 fails 3 times in 5 for an untrained character. Use 10 to 12 for tasks you want to usually succeed.
- Rolling a 1 on a d20 happens 5% of the time: once every 20 rolls, which in a long fight is about once per session per player.
All figures are straightforward probability, no house rules. Want your own dice? See the gear guide.