Rollune
Advantage raises the average d20 from 10.5 to about 13.8 and is worth roughly +3 to +5 depending on the target number (most near a DC of 11). Your crit chance almost doubles, from 5% to 9.75%. Disadvantage drops the average to about 7.2 and makes hitting a DC 15 only a 9% chance.
How it works
With advantage, roll 2d20 and keep the higher die (2d20kh1). With disadvantage, keep the lower (2d20kl1). Then add your normal modifiers. You can try both in the d20 roller.
The math
If a single d20 succeeds with probability p:
- Advantage: 1 − (1 − p)²
- Disadvantage: p²
Chance to succeed by the number you need
| Need on the d20 | Normal | Advantage | Disadvantage |
|---|---|---|---|
| 2+ | 95% | 99.75% | 90.25% |
| 5+ | 80% | 96% | 64% |
| 8+ | 65% | 87.75% | 42.25% |
| 10+ | 55% | 79.75% | 30.25% |
| 11+ | 50% | 75% | 25% |
| 13+ | 40% | 64% | 16% |
| 15+ | 30% | 51% | 9% |
| 18+ | 15% | 27.75% | 2.25% |
| 20 | 5% | 9.75% | 0.25% |
"Need" means the number on the die after you subtract your modifier from the target. With +5 against AC 15 you need a 10+.
Average result
- One d20: 10.5
- Advantage: 13.83
- Disadvantage: 7.18
The online dice roller shows these exact averages for any pool and mode.
How much is advantage "worth"?
Advantage's value in percentage points is greatest when you need an 11: +25 points, equivalent to a +5 bonus. That's why the rules treat advantage as roughly +5 for passive checks. At the extremes the benefit shrinks: needing a 2, advantage adds under 5 points; needing a 20, about 5 points.
Takeaway: grab advantage for the rolls you're uncertain about. For near-automatic successes or long shots, it matters less.
Critical hits
A natural 20 on an attack is a critical hit. With advantage the chance nearly doubles, from 5% to 9.75%, which is why advantage is so valuable for characters with big crit damage like rogues and paladins. See critical hits and fumbles.
Common sources
Advantage: attacking an unseen, prone (in melee), restrained or paralyzed target; help from an ally (the Help action); some class features and spells. Disadvantage: attacking while prone or poisoned, long-range shots, heavy armor stealth, being frightened of a visible source.
In the Rollune app you can save "Stealth with advantage" as a preset, with the mode and modifier already set.
