Advantage
Advantage and 
Disadvantage are a gameplay mechanic that can greatly affect the success of dice rolls. They can apply to Attack Rolls, Saving Throws, and Ability Checks, but not to Damage Rolls.
Advantage
When you roll with Advantage, you perform the roll twice, and use the higher result. It doesn't stack beyond 2 dice, regardless of how many sources of advantages you have.
Example: You roll two 
d20 for an Attack Roll, the results are 16 and 4. Your effective result is 16.
Advantage and Disadvantage cancels each other, and having multiple sources doesn't change this either. Even if you have three sources of Advantage, a single source of Disadvantage will cancel it.
Examples of situations that grant Advantage on attack rolls:
- Attacking an enemy that is under these conditions: Restrained, Prone, Sleeping, Entangled, Paralysed, Off balance, Enwebbed, Blinded.
- Attacking an enemy while being Hidden or invisible.
- Armor, Weapons, and Spells that grant advantage when attacking enemies of a specific Race.
Disadvantage
When you roll with Disadvantage, you perform the roll twice, and use the lower result. It doesn't stack beyond 2 dice, regardless of how many sources of disadvantages you have.
Example: You roll two d20 for an Attack Roll, because you have Disadvantage. The results are 16 and 4. Your effective result is 4.
Disadvantage and Advantage cancel each other, and having multiple sources doesn't change this either. Even if you have three sources of Disadvantage, a single source of Advantage will cancel it.
Examples of situations that grant Disadvantage on attack rolls:
- Trying to make a ranged attack against an enemy that is within 5ft and making you Threatened.
- Various spells and abilities that grant Disadvantage.
Math
The benefits of rolling with advantage (or the detriments of rolling with disadvantage) change depending on the target number you need on the 1d20 roll to succeed. The bonus from advantage can be as large as 24-25% when needing a 9, 10, 11, 12, or 13 on the 1d20 roll, and as small as 9% if one needs to roll a 19.
| Target on 1d20 | Normal Roll | Roll With Advantage | Roll With Disadvantage |
|---|---|---|---|
| 1 | 100% | 100% | 100% |
| 2 | 95% | 99.75% | 90.25% |
| 3 | 90% | 99% | 81% |
| 4 | 85% | 97.75% | 72.25% |
| 5 | 80% | 96% | 64% |
| 6 | 75% | 93.75% | 56.25% |
| 7 | 70% | 91% | 49% |
| 8 | 65% | 87.75 | 42.25 |
| 9 | 60% | 84% | 42.25% |
| 10 | 55% | 79.75 | 30.25 |
| 11 | 50% | 75% | 25% |
| 12 | 45% | 69.75% | 20.25% |
| 13 | 40% | 64% | 16% |
| 14 | 35% | 57.75% | 12.25% |
| 15 | 30% | 51% | 9% |
| 16 | 25% | 43.75% | 6.25% |
| 17 | 20% | 36% | 4% |
| 18 | 15% | 27.75% | 2.25% |
| 19 | 10% | 19% | 1% |
| 20 | 5% | 9.75% | 0.25% |
External Links
- The unexpected logic behind rolling multiple dice and picking the highest by Matt Parker
- Advantage and Disadvantage in D&D Next: The Math by The Online Dungeon Master (Michael Iachini)
- D&D 5e: Probabilities for Advantage and Disadvantage by Bob Carpenter
