The rule
When something gives you advantage, roll two d20s and use the higher. With disadvantage, use the lower. If you have both, they cancel and you roll one die, no matter how many sources of each you have. Several sources of advantage still only give you two dice.
What it is worth
Advantage matters most when you need a middling roll. The chance of rolling at least each number on the die, with one d20, with advantage (Adv.) and with disadvantage (Dis.):
| Need | 1d20 | Adv. | Dis. |
|---|---|---|---|
| 2 | 95% | 99.75% | 90.25% |
| 4 | 85% | 97.75% | 72.25% |
| 6 | 75% | 93.75% | 56.25% |
| 8 | 65% | 87.75% | 42.25% |
| 10 | 55% | 79.75% | 30.25% |
| 11 | 50% | 75% | 25% |
| 12 | 45% | 69.75% | 20.25% |
| 14 | 35% | 57.75% | 12.25% |
| 16 | 25% | 43.75% | 6.25% |
| 18 | 15% | 27.75% | 2.25% |
| 20 | 5% | 9.75% | 0.25% |
Needing an 11, advantage is worth exactly +5. Needing a 2 or a 20 it is worth less than +1, which is why it pays to hunt for it on the rolls you are most likely to fail, and why it barely matters on an easy one. Disadvantage costs the same amounts the other way. The d20 roller has the one-die odds and the dice calculator works out any other roll.
Where advantage comes from
In the SRD 5.1 rules, some common sources:
- Unseen attackers. Attacking a creature that cannot see you gives you advantage; attacking one you cannot see gives you disadvantage.
- Prone targets. Attacks from within 5 feet of a prone creature have advantage; attacks from farther away have disadvantage.
- The Help action. A friend helping you gives advantage on your next check for that task, or on your next attack against a creature within 5 feet of them.
- Class features. A barbarian's Reckless Attack gives advantage on Strength melee attacks for the turn, at the price of attacks against the barbarian having advantage too.
- Conditions. Blinded, frightened, poisoned, prone and restrained creatures roll some things with disadvantage.
Critical hits
With advantage the chance of a natural 20 nearly doubles, to 9.75%. With disadvantage the chance of a natural 1 does the same. The full story, with the maths drawn out, is in Advantage and Disadvantage in D&D.
Questions
What does rolling with advantage mean?
Roll two d20s and use the higher one, then add your modifier as usual. Disadvantage is the same with the lower one. It applies to attack rolls, ability checks and saving throws, never to damage.
How much is advantage worth?
About +5 when you need around an 11 on the die, and less towards either end: +3.75 needing a 6 or a 16, under +1 needing a 2 or a 20. Averaged over the die it adds 3.3 to the number you keep (13.83 against 10.5).
What are the odds of a nat 20 with advantage?
9.75%, nearly double the 5% of one die. A natural 1 with advantage needs both dice to show 1: 1 in 400 (0.25%). With disadvantage it is the other way round.
Do advantage and disadvantage stack?
No. However many sources you have, you roll at most two d20s. If you have advantage and disadvantage at the same time, they cancel and you roll one die, even if one side has more sources than the other.
What is 2d20 keep highest?
Dice notation for advantage: roll 2d20kh1 (keep the highest one). Disadvantage is 2d20kl1. The dice generator takes both, or use the roller above.
Is this advantage roller fair?
Yes. Both d20s come from your browser's cryptographic random number generator, with no bias towards any face, and both dice are shown, so the table sees which one you kept.