SRD 5.1 rules text (the 2014 rules), under ability scores and checks.
Sometimes a special ability or spell tells you that you have advantage or disadvantage on an ability check, a saving throw, or an attack roll. When that happens, you roll a second d20 when you make the roll. Use the higher of the two rolls if you have advantage, and use the lower roll if you have disadvantage. For example, if you have disadvantage and roll a 17 and a 5, you use the 5. If you instead have advantage and roll those numbers, you use the 17.
If multiple situations affect a roll and each one grants advantage or imposes disadvantage on it, you don't roll more than one additional d20. If two favorable situations grant advantage, for example, you still roll only one additional d20.
If circumstances cause a roll to have both advantage and disadvantage, you are considered to have neither of them, and you roll one d20. This is true even if multiple circumstances impose disadvantage and only one grants advantage or vice versa. In such a situation, you have neither advantage nor disadvantage.
When you have advantage or disadvantage and something in the game, such as the halfling's Lucky trait, lets you reroll the d20, you can reroll only one of the dice. You choose which one. For example, if a halfling has advantage or disadvantage on an ability check and rolls a 1 and a 13, the halfling could use the Lucky trait to reroll the 1.
You usually gain advantage or disadvantage through the use of special abilities, actions, or spells. Inspiration can also give a character advantage. The GM can also decide that circumstances influence a roll in one direction or the other and grant advantage or impose disadvantage as a result.
The odds
Rolling two d20s and keeping one changes your chance of reaching a number. Worked out for each target, with no modifiers:
Need on the d20
One d20
Advantage
Disadvantage
2 or higher
95%
99.7%
90.3%
4 or higher
85%
97.8%
72.2%
6 or higher
75%
93.8%
56.3%
8 or higher
65%
87.8%
42.3%
10 or higher
55%
79.8%
30.3%
12 or higher
45%
69.8%
20.3%
14 or higher
35%
57.7%
12.2%
16 or higher
25%
43.8%
6.3%
18 or higher
15%
27.8%
2.3%
20 or higher
5%
9.8%
0.3%
On average one d20 rolls 10.5, the higher of two 13.8 and the lower of two 7.2. In the middle of the range advantage is worth about +5 and disadvantage about −5, which is why a passive check adds or subtracts 5 (see Passive Checks); near the ends of the range it’s worth less.
How advantage and disadvantage work
When you have advantage on an attack roll, ability check or saving throw, you roll two d20s and use the higher. With disadvantage, you use the lower. That’s the whole mechanic, and 5e uses it in place of most small bonuses and penalties.
They don’t stack. Three reasons to have advantage still mean one extra d20. And if you have any source of advantage and any source of disadvantage on the same roll, they cancel completely and you roll one d20, even if it’s three sources against one.
If something lets you reroll, like a halfling’s Lucky trait, you reroll only one of the two dice, your choice.
What it’s worth and where it comes from
Advantage is worth about +3 to +5 on a roll where you need something in the middle of the die, and less at either end. If you need an 11, it turns a 50% chance into 75%. It also nearly doubles your chance of a critical hit, from 5% to 9.75%.
Common sources of advantage: attacking a creature that can’t see you, an ally’s Help action, a barbarian’s Reckless Attack, attacking a creature that is prone (from within 5 feet), restrained, stunned, paralyzed or blinded, and spells like Faerie Fire and Guiding Bolt.
Common sources of disadvantage: attacking a target you can’t see, shooting at long range, making a ranged attack with an enemy within 5 feet, being poisoned, frightened or prone, and a Small creature using a heavy weapon.
Watch for cancelling. Before you spend a spell to give advantage, check whether something already imposes disadvantage. Removing that disadvantage is worth just as much.
What changed in 2024
The rule itself is unchanged in the 2024 Player’s Handbook: roll two d20s, never more, and advantage and disadvantage cancel. What changed are some sources. Inspiration (now Heroic Inspiration) lets you reroll a die instead of granting advantage, exhaustion now subtracts 2 per level instead of imposing disadvantage, and being invisible gives advantage on initiative while being surprised gives disadvantage.
Common questions
Does advantage stack?
No. However many things give you advantage, you roll just one extra d20. The same goes for disadvantage.
What happens if I have advantage and disadvantage?
They cancel, and you roll a single d20. It doesn’t matter how many sources there are on each side: one source of disadvantage cancels any number of sources of advantage.
How much is advantage worth?
Roughly +5 when you need about an 11 on the die, and around +3 on average across all target numbers. It matters most in the middle of the range and least when you need a 2 or a 20.
Does advantage increase my chance to crit?
Yes. You crit if either die shows a 20, which is a 9.75% chance instead of 5%.
Can I use Lucky with advantage?
Yes, but you reroll only one of the two dice. With disadvantage and a 1 on one die, Lucky can turn it into a fair chance.
Does advantage apply to damage rolls?
No. It only applies to attack rolls, ability checks and saving throws, the rolls made with a d20.